Page 144 - Chemistry ICSE Class IX

132 ICSE Chemistry – 9

Thus, atom of the last element in any group is the largest atom in
that group.

How does atomic size vary in a period of the
periodic table
The atomic size (or atomic radius) decreases while going from left to right
in a period of the periodic table (except for He and Ne). For example, for
the elements of second and third periods, the atomic size decreases in
going from left to right.

Element Li Be B C N O F
Atomic radius/pm 152 112 89 77 74 74 72

Element Na Mg Al Si P S Cl
Atomic radius/pm 186 160 143 117 110 104 99
Atomic size increases down
a group

————— Atomic size decreases from left to right in a period o
(not to the scale)
This can be explained as follows:
Explanation: The number of protons and electrons in an atom
increases in going from left to right in a period. The added electrons enter
the same shell. Due to the increase in nuclear charge, the electrons are
pulled towards the nucleus with greater attractive force. As a result, the
electronic shell shrinks and the atomic size decreases.

Valence Electrons

In an atom, electrons are distributed in various shells (or orbits). The
electrons in the outermost shell of an atom are called valence electrons.
To calculate the number of valence electrons in the atom of any
element, write down its electronic con guration and count the number of
electrons in the outermost shell. For the elements of rst three periods of
Fig. 5.2 Sodium atom: It has only one the periodic table, we write as follows:
valence electron