Page 45 - Chemistry ICSE Class X

Chemical Bonding 33
ƒ Combination of the cation and the anion. The two ions so formed dŚĞ ĮƌƐƚ ƉƌŽĐĞƐƐ ĚŽĞƐ ŶŽƚ ĨĂǀŽƵƌ
are then held together by coulombic force of attraction. This step is ǁŚĞƌĞĂƐ ƚŚĞ ŽƚŚĞƌ ƚǁŽ ƉƌŽĐĞƐƐĞƐ
exothermic and energy equal to NCVVKEG GPGTI[ is released. ĨĂǀŽƵƌ ƚŚĞ ĨŽƌŵĂƟŽŶ ŽĨ ĂŶ ŝŽŶŝĐ
Formation of an electrovalent bond (or ionic bond) is favoured if the ;ĞůĞĐƚƌŽǀĂůĞŶƚͿ ďŽŶĚ͘
GPGTI[ TGNGCUGF KP VJG NCUV VYQ RTQEGUUGU KU OQTG VJCP VJG GPGTI[ TGSWKTGF
KP VJG ſTUV RTQEGUU. So, the factors which favour the formation of an
GNGEVTQXCNGPV QT KQPKE DQPF CTG
ƒ Low ionisation energy ƒ *KIJ GNGEVTQP CHſPKV[ ƒ High lattice energy

Which elements form electrovalent (or ionic)
compounds easily
6JG HQNNQYKPI ITQWR QH GNGOGPVU HQTO GNGEVTQXCNGPV EQORQWPFU GCUKN[
ƒ Alkali metals (Group 1 elements) have very low ionisation energies.
Therefore, atoms of group 1 elements (Li, Na, K, Rb, Cs) lose their
valence electron easily to form their cations. Therefore, alkali metals
.K 0C - 4D %U HQTO KQPKE DQPFU GCUKN[.
ƒ *CNQIGPU GNGOGPVU QH ITQWR ( %N $T + JCXG NCTIG CHſPKV[
HQT GNGEVTQPU K G NCTIG GNGEVTQP CHſPKV[ XCNWGU 5Q VJGUG GNGOGPVU
(halogens) accept an electron easily to form the corresponding anion.
Thus, JCNQIGPU ( %N $T CPF + HQTO GNGEVTQXCNGPV QT KQPKE DQPFU
easily.
ƒ Due to the reasons stated above, the elements of Group 2 and Group
16 also form ionic bonds easily.
dŚĞ ĞůĞŵĞŶƚƐ ŽĨ ŐƌŽƵƉ ϭ͕ Ϯ ǁŝƚŚ
ƚŚŽƐĞ ŽĨ ŐƌŽƵƉƐ ϭϲ ĂŶĚ ϭϳ͕ ŐĞŶĞƌĂůůLJ
Structures of Some Electrovalent Compounds ŐŝǀĞ electrovalent ;Žƌ ionicͿ
(QTOCVKQP QH UQOG GNGEVTQXCNGPV EQORQWPFU KU FGUETKDGF DGNQY compounds͘
Sodium chloride (NaCl)
6JG GNGEVTQPKE EQPſIWTCVKQP QH UQFKWO CVQO CVQOKE PQ KU 5Q KV
JCU QPG GNGEVTQP KP KVU XCNGPEG UJGNN 6JG GNGEVTQPKE EQPſIWTCVKQP QH EJNQTKPG
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
PGQP EQPſIWTCVKQP
gains an
–
Cl + e o Cl –
electron
(2,8,7) (2,8,8)
chlorine atom chloride ion
CTIQP EQPſIWTCVKQP
During the transfer of an electron, both the atoms attain the noble
ICU EQPſIWTCVKQPU UQFKWO VJCV QH PGQP CPF EJNQTKPG VJCV QH CTIQP
+
–
(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
GNGEVTQPU CTG UJQYP