Chapter 5 PERIODIC CLASSIFICATION OF ELEMENTS

The basis of arrangement is atomic mass. The mass of atom expressed in atomic mass units.

Now they are arranged on the basis of their atomic number. The number of protons in the nucleus of the atom is the atomic number of that element.

(a) This Statement is False. He divided the elements into horizontal rows of seven elements.

(b) This Statement is False. He stated that the properties of elements are a periodic function of their atomic weight.

(c) This Statement is False. They are arranged according to their atomic masses.

This Statement was given by famous Russian Chemist Dmitri Mendeleev.

(a) Periods have elements with consecutive atomic number.

(b) This Statement is Correct.

(c) This Statement is Correct.

(a) This law was given by the English chemist J.A.R Newlands in 1865.

(b) This Law was given by Dobreiner.

Mass of B is 23. According to Dobereiner the atomic weight of middle element is nearly the same as average of the atomic weight of other two elements.

There are six elements between X and Y elements.

The classification was according to the atomic mass of the elements.

Guidance to Mendeleev was provided by the following two factors:

1) The first factor was the increasing atomic mass.

2) The second factor was grouping together of elements having similar properties.

The two elements are Gallium and Scandium. Gallium was also called Eka boron and Scandium was also called Eka Alluminium.

These names were given by Mendeleev. Later on eka-boron was named as Scandium, eka-alluminium was named as Gallium and eka- silicon was named as germanium.

The two elements whose properties were predicted on the basis of their position in the periodic table are Gallium and Scandium. Gallium was also called Eka boron and Scandium was also called Eka Alluminium.

Acc. To Dobereiner the average of first and third element atomic masses is equal to the atomic mass of second element. For example: Lithium, Sodium and Potassium. Lithium is first element, sodium is middle element and potassium is third element so the average of atomic masses of lithium and potassium must be equal to Sodium. This is true. Atomic mass of Li is 7 while atomic mass of K is 39. And their average is 23 i.e. the atomic mass of sodium.

Noble gas could be placed without disturbing the original order because they are unreactive. Due to this they could be placed in a separate group.

(a) The new or modern periodic table is made on the basis of atomic number.

(b) They are called periods and the vertical rows in a periodic table are called as groups.

(c) Group 1 elements are alkali, these are the bases which can be mixed with water.

(d) They are known as Halogens. Halogens are the group of non-metallic elements.

(e) They are known as Noble gases. Noble gases are unreactive gases.

(f) This is because sulphur is the eighth element starting from oxygen. According to Newland’s statement the property of the element is similar to the element which has 8th position from the starting element.

The elements present in the Vertical rows are known as Group.

The elements present in the Horizontal Rows are known as Period.

There are 18 groups and 7 periods in the modern periodic table.

(i) Sodium and Potassium are the first group elements.

(ii) Chlorine and Bromine are the 17 group elements.

(iii) Argon and Xenon are the 18 group elements.

Lithium and Beryllium are the metals among the first ten elements of modern periodic table.

The properties of the elements in the modern periodic table are a function of their atomic number.

For example

The atomic number of the hydrogen is 1 and Lithium is 3 so both the elements have 1 valence electrons.

Isotopes of same elements have same atomic number so they are placed at same position.

In modern periodic table the elements are placed according to their atomic number so cobalt is 27 and nickel is 28, so in modern periodic table cobalt comes first and then Nickel.

Hydrogen is placed in 1^st group which belongs to alkali metals because it has a valence electron and it acts as an alkali element.

On the left side of the periodic table we will find the metals. The alkali metals and the alkaline earth metals are placed on the left side of the periodic table.

Non metals like oxygen, sulphar, phosphorus etc are placed on the right side of the modern periodic table.

Boron, Silicon, Germanium etc are called metalloids and divide metals and the non metals in the periodic table.

Hydrogen, Rubidium and Caesium have 1 electron in their outermost shells.

Barium and Radium are the two elements which have two elements in their outer most shell.

The noble gases like Argon, Krypton, Xenon have three elements with completely filled outermost shells.

When elements are arranged in oder of their atomic masses then the average of first and third element atomic masses is equal to the atomic mass of second element.

For example- In alkaline earth metals group the first element is beryllium, the middle element is magnesium and the third element is calcium, so acc. To the Debereiner’s triad the arithmetic mean of the atomic masses beryllium and calcium is equal to the atomic mass of magnesium.

Newland gave the law of octaves in 1864. According to this law, when elements are arranged in the order of increasing atomic masses then the physical and chemical property of every eighth element is similar to first element. The idea of this law came to his mind from the seven musical notes of the music.

For example- Sodium and Lithium have same property because sodium is the eighth element from Lithium.

Yes Dobereiner’s triads also exist in the columns of Newland’s law of octaves. In Newland’s Periodic table in the second column lithium, sodium and potassium are present. Starting with lithium as the first element, the eighth element will be sodium. According to Newland’s law of octaves the properties of every 8^th element should be similar to the properties of the first element. So, properties of lithium and sodium should be same. Also, if we start from sodium as the first element, then potassium is the eighth element so their properties should also be similar. Hence, according to Newland’s law of octaves, lithium, sodium and potassium have similar chemical properties. It is also known to us that lithium, sodium and potassium form a Dobereiner’s triad. Hence, we can conclude that Dobereiner’s triads also exist in the columns of Newlands octaves.

A major limitation of DObereiner’s classification was that he could not arrange all the existing elements in the form of triads which comprised of elements having same chemical properties. He could only identify three sets of triads at that time. This drawback became the reason for his classification not becoming successful.

Besides this, he failed to describe any relation between the atomic mass of elements and the chemical properties.

Limitations of Newland’s law of octaves are listed as below:

(i) This law was applicable only for lighter elements. It did not hold true for the classification of elements after calcium. After calcium, not every eighth element had properties similar to the first element.

(ii) He thought that only 56 elements existed in nature and that no more would ever be discovered. However, later many new elements were discovered and their properties did not fit into Newland’s law of octaves.

He put two elements in one slot even in the column of unlike elements having different properties. For example, he put cobalt (Co) and nickel (Ni) in a single slot with elements like fluorine, chlorine and bromine with which their properties do not match at all.

Mendeleev’s periodic law was based on the statement that physical and chemical properties of the elements are the periodic function of their atomic masses.

This law was changed because there were many demerits in this law, the position of hydrogen in Mendeleev’s periodic table was confusing. He also put three elements in the same block.

Noble gases like Argon, Hellium, Xenon are placed in the separate group because these gases are highly unreactive and their properties do not match the properties of the other elements.

The merits are:-

1) He predicted that not all elements had been discovered that time and there was a possibility of discovery of some in future.

2) According to his classification, the properties of elements could be predicted on the basis of their positions in the periodic table.

3) The most important merit of Mendeleev’s classification was that it easily accommodates noble gases after they were discovered.

The two anomalies were:-

1) It could not explain the position of isotopes. Since the elements were arranged according to the atomic masses, isotopes should have been given separate positions, which was not the case. The isotopes were not given separate places. This could not be explained by Mendeleev’s periodic law.

2) It could not explain the wrong order of atomic masses. When the elements were put in their correct group according to their chemical properties, it was discovered that the element with higher atomic mass came first and the element with lower atomic mass came later.

Eka- Alluminium and Gallium is the same element suggested by Mendeleev. The properties like density, mass, chlorine and oxide formula is almost same of eka alluminium and gallium.

(i) Scandium was named as Eka- Boron.

(ii) Gallium was named as eka- alluminium

(iii) Germanium was named as eka- silicon.

We classify elements into groups or periods in such a way that the elements of the same group possess similar properties and this makes the study of large number of elements reduced to the study of few elements.

Two criteria used by Mendeleev were:

(i) Mendeleev arranged the elements in increasing order of their atomic masses.

(ii) He put elements having similar properties in same group.

Mendeleev left some gaps in his periodic table because he wanted to make sure that the elements having similar properties came in the same groups.

Mendeleev did not mention noble gases in his periodic table because at that time noble gases were not invented, so there was no group of noble gases in Mendeleevs’s Periodic table.

It is so because they have similar chemical properties and same atomic number.

According to Mendeleev’s periodic law the properties of the elements are a periodic function of their atomic masses.

The formulae of oxides and hydrides of the elements having similar chemical properties are same, these properties helped Mendeleev to create his periodic table.

The three limitations of Mendeleev’s Classification are:

(i) The position of hydrogen was a big issue, he put hydrogen in the group of alkali metals.

(ii) He did not explain the wrong order of atomic masses of some elements.

(iii) The position of the isotopes of some elements could not be explained by him.

The other two elements for which Mendeleev had left gaps in his Periodic table are Silicon and Germanium.

The group of Noble Gases was missing from Mendeleev’s original Periodic table because at that time noble gases were not discovered.

Modern Periodic law which was given by Bohr stated that the physical and chemical properties of elements are the periodic function of their atomic number.

In the modern periodic table, the elements having same electronic configuration are placed in a group i.e have same number of valence electrons in their last shell.

When the elements are arranged on the increasing order of their atomic number, then the anomalies could be removed.

(i) The position of isotopes was resolved in the modern periodic law. All the isotopes of an element have the same number of protons, so their atomic number is also the same. Due to same atomic number, they can be put in one place.

(ii) The position of cobalt and nickel was also resolved. The atomic number of cobalt is 27 and that of nickel is 28. Now, according to modern periodic law, the cobalt with lower atomic number (27) should come first and nickel with higher atomic number (28) should come later, even if their atomic masses are in the wrong order.

(iii) The position of hydrogen is also resolved because in modern periodic law hydrogen is placed in first group of alkali metals because alkali have 1 valence electron in its last shell and hydrogen also has 1 electron in its last shell.

It is not possible to place an element with an atomic number of 1.5 between hydrogen and helium. This is because elements are placed in the increasing order of their atomic numbers. Atomic numbers are always whole numbers and cannot be in decimals, as half an electron or half a proton cannot exist .

Niels Henrik David Bohr was the physicist from Denmark who prepared the modern periodic law.

In Mendeleev’s periodic table, gap was not left for Beryllium.

The Newland’s law of octaves for the classification of elements was found to be applicable only up to Calcium.

According to Mendeleev’s periodic law, the elements were arranged in the periodic table in the order of increasing atomic masses.

The three elements having chemical symbols of Si, B and Ge are αλλμεταλλοιδσ.

In Mendeleev’s periodic table, gaps were left for the elements to be discovered later on. An element which found a vacant place in the periodic table later on is Ge.

The three imaginary elements X,Y and Z represent a Dobernenner’s triad. If the atomic mass of element X is 14 and that of element Y is 46, then the atomic mass of element Z will be 78.

The atomic numbers of four elements P, Q, R and S are 6, 8, 14 and 16 respectively. Out of these, the element known as metalloid is R.

Regarding to the classification of elements, In modern periodic table, the isotopes of chlorine having different atomic masses are kept in the same group

.

The element was placed in the third group of Mendeleev’ periodic table.

The modern periodic table was prepared by Bohr.

The atomic particle whose number in the atoms of an element always remains the same and which forms the real basis for the modern classification of elements is Proton.

(a) We have to calculate the average atomic mass of elements X and Z

Avg. atomic mass= Atomic mass of X + Atomic mass of Z

Avg. atomic mass= 7+39/2

Avg. atomic mass= 23

(b) The average atomic mass of elements X and Z is equal to the element Y.

So X, Y and Z elements follow the law of Doberenier’s.

(c) The given elements follow the law of Doberenier.

(d) The atomic mass of X element is 7, so this element is Lithium. The atomic mass of Y element is 23, so this element is Sodium and the atomic mass of Z element is 39 so this element is Potassium.

(e) The three elements chlorine, bromine and iodine also follow the law of doberenier.

In the above elements Sodium does not belong to this group because Sodium belongs to the group of alkali metals and the rest three belong to the alkaline earth metals group.

From the given elements chlorine does not belong to the set because all the remaining elements belong to the 2^nd period of the modern periodic table but chlorine belongs to the 3^rd period.

(a) Na, Si and Cl, these three elements have different physical as well as chemical properties so they do not belong to dobereiner’s triads, even if the atomic mass of Si is equal to the arithmetic mean of atomic mass of Na and Cl.

(b) Be, Mg and Ca, these three elements have similar properties and the arithmetic mean of the atomic mass of Be and Ca is equal to the atomic mass of Mg. Therefore these elements belong to the Doberenier’s Triads.

(a) The elements Na, Mg and Al belong to the same period. They belong to the 2^nd period of the modern periodic table.

(b) The elements Li, Na and K belong to the same group, they belong to the first group of the modern periodic table.

(a) So in this electronic configuration there are three shells and there are 2 electrons in its last shell, so the total electrons are 12. So this element is Alluminium.

(b) The element which has two shells and both are completely filled with electrons is Neon. Neon has 2 electrons in its first shell and 8 electrons in the second.

(c) The element which has three shells and 4 electrons in its valence shell is Silicon.

(d) The element which has two shells and two electrons in its valence shell is Boron (2,3).

(e) As we know only 2 electrons can exist in the first shell, so it means there are 4 electrons in the valence shell of this element. Therefore this element is Carbon (2,4).

Li, Na and K, these elements from the above elements are the alkali metals and they have 1 electron in its valence shell.

Ca, Sr and Ba, these three elements from the above elements are the alkaline earth metals and having 2 electrons in its valence shell.

Cl, Br and I, these elements from the above elements are the non metals and also termed as Halogens.

(a) (i) Gallium is the new element which has taken the place of eka – aluminium.

(ii) Germanium is the new element which has taken the place of eka silicon.

(b) These elements are placed in the 4^th period of the modern periodic table.

(c) As we know that in musical scale the eight notes are repeating so Newland compared it with the musical scale because in his law the property of the 1^st element is same as the 8^th.

(d) This classification of the element was only applicable to the Calcium.

As we move horizontally from left to right the metallic character is decreasing.

The atomic size of the element is decreasing as we move from left to right.

The tendency to gain electrons increases on moving from left to right in a period of the periodic table.

The tendency to lose electrons decreases as we go from left to right across a period of the periodic table.

The alkali metals group is the first group of the periodic table so as we move top to dowm from lithium to francium the chemical reactivity increases.

Group 17 is the group of halogen elements of the periodic table, as we move top to down in the group 17 of halogens the chemical reactivity decreases from fluorine to iodine.

The property of valence electron is same in a group of periodic table, the number of valence electron in the last shell of the elements in a same group is same.

Flourine belongs to the halogen family of the periodic table which comes in 17^th group. Flourine has 1 valence electron in its last shell so all the elements of the 17^th group have 1 valence electron in its last shell.

One valence electron in the atoms of the first element in the period.

Usually 8 valence electrons are present in the atoms of the last element in a period.

This statement is false because as we go down in a group of the periodic table the number of valence electrons in the last shell of the elements remain same.

The first elements in the periods of the modern periodic table are called the alkali metals and the major characterstic of the first elements in the periods of the table is that they have one valence electron in their last shell.

When we move left to right in a period of the periodic table the atomic radii of elements decreases.

The metallic character of the elements increases as we go down in a group of the periodic table.

(i) In the first period of the periodic table when we move left to right the number of valence electron increases from 1 to 2.

(ii) In the second period of the periodic table when we move left to right the number of valence electron increases from 1 to 8.

As we move left to right in the third period of the periodic table the valency of the element increases from 1 to 4 then decreases to zero.

When we move from top to bottom in any group of the modern periodic table the valency of all the elements in that group remains same.

(a) The element in the first group and third period is Sodium.

(b) The element in the 17^th group and 2^nd period is Fluorine.

Electronic configuration of elements are as follows:

Lithium (2,1), Beryllium(2,2), Boron(2,3), Carbon(2,4), Nitrogen(2,5), Oxygen(2,6), Fluorine(2,7) and Neon(2,8).

Here Flourine has the least atomic raddi then Nitrogen then Beryllium and Lithium has the largest atomic radii among these.

These are arranged as follows:-

Ga<Mg<Ca<K

(i) This statement is wrong. The elements in the same group have equal valency.

(ii) This statement is wrong. The metallic character of elements in a period decreases gradually on moving from left to right.

(a) Periods

The horizontal rows are the periods.

(b) Increases

In going across a period (right to left) in periodic table, the atomic size of the atom increases.

(c)decreases

On moving from right to left in the second period, the number of valence electrons decreases.

(d) On going down in a group in the periodic table, the metallic character of elements increases.

(e)The tendency to gain an electron decreases on moving down in a group of the periodic table.

Electronic Configuration of Nitrogen is (2,5) and Phosphorous is (2,8,5). Among these two nitrogen is more electronegative because nitrogen is smaller in size so the attraction between the nucleus and the incoming electron of nitrogen is more.

(a) Electron X belongs to the group 2^nd of the alkali metals so the number of valence electrons in element X is 2.

(b) The valency of the element X is 2.

(c) The element belongs to the 15 group so the number of valence in Y is 5.

(d) Valency of electron Y is 5.

Periods are the horizontal rows present in the periodic table. As we move from left to right in a period, the atomic number of elements increases which means that the no. of protons and electrons in the atom increases. Due to large positive charge on the nucleus, the electrons are pulled in more close to the nucleus and the size of the atom decreases.

(i) When we move left to right in a period of the periodic table the chemical reactivity of elements first decreases and then increases.

For example- In the 3rd period of elements, sodium is a very reactive element, magnesium is less reactive whereas aluminium is still less reactive. Silicon is the least reactive in the third period. Now, phosphorus is quite reactive, sulphur is still more reactive whereas chlorine is very reactive.

(ii) When we move from left to right in a period the nature of oxides of elements decreases and the acidic nature of oxides increases.

Example- In the 3rd period of the periodic table, sodium oxide is highly basic in nature and magnesium oxide is comparatively less basic. The aluminium and silicon oxides are amphoteric in nature. Phosphorus oxides are acidic, sulphur oxides are more acidic whereas chlorine oxides are highly acidic in nature.

As we know that in modern periodic table the atomic number increases as we move from left to right in a period which means that the no. of protons and electrons also increases .When the amount of protons and electrons increases in the elements then the positive charge also increases in the nucleus of the elements, so nucleus pulls electrons and the size of the atom decreases.

The metallic character of the elements decreases as we move from left to right in a period. In group 1 and 2 maximum amout of metals are placed in the modern periodic table.

(i) All the elements of a group have similar chemical properties because the elements which are placed in a same group have same amount of electrons in its valence shell or we can say that the valency of the elements in a same group are same.

(ii) Periods are the horizontal rows present in the modern periodic table. So the elements which are present in the same periods have different valency that’s why the chemical properties of the elements in a same period are different.

In the modern periodic table as we move in a periof left to right the atomic number of the element increases and the size of the atom is decreased.

X<Z<Y

These elements are arrange in this manner, X comes first because its atomic radii is greatest among these three and then Z and lastly Y is placed because it has least atomic radii among all.

The electropositive character of elements increases on going down in a group of periodic table because electro positivity means how muchelectrons an element can lose. so as we move down in a group the number of shell increases and more easily the element can lose electrons.

(i) In a group, the elements have same number of electron in its last shell so the valency of electrons in the group is same.

(ii) In a period as we move left to right, first the valency of the electron increases from 1 to 4 and then decreases to zero.

As we know that group 1 elements are called alkali metals and group 2 elements are called alkaline earth metals, in group 1 the number of valence electrons in the valence shell is 1 and in group 2 the electrons in valence shell are 2.

(i) The elements whose valence shell electrons are same, it means that those elements have similar chemical properties.

(ii) The first element can be identified by checking its valence shell electrons, elements which have 1 electron in their valence shell then these elements are the first element of the period.

the elemwants in the 18^th group of the periodic table are noble gases. So the valence electron in its shell is 8 and the valency is 0.

As we move down in the group the valence electrons in the element of the same group is same.

The last element of the periodic table means the elements which are present in the 18^th group of the periodic table and as we know that the 18^th group elements are known as the noble gases. Except helium all the noble gases are filled with 8 electrons in their valence shell.

(a) Number of elements in the 1^st periods are 2.

(b) Number of elements in the 3^rd period are 8.

As we move top to bottom in the group a new shell is added in every element so the atomic size of the element increases.

Lithium, sodium and potassium are the elements of the same group so the number of valence electron in their shell are same so these elements are similar in the chemical properties.

As we move down in the group a new shell increases in every element of the group so as the shell is added the distance between the nucleus of the atom and the valence electron increases, so tendency of loosing electrons increases as we move down in a group.

The tendency to gain electrons decreases as we move down in the group 17 because at every step the shell is included and because of this the distance between the nucleus and the valence electron increases and the size of atom increases.

The size of the atoms progressively become smaller when we move from sodium to chlorine in the third period of the periodic table because as we move left to right in a period the positive charge on the nucleus increases and tendency to pull the electron increases so the size decreases.

Both the elements are noble gases and the valence shell is fully filled with the valence electron.

Cobalt comes first because as we know the modern periodic table is based on the increasing order of atomic number. So the atomic number of cobalt is 27 and nickel is 28. That’s why cobalt comes first in the modern periodic table.

The position of hydrogen is a big issue in Mendeleev’s periodic table, hydrogen is placed in the group of alkali metals because it also combines with halogens, oxygen and sulphar to form compounds having similar formulae.

In the periodic table, the horizontal rows of elements are called periods and the vertical rows of elements are called groups.

Characteristics of a Period:

i) Elements have consecutive atomic numbers.

ii) Number of elements in period is fixed a andhe maximum number of electrons which can be accommodated in various shells.

Characteristics of a Group:

i) Elements do not have consecutive atomic numbers.

ii) All elements have similar electronic configuration and show similar properties.

In a Period, the atomic size of elements decreases as we move from left to right because the number of protons and electrons increases which results in the force of attraction leading to the reduction of size of atom.

While in a Group, the number of shells increases as we go down the group resulting in the increase of atomic size.

(i) Atomic size increases gradually from Lithium to Francium.

(ii) It increases from Lithium to Francium.

As we go down the group, the atomic size and the metallic character of elements increases. Thus, the size of atom increases from Lithium to Francium. Also, Lithium is least metallic while Fancium is the most metallic element.

This Statement is wrong. The correct statement is that Period have elements with consecutive atomic number.

The first period of the modern periodic table has only two elements because there are two elements in the first period Hydrogen and Helium. In these two elements there is only one shell present and it can only take two elements. Second period has 8 elements because it has 2 shells and 2^nd shell will not take electrons more than 8.

Elements in the same group have same number of electrons in their valence shell but the elements of different groups have different number of electrons in their valence shell.

(i) Among these three elements Flourine has least atomic radius and Bromine is chemically least reactive.

(ii) Among these three elements Lithium has least atomic radius and it is also least chemically reactive.

Both the elements Flourine and Chlorine are the halogens so they are in the group of halogens family and they both have 7 electrons in their valence shell so they are kept in the same group of periodic ttable.

(i) The main advantage that it shows that why elements in a group show similar properties and elements in different groups show different properties.

(ii) The modern periodic table is based on the increasing order of their atomic number.

The elements which are present in the vertical column is called the group.

(i) The lowest part of the table have the greatest metallic character.

(ii) The lowest part of the table have the largest atomic size.

The elements present in the 1^st group of the periodic table have 1 electron in its valence shell.

For Example- Lithium has 1 electron in its valence shell.

The element present in the 17^th group have 7 electrons in its valence shell.

For example- Fluorine has 7 electrons in its valence shell.

which element will be the first and which will be the last is decided by checking the valence electron in its last shell.

This is the standard way of electronic configuration which is repeated after 2,8,18 and 32 elements in the periodic table. Hence the elements show similar properties.

The advantages of periodic table:-

(i) This periodic table makes the study of chemistry easy.

(ii) If the position of the element in the periodic table is known then it is easy to study about that element.

(iii) Compounds that are formed can be predicted by knowing the position of the elements.

The atoms lose their electrons more easily. This statement about the trends when going from left to right across the periods of the periodic table is not correct.

The Electronic configuration of the atom of an element X is 2,8,4.In modern periodic table, the element X is placed in 14^th group.

The atomic number of an element is 20. In modern periodic table, this element is placed in 4^th period.

Five elements A,B,C,D and E have atomic numbers of 2,3,7 10 and 18 respectively.The elements which belong to the same group of the periodic table is B,C,D

The elements A, B, C, D and E have atomic numbers 9, 11, 17, 12 and 13 respectively. The pair of elements which belongs to the same group of the periodic table is A and C.

Potassium would lose an electron easily.

Fluorine does not lose electron easily.

The element with electronic configuration 2, 8 in the modern periodic table is located in group 18

An element which is an essential constituent of all organic compouds belongs to group 14 of modern periodic table

L shell is the valence shell for the elements of second period of the modern periodic table.

The element which has the maximum number of valence electrons is P.

The correct increasing order of the atomic radii of the elements oxygen, fluorine and nitrogen is F, O, N.

The atomic numbers of the elements Na, Mg, K and Ca are 11, 12, 19 and 20 respectively. The elements having the largest atomic radius is K.

The correct characteristics of isotopes of an element is that they have same atomic number and chemical properties.

The correct formula of the oxide of Eka-aluminium element predicted by Mendeleev was Ea₂O_3.

The element which can form an acidic oxide should be the one whose atomic number is 16.

The element which forms a basic oxide has the atomic number of 19.

Valence electrons do not increase while moving down the group of the periodic table

On moving from left to right in a period of the periodic table, the atomic number of elements increases. The size of atoms of elements on moving from left to right in a period is decreases.

Be, Mg, Ca, This set of elements is written correctly in the order of their increasing metallic character.

(i) X and Z elements have atomic number 2 and 10. They have0 valency so they belong to the group of noble gas.

(iii) Y and Z have two valence electrons in their last shell so they belongs to the same period.

(a) The election structure is (2,7) so we have to add these electrons to get the atomic number. Therefore the atomic number is 9.

(b) Chlorine which has atomic number 17 is chemically similar.

(c) They expect to be similar because both have same number of valence electron 7.

(i) Argon and Hellium are the most stable among these all.

(ii) Calcium and Beryllium have 2 valence electrons so they belong to 2^nd group.

(iii) Oxygen and Sulphar have 6 valence electrons so the belong to 16 group.

(a) Mg is bigger in size because as we move left to right in period the atomic size decreases.

(b) K is bigger in size because as we move down the group the atomic size increases.

(i) Element C belongs to group 10 because it has 8 elements in the last shell and its valency is 0.

(ii) elements belongs to group 15 because it has 5 electrons in its last shell.

(iii) an element belongs to the group 13 because it has 3 electron in its valence shell.

(iv) These elements belong to the 2^nd period because all the 2 shells are filled.

(a) This Element X has 2 valence electrons.

(b) The valency of X element is 2.

(c) The element X is metal.

(d) Element X is known as Magnesium.

(i) an element has a bigger atom because when we move left to right the size of the atom decreases.

(ii)The valency of o element is 1 and k element is 3.

(iii) When we move from left to right the metallic character decreases, so element i is more metallic.

(iv) When we move left to right in the period then the non- metallic character increases, so element g is more non metallic.

(v) letter b represents a metal of valency 2.

(vi) letter f represents a non metal of valency 2.

(a) The formula of its chloride is XCl₂ because it has two electrons in its valence shell.

(b) The formula of its oxide is XO.

(i) This metal is surely non metal because metals are represented in the left side of periodic table.

(ii) Its electronic configuration is (2,6), so it has 6 valence electrons.

(iii) It has 6 electrons in its valence shell therefore the valency is 2.

(iv) The element which has configuration (2,6) is Oxygen.

(v) The formula with sodium id Na2Y.

This element X belongs to the 1^st group of the periodic table. The electronic configuration is (2,8,8,1) of this element.

The element is Oxygen because X element is monovalent and Y has to be divalent to form compound X2Y.

(a) 2 electrons would be there in the outermost shell of the element X.

(b) Group 2 of the periodic table does the element X belong.

(c) The formula of the compound formed when X reacts with chlorine is XCl2.

(a) The element A is present in group 14 and the element B is present in group 17.

(b) Covalent bond is formed between A and B

(c) The formula should be AB4 of this compound.

(ii)

Element C and D produce a covalent bond.

(a) The formula should be XY of the compound.

(b) The nature of bond in the compound formed is Ionic bond.

In the first group of the periodic table there is only one electron in the last shell of the elements, hence valency of 1^st group element is 1. So they react with oxygen which have valency 2.

The formula will be X2O.

(i) The element A and element B are the two non metals. Therefore covalent bonds are formed between the two non metals.

(ii) The formul should be AB2.

(a) The nature of the compound is Ionic.

(b) The compound formed will conduct electricity.

(c) The formula should be XY2.

(d) The valency of the element X is 2 because it belongs to the second group of the periodic table.

(e) The Y element belongs to 17^th group so there are 7 electrons in the outermost shell.

(i) the letter d which representsan alkali metal

(ii) the letter c which represents a noble gas

(iii) the letter e which represents a halogen

(iv) The bond formed between a and e is a covalent bond.

(v) The bond formed between d and e is ionic bond.

(a) The two elements B and C will form a covalent compound.

(b) Two elements A and C will form an ionic compound.

The neutral atom in the periodic table which has the same number of electrons as K+ and Cl- is Argon atoms. Argon atoms have 18 electrons.

(a) These elements belong to the 3^rd period of the periodic table.

(b) The nature of compound formed by a combination of elements B and F is Ionic Compound.

(c) Elements A and Elements B could definitely be metals.

(d) H element is most likely to be found in gaseous state at room tempreture.

(e) The formula of the compound formed by the combination of C and G is CG2.

The two very reactive metals are sodium (Na) and Potassium(K) belonging to group 1 of the Periodic Table.

Sodium is a reactive metal, it react with halogen to form an ionic chloride called sodium chloride.

The bond formed in this compound is ionic bond and the compound is called as ionic compound.

The physical properties of Ionic Compounds are:-

(i) These compounds are hard and brittle.

(ii) They can conduct electricity

(iii) The melting and boiling point of this compound is high.

(iv) Most of the ionic compound are soluble in water.

(a) A is Carbon(C), B is Carbon monoxide(CO) and C is carbon dioxide (CO₂).

(b) A belongs to the 14^th group of the periodic table.

(c) Silicon can be placed in place of A.

(a) X is Nitrogen Gas (N₂), Y is Ammonia (NH₃) and Z is ammonium sulphate (NH₄)₂SO₄.

(b) X belongs to the 15^th group of the periodic table.

(c) X is placed in 2^nd period of the periodic table .

(d) Carbon is placed just before X in the Period

(e) Oxygen is placed just after X in the period.