Page 119 - Chemistry ICSE Class IX

Atomic Structure and Chemical Bonding 107
Electrovalent Bond
Ŷ ĞůĞĐƚƌŽǀĂůĞŶƚ ;Žƌ ŝŽŶŝĐͿ ďŽŶĚ
The coulombic force of attraction between two oppositely charged ions is ŝƐ ĨŽƌŵĞĚ ďLJ Ă complete transfer
called an electrovalent (or ionic) bond. ŽĨ ŽŶĞ Žƌ ŵŽƌĞ ĞůĞĐƚƌŽŶƐ ĨƌŽŵ
An electrovalent bond is formed by a complete transfer of one or more ƚŚĞ ĂƚŽŵ ŽĨ ĂŶ ĞůĞŵĞŶƚ ƚŽ ƚŚĂƚ ŽĨ
electrons from the atom of an element to that of another element. ĂŶŽƚŚĞƌ ĞůĞŵĞŶƚ͘ ͻ
An electrovalent (or ionic) bond has the following characteristics. dŚĞ ĂƚŽŵ ǁŚŝĐŚ ůŽƐĞƐ ŽŶĞ Žƌ ŵŽƌĞ
ƒ An electrovalent bond is formed due to the coulombic attraction ĞůĞĐƚƌŽŶƐ ŐŝǀĞƐ Ă caƟon͕ ǁŚŝůĞ ƚŚĞ
between positively and negatively charged ions. ĂƚŽŵ ǁŚŝĐŚ ŐĂŝŶƐ ŽŶĞ Žƌ ŵŽƌĞ
ƒ An electrovalent bond is non-directional. This is because coulombic ĞůĞĐƚƌŽŶƐ ŐŝǀĞƐ ĂŶ anion.
forces act equally in all directions.
ƒ An electrovalent bond gets broken when an electrovalent compound
is dissolved in a polar solvent such as water, or when melted.
/ƚ ƐŚŽƵůĚ ďĞ ƌĞŵĞŵďĞƌĞĚ ŚĞƌĞ
Structures of Some Electrovalent Compounds ƚŚĂƚ ĚƵƌŝŶŐ ƚŚĞ ĨŽƌŵĂƟŽŶ ŽĨ
ĞůĞĐƚƌŽǀĂůĞŶƚ ďŽŶĚ͕ ƚŚĞ ƚŽƚĂů
Formation of some electrovalent compounds is described below: ŶƵŵďĞƌ ŽĨ ĞůĞĐƚƌŽŶƐ ůŽƐƚ ďLJ ŽŶĞ
Žƌ ŵŽƌĞ ĂƚŽŵƐ ŽĨ ĂŶ ĞůĞŵĞŶƚ ŝƐ
Sodium chloride (NaCl) ĂůǁĂLJƐ ĞƋƵĂů ƚŽ ƚŚĞ ƚŽƚĂů ŶƵŵďĞƌ
The electronic con guration of sodium atom (atomic no. 11) is 2, 8, 1. So, it ŽĨ ĞůĞĐƚƌŽŶƐ ŐĂŝŶĞĚ ďLJ ŽŶĞ Žƌ ŵŽƌĞ
ĂƚŽŵƐ ŽĨ ƚŚĞ ŽƚŚĞƌ ĞůĞŵĞŶƚ͘
has one electron in its valence shell. The electronic con guration of chlorine
atom (atomic no. 17) is 2, 8, 7. So, it has 7 valence electrons.
When the two combine, there is a transfer of one electron from sodium
+
–
atom to chlorine atom forming Na and Cl .
loses an
Na o Na + + e –
(2,8,1) electron (2,8)
sodium atom sodium ion
(neon con guration)
gains an
–
Cl + e o Cl –
electron
(2,8,7) (2,8,8)
chlorine atom chloride ion
(argon con guration)
During the transfer of an electron, both the atoms attain the noble
gas con gurations: sodium that of neon (2, 8) and chlorine that of argon
+
–
(2, 8, 8). The two ions, Na and Cl are then held together by the electrostatic
+
attraction to form the ionic compound, sodium chloride (Na Cl).
We can also show it diagrammatically as follows (only the valence
electrons are shown):

Magnesium Chloride (MgCI )
2
The electronic con guration of magnesium and chlorine atoms are,
Magnesium (atomic no. 12) Chlorine (atomic no. 17)
No. of electrons = 12 No. of electrons = 17
Electronic con guration: (2,8,2) Electronic con guration: (2,8,7) dŚĞ ĐŽŵƉŽƵŶĚ ĨŽƌŵĞĚ ĚƵĞ ƚŽ ƚŚĞ
Thus, magnesium atom has two electrons in its valence shell. The ƚƌĂŶƐĨĞƌ ŽĨ ĞůĞĐƚƌŽŶƐ ĨƌŽŵ ƚŚĞ ĂƚŽŵ
chlorine atom has seven valence electrons. Thus, magnesium has two ŽĨ ĂŶ ĞůĞŵĞŶƚ ƚŽ ƚŚĂƚ ŽĨ ĂŶŽƚŚĞƌ ŝƐ
electrons in excess of the neon con guration (2, 8), and chlorine is one ĐĂůůĞĚ ĂŶ electrovalent compound.
electron short of argon con guration (2, 8, 8). So one atom of magnesium
will transfer its two valence electrons to two chlorine atoms, (one to each)
as shown here: