Page 112 - Chemistry ICSE Class IX

100 ICSE Chemistry – 9
For example,
Sodium atom has an electronic con guration of 2, 8, 1. The electronic
con guration of sodium atom is shown in Fig. 4.14(a). The outermost shell,
i.e., M-shell of sodium contains only one electron (2, 8, 1). So, the number
of valence electron in sodium is one.
Similarly, in the case of chlorine, there are seven electrons (2, 8, 7)
in its outermost shell (M-shell). So, the number of valence electrons in
chlorine is seven, Fig. 4.14(b).
(a)
Sodium atom According to the electronic theory of valency, the valency of an
(It has only one valence electron) element is de ned as the number of electrons it transfers to some other
atom or it shares with some other atom, during any chemical reaction is
called its valency.
In general, the valency of an element is equal to the number of valence
electrons or equal to eight minus the number of valence electrons. The
valency of an element depends upon the nature of the reaction in which
that element is involved.
The modern approach to the concept of valency is based on the electronic
concept of the atom.

(b) How can the electronic configuration of an element
Chlorine atom
(It has seven valence electrons) explain its chemical reactivity
Fig. 4.14 Electronic con¿gurations of The GNGEVTQPKE EQPſIWTCVKQP of an element gives an idea about its reactivity.
sodium and chlorine describing valence The elements having a completely lled outermost orbit (or shell) will
electrons
be chemically inert (non-reactive). For example, the outermost shell in the
case of helium (He), neon (Ne) and argon (Ar) are completely lled. As
a result, helium, neon and argon are chemically inactive (or inert). These
elements do not form compounds with other elements.
Total number of Distribution of electrons in various shells
Element
electrons K L M
He 2 2
Ne 10 2 8
Ar 18 2 8 8
Because of this chemical inactivity, these gases are called noble gases
(earlier these were called inert gases).
The elements containing only one or seven electrons in their
outermost shell show greater chemical reactivity, i.e., such elements react
very fast with other elements. For example, sodium and chlorine having
the following electronic con gurations are highly reactive:
Sodium 2, 8, 1 — Here, the outermost shell has only one electron:
one more than the completely lled shell.
Chlorine 2, 8, 7 — Here, the outermost shell has seven electrons:
one less than required to ll the shell completely.

Illustrative Questions

Q.1. What is the ground state of an atom?
Ans. The state of an atom in which all the electrons in the atom are in their lowest energy levels is called the ground
state of the atom.
Q.2. Which electrons of an atom are involved in the bond formation between different atoms?
Ans. In bond formation, only the valence electrons (the electrons in the outermost shell) of an atom are involved.