CBSE CLASS 10 CHAPTER 5 PERIODIC CLASSIFICATION OF ELEMENTS

Scientists tried to classify the elements multiple times, earliest attempt to classify the elements resulted in grouping the then known elements as metals and non-metals.

Döbereiner’s Triads

Johann Wolfgang Döbereiner, a German

chemist, tried to arrange the elements with similar properties into groups. He showed that when the three elements in a triad were written in the order of increasing atomic masses; the atomic

mass of the middle element was roughly the average of the atomic masses

of the other two elements.

triad consisting of lithium (Li), sodium (Na)

and potassium (K) with the respective atomic masses 6.9, 23.0 and 39.0

Li	Ca	Cl
Na	Sr	Br
K	Ba	I

triads of elements which led to

the development of the Periodic Table of elements

Newlands’ Law of Octaves

John Newlands,

an English scientist, arranged the then known elements in the order of

increasing atomic masses. He started with the element having the lowest

atomic mass (hydrogen) and ended at thorium which was the 56th

element. He found that every eighth element had properties similar to

that of the first

properties of lithium and

sodium were found to be the same. Sodium is the eighth element after

lithium. Similarly, beryllium and magnesium resemble each other

sa	re	ga	ma	pa	da	ni
(do)	(re)	(mi)	(fa)	(so)	(la)	(ti)
H	Li	Be	B	C	N	O
F	Na	Mg	Al	Si	P	S
Cl	K	Ca	Cr	Ti	Mn	Fe
Co	and	Ni	Cu	Zn	Y	In	As	Se
Br	Rb	Sr	Ce	and	La	Zr	—	—

Law of Octaves was applicable only upto

calcium, as after calcium every eighth element did not possess

properties similar to that of the first

Note that cobalt

and nickel are in the same slot and these are placed in the same

column as fluorine, chlorine and bromine which have very different

properties than these elements. Iron, which resembles cobalt and

nickel in properties, has been placed far away from these elements

1. Did Döbereiner’s triads also exist in the columns of Newlands’ Octaves? Compare

and find out.

Answer:

Döbereiner’s triads did exist in the columns of Newlands’ Octaves; For example, the

elements

Lithium (Li), Potassium (K) and Sodium (Na) constitute a Dobereiner’s Triad but are also

found in the second column of Newland’s Octaves.

2. What were the limitations of Döbereiner’s classification?

Answer:

(i) They were not applicable for very low mass or very high mass elements.

(ii) All the elements couldn’t fit into Dobereiner’s triads.

(iii) As the methods to calculate atomic mass improved, Dobereiner’s triads validity began to decrease. For example, in the triad of F, Cl and Br, the arithmetic mean of

atomic masses of F and Br are not equal to the atomic mass of CI.

3. What were the limitations of Newlands’ Law of Octaves?

Answer:

Limitations of Newlands’ Law of Octaves are as follows

Newlands’ Law of Octaves applicable to elements up to Calcium

Newland assumed there are 56 elements in the nature and no more elements would be discovered in the future.

To fit elements into table Newland put two elements into one slot. Newland

introduced unlike elements with different properties into one column.

Iron (Fe) was placed away from elements that resembles in properties. Ex: Nickel

and cobalt

MENDELÉEV’S

PERIODIC TABLE

Dmitri lvanovich Mendeléev

examined the relationship between the atomic masses of the elements

and their physical and chemical properties formulae

of the hydrides and oxides formed by an element were treated as one of

the basic properties of an element for its classification. He noticed elements got a place in

a Periodic Table and were arranged in the order of their increasing atomic

masses,

Mendeléev formulated a Periodic Law, which states that ‘the properties

of elements are the periodic function of their atomic masses

letter ‘R’ is used to represent any

of the elements in the group. Note the way formulae are written. For example, the hydride

of carbon, CH4, is written as RH4 and the oxide CO2, as RH4

2

 

Mendeleev considered formulae of hydrides and oxides formed by an element as one of the basic properties to classify the elements in his Periodic Table.

Hydrogen makes oxides similar to alkali metals, such as H₂O (Hydrogen oxide i.e. water) and hydrides similar to halogen such as LiH (Lithium hydride), so it could be placed with halogens in the same group i.e. in VII^th group. But, hydrogen was placed with alkali metals in I^st group in Mendeleev's Periodic Table.

It appears that hydrogen is placed as starting element in 1^st period and I^st group in the Mendeleev's Periodic Table. Thus, I think it was the correct position as hydrogen has been placed the same position even in Modern Periodic table.

Hydrogen should be assigned in 1^st period and I^st group. As I think Mendeleev also thought and considered hydrogen as starting element and gave the first place in his Periodic Table.

Activity 5.2

Consider the isotopes of chlorine, Cl-35 and Cl-37.
Question 1. Would you place them in different slots because their atomic masses are different?
Answer. No, the more fundamental base of classification is atomic number and not atomic mass
Question 2. Or would you place them in the same position because their chemical properties are the same?
Answer.Yes, both the isotopes are placed in same position because they have same chemical properties and similar atomic number.

Questions

1. Use Mendeleev’s Periodic Table to predict the formulae for the oxides of the

following elements: K, C, AI, Si, Ba.

Answer:

K- K2O

C-C2O4 or CO2

Al- Al2O3

Si-Si2O4 or SiO2

Ba2O2 or BaO

Oxygen is a member of group VI A in Mendeleev's periodic table. Its valency is 2.

Similarly. The valencies of all the elements listed can be predicted from their respective

groups. This will help in writing the formulae of their oxides.

(i) Potassium (K) is a member of group IA. Its valency is 1. Therefore, the formula of it is

K2O.

(ii) Carbon (C) is a member of group IV A. Its valency is 4. Therefore, the formula of it is

C2O4 or CO2.

(iii) Aluminium (Al) belongs to groups III A and its valency is 3. The formula of its oxide

is Al2O3.

(iv) Silicon (Si) is present in group IV A after carbon. Its valency is also 4. The formula

oxide is Si2O4 or SiO2.

(v) Barium (Ba) belongs to group II A and the valency of the element is 2. The formula

of oxide of the element is Ba2O2 or BaO.

2. Besides gallium, which other elements have since been discovered that were left

by Mendeleev in his Periodic Table? (Any two)

Solution:

Germanium and Scandium are the element that are left by Mendeleev in his Periodic

Table since its discovery.

3. What were the criteria used by Mendeleev in creating his Periodic Table?

Answer:

Mendeleev concentrated on various compounds formed by the elements with Hydrogen

and

Oxygen. Among physical properties, he observed the relationship between the atomic

masses

of various elements while creating his periodic table.

4. Why do you think the noble gases are placed in a separate group?

Answer:

Noble gases are placed in a separate group because of their inert nature and low

concentration in our atmosphere. They are kept in a separate group called Zero group so

that they don’t disturb the existing order.

 

Activity 5.3

Question 1. How were the positions of cobalt and nickel resolved in the Modern Periodic Table?
Answer. Modern Periodic Table is based on the atomic number. Cobalt (27) is placed before Nickel (28) in Modern Periodic Table

Question 2. How were the positions of isotopes of various elements decided in the Modern Periodic Table?
Answer.The isotopes have same atomic number so they are placed in the same group in Modern Periodic Table

Question 3. Is it possible to have an element with atomic number 1.5 placed between hydrogen and helium?
Answer.No, it is not possible because atomic number is a whole number

Question 4. Where do you think should hydrogen be placed in the Modern Periodic Table?
Answer.I think that the place of hydrogen in the Modern Periodic Table is correct

 

Activity 5.4

Question 1. Look at the group 1 of the Modern Periodic Table, and name the elements present in it.
Answer. The elements in group 1 are:
Hydrogen (H), Lithium (Li), Sodium (Na), Potassium (k), Rubidium (Rb), Cesium (Cs) and Francium (Fr).

Question 2. Write down the electronic configuration of the first three elements of group 1.
Answer.
(i) H (1) --> 1
(ii) Li(3) -->2, 1
(iii) Na (11) --> 2, 8, 1

Question 3.What similarity do you find in their electronic configurations?
Answer. All the elements have same number of valence electron, i.e. 1

Question 4. How many valence electrons are present in these three elements?
Answer. One (1)

Question 1. If you look at the long form of the Periodic Table, you will find that the elements Li, Be, B, C, N, O, F, and Ne are present in the second period. Write down their electronic configuration.
Answer.

Li (3)	--->	2,1
Be (4)	--->	2, 2
B (5)	--->	2, 3
C (6)	--->	2, 4
N (7)	--->	2, 5
O (8)	--->	2, 6
F (9)	--->	2, 7
Ne (10)	--->	2, 8

Question 2. Do these elements also contain the same number of valence electrons?
Answer. No

Question 3. Do they contain the same number of shells?
Answer. Yes
The elements of same periods have same number of shells but not same number of valence electrons, which increases by 1

 

Activity 5.6

 

Question 1. How do you calculate the valency of an element from its electronic configuration?
Answer.
Valency of metal : It is same as the number of valence electron i.e. 1, 2, and 3.
Valency of non-metal : Valency of non-metals can be calculated by subtracting number of valence electrons from 8 (i.e. 8- number of valence electrons).
For example :

8 - 4 = 4	8 - 5 = 3	8 - 6 = 2
8 - 7 = 1	8 - 8 = 0

Question 2.What is the valency of magnesium with atomic number 12 and sulphur with atomic number 16?
Answer.Magnesium (12) --> 2, 8, 2
Sulphur (16) --> 2, 8, 6
The valency of Magnesium is same as valence electron, i.e. 2
The valency of Sulphur is 8-6 = 2 because it is non-metal.

Question 3. Similarly find out the valencies of the first twenty elements.
Answer.

Sr No	Elements	Atomic No.	Configuration	Valency
1.	H	1	1	1
2.	He	2	2	0
3.	Li	3	2, 1	1
4.	Be	4	2, 2	2
5.	B	5	2, 3	3
6	C	6	2, 4	8-4 = 4
7.	N	7	2, 5	8-5 = 3
8.	O	8	2,6	8-6 = 2
9.	F	9	2, 7	8-7 = 1
10	Ne	10	2, 8	8-8 = 0
11.	Na	11	2, 8, 1	1
12.	Mg	12	2, 8, 2	2
13.	Al	13	2, 8, 3	3
14.	Si	14	2, 8, 4	8-4= 4
15.	P	15	2, 8, 5	8-5= 3
16.	S	16	2, 8, 6	8-6= 2
17.	Cl	17	2, 8, 7	8-7= 1
18.	Ar	18	2, 8, 8	8-8= 0
19.	K	19	2, 8, 8, 1	1
20.	Ca	20	2, 8, 8, 2	2

Question 4. How does the valency vary in a period on going from left to right?
Answer. Valency first increases 1 to 4 then decreases from 4 to 0 (1, 2, 3, 4, 3, 2, 1, 0).

Question 5. How does the valency vary in going down a group?
Answer. Valency remains the same because valence electrons do not change on going down in a group.


Activity 5..7

Atomic radii of the elements of the second period are given below:

Period II elements :	B	Be	O	N	Li	C
Atomic radius (pm) :	88	111	66	74	152	77

Answer.

Question 1. Arrange them in decreasing order of their atomic radii.
Answer.

Period II elements :	Li	Be	B	C	N	O
Atomic radius (pm) :	152	111	88	77	74	66

Question 2. Are the elements now arranged in the pattern of a period in the Periodic Table?
Answer. Yes

Question 3.Which elements have the largest and the smallest atoms?
Answer. Lithium (Li) has Largest atoms (152 pm)
Oxygen (O) has smallest atoms (77 pm)

Question 4. How does the atomic radius change as you go from left to right in a period?
Answer. The atomic radius decreases as we go left to right in a period
Activity 5.8 | Page 89 | Chapter 5. Periodic Classification of Elements| CBSE Class 10th Science

Question 1. Study the variation in the atomic radii of first group elements given below and arrange them in an increasing order.

Group 1 Elements :	Na	Li	Rb	Cs	K
Atomic Radius (pm) :	186	152	244	262	231

Answer.

Group 1 Elements :	Na	Li	K	Rb	Cs
Atomic Radius (pm) :	86	152	231	244	262

Question 2. Name the elements which have the smallest and the largest atoms.
Answer. Na (Sodium) has smallest atom
Ca (Calcium) has largest atom.
Question 3. How does the atomic size vary as you go down a group?
Answer.Atomic size increases as we go down a group


Activity 5..9

Question 1. Examine elements of the third period and classify them as metals and non-metals.
Answer. Elements of third period are : Na(11), Mg (12), Al (13), Si (14), P (15), S (16), Cl (17), Ar (18)

Elements	Atomic No.	Configuration	Metals / Non Metals
Na	11	2, 8, 1	Metal
Mg	12	2, 8, 2	Metal
Al	13	2, 8, 3	Metal
Si	14	2, 8, 4	Non-Metal
P	15	2, 8, 5	Non-Metal
S	16	2, 8, 6	Non-Metal
Cl	17	2, 8, 7	Non-Metal
Ag	18	2, 8, 8	Non-Metal
The elements having 1,2,3 valence electrons are metals while the elements having 4,5,6,7,8 valence electrons are non- metals.

Question 2. On which side of the Periodic Table do you find the metals?
Answer.On the left side
Question 3. On which side of the Periodic Table do you find the non- metals?
Answer. On the right side

Activity 5.10

Question 1. How do you think the tendency to lose electrons will change in a group?
Answer. The tendency of lose electrons increases in a group on going down

Question 2. How will this tendency change in a period?

Answer. The tendency of lose electrons decreases as we go left to right in a period.


Activity 5..11

Question 1.How would the tendency to gain electrons change as you go from left to right across a period?
Answer. The tendency of gaining electrons increases as we go left to right in a period upto 17th group. It decreases in 18th group.

Question 2. How would the tendency to gain electrons change as you go down a group?
Answer. The tendency of gaining the electrons decreases as we go down a group.

 

 

Questions

1. How could the Modern Periodic Table remove various anomalies of Mendeleev’s

Periodic Table?

Answer:

(a) In the Modern Periodic table elements are arranged in the increasing order of their

atomic number. This removes the anomaly regarding certain pairs of elements in

Mendeleev’s periodic table.

(b) Atomic number of cobalt is 27 and nickel is 28. Hence cobalt will come before nickel

even though its atomic mass is greater.

c) All isotopes of the same elements have different atomic masses, but same atomic

number; therefore they are placed in the same position in the modern periodic table.

2. Name two elements you would expect to show chemical reactions similar to

magnesium. What is the basis for your choice?

Solution:

Calcium and Beryllium are similar to Magnesium because all the three elements belong

to the same group and have 2 valence electrons in their outer shell.

3. Name

(a) Three elements that have a single electron in their outermost shells.

(b) Two elements that have two electrons in their outermost shells.

(c) Three elements with filled outermost shells

Answer:

(a) Lithium (Li), Sodium (Na) and potassium (k) have a single electron in their

outermost shells.

(b) Magnesium (Mg) and Calcium (Ca) have two electrons in their outermost shells

(c) Neon (Ne), Argon (Ar and Xenon (Xe) filled outermost shells

4. a) Lithium, sodium, potassium are all metals that react with water to liberate

hydrogen gas. Is there any similarity in the atoms of these elements?

(b) Helium is an unreactive gas and neon is a gas of extremely low reactivity. What,

if anything, do their atoms have in common?

 Answer:

They’ve one valence electron in their outermost shells and as a result of this, they are

very unstable. So, they readily react with water to liberate hydrogen. They are also called

alkali metals.

Their outermost shells are full leading to high stability. They react only in extreme

circumstances and hence are called noble gases.

5. In the Modern Periodic Table, which are the metals among the first ten elements?

Answer:

Lithium and Beryllium are the metals among the first ten elements in Modern Periodic

Table.

6. By considering their position in the Periodic Table, which one of the following

elements would you expect to have maximum metallic characteristic? Ga Ge As Se

Be

Answer:

Among the elements listed in the question. Be and Ga are expected to be most

metallic. Out of Be and Ga, Ga is bigger in size and hence has greater tendency to

lose electrons than Be. Therefore, Ga is more metallic than Be.

 

1. Which of the following statements is not a correct statement about the trends

when going from left to right across the periods of periodic Table.

(a) The elements become less metallic in nature.

(b) The number of valence electrons increases.

(c) The atoms lose their electrons more easily.

(d) The oxides become more acidic

Answer:

Correct answer is c .The atoms lose their electrons more easily.

The atoms lose their electrons more easily is a wrong statement because as we move from

left to right across the periods of the periodic table, the non-metallic character increases.

Therefore tendency to lose an electron decreases.

2. Element X forms a chloride with the formula XCl2, which is a solid with a high

melting point. X would most likely be in the same group of the Periodic Table as (a)

Na (b) Mg (c) AI (d) Si

Answer:

Answer is Magnesium because Mg has the valency 2 which is same as the group (a) Na

(b) Mg (c) AI (d) Si

Also Mg when combines chloride forms MgCl2.

3. Which element has?

(a) Two shells, both of which are completely filled with electrons?

(b) The electronic configuration 2, 8, 2?

(c) A total of three shells, with four electrons in its valence shell?

(d) A total of two shells, with three electrons in its valence shell?

(e) twice as many electrons in its second shell as in its first shell?

Answer:

a) Neon has two shells which are completely filled.

b) Silicon has the electronic configuration 2, 8, 4

c) Carbon has a total of 2 shells, with four electrons in its valence shell

d) Boron a total of two shells, with three electrons in its valence shell

e) Carbon has twice as many electrons in its second shell as in its first shell

 4. (a) What property do all elements in the same column of the Periodic Table as

boron have in common?

(b) What property do all elements in the same column of the Periodic Table as

fluorine have in common?

Answer:

(a)All the elements which lie in me same column as that of boron belong to group 13.

Therefore, they have three electrons in their respective valence shells. Except, boron

which is a non-metal, all other elements (i.e., aluminum, gallium, indium and thallium) in

this group are metals.

(b) All elements in the same column of the Periodic Table as fluorine have in common

three electrons in their valence shell and they all are belong to group thirteen.

5. An atom has electronic configuration 2, 8, 7.

(a) What is the atomic number of this element?

(b) To which of the following elements would it be chemically similar? (Atomic

numbers are given in parentheses.) N(7), F(9), P(15), Ar(18)

Answer:

(a)The element with electronic configuration (2+8+7) 17 is chlorine.

The no. of atomic number = no. of electrons

Therefore, atomic number is 17.

(b) An atom with electronic configuration 2, 8, 7 would be chemically similar to F (9)

6. The position of three elements A, B and C in the Periodic Table are shown below–

Group 16 Group 17

- -

- A

- -

B C

(a) State whether A is a metal or non-metal.

(b) State whether C is more reactive or less reactive than A.

(c) Will C be larger or smaller in size than B?

(d) Which type of ion, cation or anion, will be formed by element A?

Answer:

(a) Element A is a non-metal

(b) Element C is less reactive than Element A

(c) C is smaller in size than B

(d) A will form anion

7. Nitrogen (atomic number 7) and phosphorus (atomic number 15) belong to group

15 of the Periodic Table. Write the electronic configuration of these two elements.

Which of these will be more electronegative? Why?

Answer:

Atomic number of Nitrogen is 7 hence Electronic configuration of Nitrogen is 1s2 2s2 2p3 Atomic number of Nitrogen is 15 hence Electronic configuration of Phosphorous is 1s2 2s2 2p6 3s2 3p3

On moving down a group in the periodic table, the number of shell increases. Because of which valence electrons move away from the electrons and the effective nuclear charge decreases. This causes the decrease in the tendency to attract electron and hence electro negativity decreases. Because of all these reasons Nitrogen is more electronegative than phosphorus.

8. How does the electronic configuration of an atom relate to its position in the

Modern Periodic Table?

Answer:

The number of valence electrons decides an atom’s position in the periodic table while the electronic configuration decides the number of valence electrons.

9. In the Modern Periodic Table, calcium (atomic number 20) is surrounded by elements with atomic numbers 12, 19, 21 and 38. Which of these have physical and chemical properties resembling calcium?

Answer:

Calcium has an atomic number of 20, and thus has an electronic configuration of 2, 8, 8,

2. Thus, calcium has 2 valence electrons. The electronic configuration of the element

having atomic number 12 is 2, 8.2. Thus, this element with 2 valence electrons resemble

calcium the most.

10. Compare and contrast the arrangement of elements in Mendeleev’s Periodic

Table and the Modern Periodic Table.

Answer:

Mendeleeve’s periodic table.	Modern periodic table.
Elements are arranged in the increasing order of their atomic masses.	Elements are arranged in the increasing order of their atomic numbers.
There are eight groups.	There are 18 groups.
Each group are subdivided into subgroup A&B.	Groups are not subdivided into subgroups.
Groups for Noble gas was not present. As Noble gases were not discovered at that time.	A separate group. Is meant For Noble gases.
There was no place for isotopes.	This problem has been rectified. As slots are determined according to atomic numbers.