Page 117 - Chemistry ICSE Class IX

Atomic Structure and Chemical Bonding 105
All other elements having less than eight electrons in their outermost
shell (except helium which has two electrons in its outermost shell) show
a reasonable chemical activity. For example, sodium and chlorine are
quite reactive because their outermost shells are incomplete, i.e., Na and Cl
contain less than eight electrons in their outermost shell.
K L M
Na (atomic no. 11) 2, 8, 1 outermost shell contains only one electron
Cl (atomic no. 17) 2, 8, 7 outermost shell contains seven electrons
Thus, we see that the elements having less than eight electrons in
their outermost shell (except helium) are chemically reactive.

Octet Rule

Atoms play fair game! The atoms react with each other according to certain
rules. These rules are followed by most of the elements in most of the
reactions.

Why do atoms tend to acquire noble gas configurations
W. Kossel and G. N. Lewis (1916) observed mat during chemical reactions,
atoms of all elements tend to achieve noble gas con gurations. Based on this
observation, they proposed a theory for the formation of chemical bonds.
According to this theory,
“During any chemical reaction, the atoms of all the elements tend
VQ ICKP UVCDKNKV[ D[ CESWKTKPI VJG GNGEVTQPKE EQPſIWTCVKQP QH VJG PGCTGUV
noble gas element.” ƚŽŵƐ ŽĨ Ăůů ŶŽďůĞ ŐĂƐ ĞůĞŵĞŶƚƐ
Thus, during chemical reactions, atoms of all elements tend to achieve ĞdžĐĞƉƚ ŚĞůŝƵŵ͕ ĐŽŶƚĂŝŶ ϴ ĞůĞĐƚƌŽŶƐ
eight electrons in their outermost shell. This is known as the octet rule. ŝŶ ƚŚĞŝƌ ŽƵƚĞƌŵŽƐƚ ƐŚĞůů͘ ƚŽŵƐ ŽĨ Ăůů
Hydrogen atom, however, gains stability by either losing its only ŽƚŚĞƌ ĞůĞŵĞŶƚƐ ĐŽŶƚĂŝŶ ůĞƐƐ ƚŚĂŶ
electron or by gaining one to have two electrons (helium structure) in its ϴ ĞůĞĐƚƌŽŶƐ ŝŶ ƚŚĞŝƌ ŽƵƚĞƌŵŽƐƚ ƐŚĞůů͘
outermost shell, viz.,
H o H + + e –
1 electron in proton
K-shell no electron in K-shell
–
H + e o H –
1 electron in K-shell is complete with
K-shell 2 electrons: Helium con guration
Thus, CNN CVQOU VGPF VQ CESWKTG PQDNG ICU EQPſIWTCVKQP VQ ICKP
stability.
The octet rule is found useful for describing bonding in a large
number of cases. However, there are some examples where the octet rule
is not found applicable. For example, in compounds like phosphorus
pentachloride (PCl ) and sulphur hexa uoride (SF ), there are more than
6
5
8 electrons around the central atom as shown alongside. Thus, PCl and SF
5
6
do not follow octet rule.
How do atoms acquire stable noble gas configuration
The atoms in which the outermost shell is completely lled are more stable.
For example, the distribution of electrons in uorine, neon and sodium
atoms is shown below:

Element K L M
F (Atomic no. 9) 2 7
Ne (Atomic no. 10) 2 8
Na (Atomic no. 11) 2 8 1