Activity 1.9


1
Atomic radii of elements of third period are given below:


Period 3  elements
S
Na
Cl
P
Mg
Si
Al
Atomic radius (pm) :
127
190
99
128
160
132
143
(a)
Arrange them in the decreasing order of their atomic radii.
Ans:
They are arranged in the decreasing order of their atomic radii as follows:
Period 3  elements
Na
Mg
Al
Si
P
S
Cl
Atomic radius (pm) :
190
160
143
132
128
127
99
(b)
Are they arranged as in the periodic table?
Ans:
Originally they are not arranged as in the periodic table. But, after arranging them in the decreasing order of theit atomic radii, they are arranged as in the periodic table.
(c)
Which are the atoms of highest and lowest atomic radii?
Ans:
The atom with highest atomic radius is Sodium (190 pm) and the atom with lowest atomic radius is Chlorine (99 pm).
(d)
What trend is observed in the atomic radii in a period from left to right?
Ans:
As we move from left to right in a period, the atomic radius gradually decreases. As we move from left to right, electrons are added. This increases the nuclear attraction and electrons tend to pull closer. This decreases the atomic radius.

2
Atomic radii of group 17 are given below:



Group 17 elements
I
F
Cl
Br
Atomic radius (pm) :
133
72
99
114
(a)
Arrange them in the increasing order of their atomic radii.
Ans:
They are arranged in the increasing order of their atomic radii as follows:
Group 17 elements
F
Cl
Br
I
Atomic radius (pm) :
72
99
114
133
(b)
Are they arranged as in the periodic table?
Ans:
Originally they are not arranged as in the periodic table. But, after arranging them in the increasing order of theit atomic radii, they are arranged as in the periodic table.
(c)
Which are the atoms of highest and lowest atomic radii?
Ans:
The atom with highest atomic radius is Iodine (133 pm) and the atom with lowest atomic radius is Fluorine (72 pm).
(d)
What trend is observed in the atomic radii down the group?
Ans:
As we move from top to bottom in a group, the atomic radius gradually increases.. As we move from top to bottom in a group, new shells are added. This increases the atomic radius.