Page 59 - Chemistry ICSE Class X

Chemical Bonding 47
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).
The compounds having polar covalent bonds undergo ionisation
when dissolved in any polar solvent, such as water. Such polar compounds /ƚ ŵĂLJ ďĞ ŶŽƚĞĚ ŚĞƌĞ ƚŚĂƚ ŝŶ Ă ƉŽůĂƌ
in solutions produce free ions. For example, when hydrogen chloride (HCl) ĐŽǀĂůĞŶƚ ďŽŶĚ͕ ƚŚĞƌĞ ŝƐ ŶŽ ƚƌĂŶƐĨĞƌ ŽĨ
ĞůĞĐƚƌŽŶ ĨƌŽŵ ŽŶĞ ĂƚŽŵ ƚŽ ĂŶŽƚŚĞƌ͘
–
+
is dissolved in water, it ionises to produce H O (hydronium ion) and Cl ƉŽůĂƌ ĐŽǀĂůĞŶƚ ďŽŶĚ ŝƐ Ă ĐŽǀĂůĞŶƚ
3
(chloride ion) in solution. ďŽŶĚ ŚĂǀŝŶŐ ƐŽŵĞ ŝŽŶŝĐ ĐŚĂƌĂĐƚĞƌ͘
+G –G
, o ů
^ƵĐŚ ŵŽůĞĐƵůĞƐ ĂƌĞ ĐĂůůĞĚ ĚŝƉŽůĂƌ
ŵŽůĞĐƵůĞƐ͘
How can the electronegativity values of combining
elements be used for predicting the polarity in a bond

The difference between the electronegativities of combining elements can
be used for predicting the polarity (or ionic character) in the bond.
The ionic character in a bond increases with the increase in the
FKHHGTGPEG DGVYGGP VJG GNGEVTQPGICVKXKVKGU QH VJG EQODKPKPI CVQOU

This is illustrated for hydrohalic acids (HX).
The electronegativities of hydrogen and halogens are,
H (2.1), F (4.0), Cl (3.0), Br (2.8) and I (2.5)
Then, H – F H – Cl H – Br H – I
'NGEVTQPGICVKXKV[ (4 – 2.1) (3.0 – 2.1) (2.8 – 2.1) (2.5 – 2.1)
difference: 1.9 0.9 0.7 0.4
Therefore, the polarity (or ionic character) in the H – X bond is
H – F > H – Cl > H – Br > H – I

Coordinate Bond
There is another way by which certain atoms combine with each other and
form a bond. This is due to QPG UKFGF UJCTKPI of electrons. dŚĞ ĂƚŽŵ ǁŚŝĐŚ ƉƌŽǀŝĚĞƐ ĂŶ ĞůĞĐƚƌŽŶ
ƉĂŝƌ ŝƐ ĐĂůůĞĚ ƚŚĞ donor atom͕ ǁŚŝůĞ
ƚŚĞ ĂƚŽŵ ǁŚŝĐŚ ĂĐĐĞƉƚƐ ƚŚĞ ĞůĞĐƚƌŽŶ
What is a coordinate bond
ƉĂŝƌ ŝƐ ĐĂůůĞĚ ƚŚĞ acceptor atom͘
The bond formed when one-sided sharing of electrons take place is dŚƵƐ͕ Ă ĐŽŽƌĚŝŶĂƚĞ ďŽŶĚ ŝƐ ĨŽƌŵĞĚ ďLJ
called a coordinate bond. Such a bond is also known as dative bond. one-sided sharing ŽĨ ĞůĞĐƚƌŽŶƐ͘
A coordinate bond is denoted by an arrow (o) pointing towards the
acceptor atom. Formation of a coordinate bond is illustrated through the
HQNNQYKPI GZCORNGU
+
Formation of hydronium (H O ) ion
3
+
Hydronium ion is formed when a proton (H ) is added to a water molecule
(H O). Water molecule (H O) is a polar covalent molecule. In H O molecule,
2
2
2
the oxygen atom is bonded to two hydrogen atoms by mutual sharing of
electrons. Thus, two hydrogen atoms are bonded to the oxygen atom by
covalent bonds.