Page 23 - Chemistry ICSE Class X

Periodic Properties and Variations of Properties (Physical and Chemical) 11
The effect of increase in atomic size outweighs the effect of increased
PWENGCT EJCTIG #U C TGUWNV VJG KQPKUCVKQP GPGTI[ FGETGCUGU CU YG IQ FQYP
VJG ITQWR

How does ionisation energy vary in a period
6JG KQPKUCVKQP GPGTI[ KPETGCUGU CU YG IQ HTQO NGHV VQ TKIJV KP C RGTKQF 6JG
ſTUV KQPKUCVKQP GPGTI[ XCNWGU QH VJG UGEQPF RGTKQF GNGOGPVU CTG IKXGP DGNQY
CPF UJQYP KP (KI

Element Li Be B C N O F Ne
–1
Ionisation energy (kJ mol ) 520 899 800 1086 1402 1314 1681 2080
Explanation: When we go from left to right in a period, the following
changes take place:
ƒ 6JG PWENGCT EJCTIG KPETGCUGU D[ QPG WPKV CV GCEJ GNGOGPV
ƒ 6JG CFFGF GNGEVTQPU GPVGT KPVQ VJG UCOG UJGNN
Since, the electrons in the same shell do not shield each other from the
nucleus effectively, the atomic size decreases while going from left to right
KP C RGTKQF
Due to the combined effect of increased nuclear charge and decreased
atomic size, the ionisation energy increases in going from left to right in
a period. Fig. 1.5 First ionisation energies of the
elements of second period as a function of
their atomic number
Electron Affinity

What is electron affinity
6JG GNGEVTQP CHſPKV[ KU VJG EJCPIG KP GPGTI[ YJGP CP KUQNCVGF ICUGQWU CVQO ůĞĐƚƌŽŶ ĂĸŶŝƚLJ ŝƐ ŶŽǁ ĐĂůůĞĚ ďLJ ƚŚĞ
(or anion) in its ground state adds an electron to form the corresponding ŶĂŵĞ ŽĨ ĞůĞĐƚƌŽŶ ŐĂŝŶ ĞŶƚŚĂůƉLJ͘
CPKQP 6JWU VJG GPGTI[ EJCPIG HQT VJG TGCEVKQP
–
–
: g) + e o : (g)
is called the GNGEVTQP CHſPKV[ QH VJG GNGOGPV : 6JG GNGEVTQP CHſPKV[ OC[ DG WůĞĂƐĞ ŶŽƚĞ ƚŚĂƚ ƐŽŵĞ ďŽŽŬƐ ĂĚŽƉƚ
RQUKVKXG QT PGICVKXG 6JG GNGEVTQP CHſPKV[ XCNWGU CTG GZRTGUUGF KP VJG WPKVU ŽƉƉŽƐŝƚĞ ƐŝŐŶ ĐŽŶǀĞŶƟŽŶ ĨŽƌ ĞůĞĐƚƌŽŶ
–1
QH M, OQN QT KP VJG WPKV QH G8 CVQO ĂĸŶŝƚLJ͕ ƚŚĂƚ ŝƐ͕ ǁŚĞŶ ĞŶĞƌŐLJ ŝƐ
For elements whose atoms have strong tendency to accept an electron, ƌĞůĞĂƐĞĚ ĚƵƌŝŶŐ ƚŚĞ ĨŽƌŵĂƟŽŶ ŽĨ
energy is released during the formation of anion and, therefore, the electron ĂŶ ĂŶŝŽŶ ĨƌŽŵ Ă ŐĂƐĞŽƵƐ ĂƚŽŵ͕ ƚŚĞ
CHſPKV[ XCNWGU CTG negative (QT GZCORNG KP VJG TGCEVKQP ĞůĞĐƚƌŽŶ ĂĸŶŝƚLJ ŽĨ ƚŚĂƚ ĂƚŽŵ ;Žƌ
ĞůĞŵĞŶƚͿ ŝƐ ƚĂŬĞŶ ĂƐ ƉŽƐŝƟǀĞ͘ dŚŝƐ
–
–
Cl(g) + e o Cl (g M, OQN –1 ŝƐ ĐŽŶƚƌĂƌLJ ƚŽ ƚŚĞ ƚŚĞƌŵŽĚLJŶĂŵŝĐ
–1
GPGTI[ GSWKXCNGPV VQ M, OQN KU TGNGCUGF 6JGTGHQTG VJG GNGEVTQP CHſPKV[ ĐŽŶǀĞŶƟŽŶ͘ /Ŷ ƚŚŝƐ Ŭ͕ ǁĞ ŚĂǀĞ
–1
of chlorine is – 340 kJ mol ĨŽůůŽǁĞĚ ƚŚĞ ůĂƚĞƐƚ ƚŚĞƌŵŽĚLJŶĂŵŝĐ
ĐŽŶǀĞŶƟŽŶ ĚĞƐĐƌŝďĞĚ ĂůŽŶŐƐŝĚĞ͘
+V OC[ DG PQVGF JGTG VJCV VJG GNGEVTQP CHſPKV[ KU not reverse of the
ionisation energy in which electrons are removed from a neutral gaseous
atom to form gaseous cation viz
+
A(g) o A (g) + e –
What is meant by the successive electron affinity
'NGEVTQP CHſPKV[ XCNWGU HQT VJG CFFKVKQP QH ſTUV UGEQPF VJKTF GNGEVTQP CTG
called ſTUV UGEQPF and third GNGEVTQP CHſPKV[ TGURGEVKXGN[ (QT GZCORNG
the energy change in,
–
: g) + e o : – (g) is called ſTUV GNGEVTQP CHſPKV[
2–
–
–
: (g) + e o : (g) is called UGEQPF GNGEVTQP CHſPKV[
–
3–
2–
: (g) + e o : (g) is called VJKTF GNGEVTQP CHſPKV[