Page 122 - Chemistry ICSE Class IX

110 ICSE Chemistry – 9

Methane molecule (CH )
4
Methane molecule contains one atom of carbon and four atoms of hydrogen,
i.e., the molecular formula of methane is CH . The electronic con guration
4
of carbon is 2, 4 and that of hydrogen is 1.
Carbon atom in CH thus shares 4 pairs of electrons, one pair with
4
each hydrogen atom, i.e.,

In methane, carbon atom is linked to four hydrogen atoms by four
C – H single bonds.

Carbon tetrachloride molecule (CCl )
4
The electronic con guration of carbon is ( 2 K , L 4 ) and that of chlorine is
/ƚ ŵĂLJ ďĞ ŶŽƚĞĚ ŚĞƌĞ ƚŚĂƚ ŝŶ Ă ƉŽůĂƌ K L M . The carbon atom needs four electrons to complete its octet, and
ĐŽǀĂůĞŶƚ ďŽŶĚ͕ ƚŚĞƌĞ ŝƐ ŶŽ ƚƌĂŶƐĨĞƌ ŽĨ 2 8 7
each chlorine atom requires only one electron to complete its octet. Thus,
ĞůĞĐƚƌŽŶ ĨƌŽŵ ŽŶĞ ĂƚŽŵ ƚŽ ĂŶŽƚŚĞƌ͘
ƉŽůĂƌ ĐŽǀĂůĞŶƚ ďŽŶĚ ŝƐ Ă ĐŽǀĂůĞŶƚ each chlorine atom shares one electron pair with carbon atom, i.e.,
ďŽŶĚ ŚĂǀŝŶŐ ƐŽŵĞ ŝŽŶŝĐ ĐŚĂƌĂĐƚĞƌ͘
, +G o ů ʹG
^ƵĐŚ ŵŽůĞĐƵůĞƐ ĂƌĞ ĐĂůůĞĚ polar
;or dipolarͿ molecules.

Formation of Polar Covalent Bond

The covalent bond between the atoms of two elements having different
electronegativities is called a polar covalent bond.
When a covalent bond is formed between two atoms of different
elements, the shared pair of electrons does not lie exactly midway
between the two atoms. In fact, it lies more towards the atom which is
more electronegative. The atom with higher af nity for electrons, develops
a slight negative charge and the atom with lesser af nity for electrons
develops a slight positive charge.

Hydrogen chloride (HCl)
In the hydrogen chloride (HCl) molecule, hydrogen and chlorine atoms are
bonded through electron sharing.

In HCl, the shared pair of electrons lies more towards Cl atom
(because Cl is more electronegative). Therefore, Cl atom acquires a slight
negative charge (–G) and H atom, a slight positive charge (+G).