Question 1

What is the significance of atomic number in the modern periodic table ?

Answer

Modern periodic table is based on Henry Moseley's modern periodic law which
states that "the physical and chemical properties of elements are the periodic
functions of their atomic number".

Question 2

Arrange the following in order of :

(i) increasing radii.

(a) Cl-, Cl
(b) Mg2+, Mg, Mg+
(c) N, O, P

(ii) increasing ionisation energy.

(a) P, Na, Cl
(b) F, O, Ne
(c) Ne, He, Ar

Explain your choice.

Answer

(i) In order of increasing radii:

(a) Cl < Cl-.

Reason — Anion is greater in size than its parent atom.

(b) Mg2+ < Mg+ < Mg.

Reason — Cation is smaller in size than its parent atom.

(c) O < N < P.

Reason — Between Oxygen (O) and Nitrogen (N), atomic size of Nitrogen (N) is
more as it is to the left of Oxygen (O) in period 2. Between Nitrogen (N) and
Phosphorus (P), Phosphorus (P) has larger atomic size as it comes after Nitrogen
(N) in group 5A.

(ii) In order of increasing ionisation energy:

(a) Na < P < Cl

Reason — Ionisation energy increases when moving across a period from left to
right.

(b) O < F < Ne

Reason — Ionisation energy increases when moving across a period from left to
right.
(c) Ar < Ne < He
Reason — Ionisation energy decreases when moving down the group.

Question 3

Atomic numbers of elements A, B, C, D, E, F, are 8, 7, 11, 12, 13 and 9 respectively.

State the type of ions they form.

Answer

The electronic configuration of the given elements are:

A(8) = (2, 6 ) — A forms anion.

B(7) = (2, 5 ) — B forms anion.

C(11) = (2, 8, 1) — C forms cation.

D(12) = (2, 8, 2) — D forms cation.

E(13) = (2, 8, 3) — E forms cation.

F(9) = (2, 7 ) — F forms anion.

Question 4

Give reasons for the following assertions :

(a) The oxidising power of elements increases from left to right along a period.

(b) Ionisation potential of elements increases across a period from left to right.

(c) Alkali metals are good reducing agents.

Answer

(a) The oxidising power of elements increases from left to right along a period
because electro-negativity and the non metallic character increases from left to
right. As oxidising power depends on tendency to gain electrons and non-metals
are good oxidising agents hence oxidising power of elements increases across a
period.

(b) The ionisation potential of element increases across a period because the
atomic size decreases due to an increase in nuclear charge and electrons in the
outermost shell are more strongly held because of which greater energy is required
to remove the electron.

(c) Alkali metals have one electron in their valence shell. In order to be stable, they
easily lose this electron and get oxidised. Hence, they are good reducing agents.

Exercise 1 — Long Answer Type

Question 1
Chlorine in the Periodic Table is surrounded by the elements with atomic number 9,
16, 18, and 35.

(a) Which of these have Physical and Chemical properties resembling chlorine ?

(b) Which is more electronegative than chlorine ?

Answer

(a) The elements with atomic number 9 (fluorine) & 35 (bromine) have physical
properties resembling chlorine because they belong to the same group.

(b) The element with atomic number 9 (fluorine) is more electronegative than
chlorine because on moving down the group electronegativity decreases.

Question 2

First ionisation enthalpy of two elements X and Y are 500 kJ mol -1 and 375 kJ mol-
1
respectively. Comment about their relative position in a group as well as in a
period.

Answer

It is given that,

X = 500 kJ mol-1

Y = 375 kJ mol-1

Their relative position in a group is X & Y as ionisation enthalpy decreases on

moving down the group.

Their relative position in a period is Y & X as ionisation enthalpy increases on

moving across a period.

Question 3

The elements of one short period of the Periodic Table are given below in order
from left to right :

Li Be B C O F Ne

(a) To which period do these elements belong ?

(b) One element of this period is missing. Which is the missing element and where
should it be placed ?

(c) Place the three elements fluorine, beryllium and nitrogen in the order of
increasing electronegativity.

(d) Which one of the above elements belongs to the halogen series ?

Answer

(a) These elements belong to the 2nd period.

(b) The missing element is Nitrogen. It belongs to the Group 15 and should be
placed as shown below:
Li Be B C N O F

(c) Beryllium < Nitrogen < Fluorine.

(d) Fluorine.

Question 4

The atomic number of an element Z is 16. State

(a) the period to which it belongs.

(b) the number of valence electron(s) in the element.

(c) whether the element is a metal or a non-metal.

(d) State the formula of the compound between Z and hydrogen.

Answer

(a) The element belongs to 3 rd period.

Reason — The element Z has atomic number 16 and so the electronic
configuration will be 2, 8, 6. The number of shells present in an atom determines
it's period. Hence, Z will belong to 3 rd period as it has three shells.

(b) 6 [∵ electronic configuration is 2, 8, 6]

(c) Z is a non metal.

(d) H2Z

Question 5

In the activity series of metals — M is a metal above hydrogen in the activity series
and it's oxide has the formula M 2O. M2O when dissolved in water forms the
corresponding hydroxide which is a good conductor of electricity. In the above
context, answer the following :

(i) What kind of combination exists between M and O?

(ii) State the number of electrons in the outermost shell of M ?

(iii) Name the group to which M belongs.

Answer

Given, M is a metal

(i) Electrovalent bond exits between M and O because the bond is formed
between a metal and non-metal due to oppositely charged ions.
(ii) Number of electrons in the outer most shell of M is 1. It is so because the
valency of O is -2 and as 2 atoms of M combine with O to form M 2O, hence we can
say that M has 1 valence electron.

(iii) M belongs to group 1 [1A] because there is 1 electron in the outer most shell.

Question 6

The metals of Group 2 in the periodic table from top to bottom are — Be, Mg, Ca,
Sr, and Ba.

(i) Which one of these elements will form ions most readily and why?

(ii) State the common feature in the electronic configuration of all these given
elements.

Answer

(i) Ba - Elements at the bottom of a group are most metallic, have large atomic size
and lowest ionisation potential. So, the outer electrons are loosely held and will
form ions from metals most readily and thus are more reactive.

(ii) As the elements belong to group 2 thus they all have 2 electrons in the valence
shell.

Question 7

Study the extract of the Periodic Table given below and answer the questions. Give
the alphabet corresponding to the element in question. Do not repeat an element.

C D E

B G F

(i) Which element forms electrovalent compound with G.

(ii) The ion of which element will migrate towards the cathode during electrolysis.

(iii) Which non-metallic element has the valency of 2?

(iv) Which is an inert gas?

Answer

(i) B forms an electrovalent compound with G.

(ii) A has positive ions which migrate towards cathode.

(iii) E is non-metallic and has a valency of 2 as it belongs to group 16.

(iv) F is an inert gas as it belongs to the zero group.

Question 8

In the table below, H does not represent hydrogen. Some elements are given in
their own symbol and position in the periodic table while others are shown with a
letter.

III IV V VI
1 II VI O
A A A A
A A IA 1
1 1 1 1
1 2 17 8
3 4 5 6

N
Li D O J
e

M
A E Si H K
g

B C F G L

Answer the following questions.

1. Identify the most electronegative element.

2. Identify the most reactive element of group IA or 1.
3. Identify the element from period 3 with the smallest atomic size.
4. Identify the element with the highest ionization potential.
5. How many valence electrons are present in G ?
6. Which element from group 2 will have the least ionization energy ?
7. Identify the noble gas of the fourth period.
8. In the compound between A and H, what type of bond is formed ? Give its
molecular formula.
9. Draw the electron dot structure for the compound formed between C and K.

Answer

1. J is the most electronegative element.

2. B is the most reactive element of group IA or 1.
3. K is the element from period 3 with smallest atomic size.
4. Ne the highest ionization potential.
5. 5 valence electrons present in G.
6. C is the element from group 2 which has least ionization energy.
7. L is the noble gas of the fourth period.
8. ionic bond is formed between A and H.

2A + H ⟶ A2H
The molecular formula is :

9. Electron dot structure for the compound formed between C and K is shown
below:
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