Page 25 - Chemistry ICSE Class X

Periodic Properties and Variations of Properties (Physical and Chemical) 13

How does the electron affinity vary in a group
6JG OCIPKVWFG QH VJG GNGEVTQP CHſPKV[ QH GNGOGPVU FGETGCUGU KP IQKPI HTQO
VQR VQ DQVVQO KP C ITQWR *QYGXGT GNGEVTQP CHſPKV[ QH ƀWQTKPG KU NQYGT
VJCP VJCV QH EJNQTKPG
'NGEVTQP CHſPKV[ XCNWGU HQT CNMCNK OGVCNU CPF JCNQIGPU CTG IKXGP DGNQY
CPF UJQYP HQT JCNQIGPU KP (KI
Element Li Na K Rb
–1
'NGEVTQP CHſPKV[ M, OQN ) Ō Ō Ō Ō
Element F Cl Br I
–1
'NGEVTQP CHſPKV[ M, OQN ) –328 –349 –324 –295
Explanation: 1P OQXKPI HTQO VQR VQ VJG DQVVQO QH C ITQWR
ƒ The atomic size increases
ƒ 6JG PWENGCT EJCTIG KPETGCUGU
6JG GHHGEV QH VJGUG HCEVQTU KU QRRQUKVG VQ GCEJ QVJGT +P C ITQWR VJG
effect of increase in the atomic size outweighs the effect of increased
PWENGCT EJCTIG #U C TGUWNV VJG VGPFGPE[ VQ CEEGRV CP GNGEVTQP KP KVU
XCNGPEG UJGNN FGETGCUGU #U C TGUWNV VJG OCIPKVWFG QH GNGEVTQP CHſPKV[ Fig. 1.6 (OHFWURQ DI¿QLW\ RI KDORJHQV
decreases as we go down the group

How does the electron affinity vary in a period
6JG OCIPKVWFG QH GNGEVTQP CHſPKV[ KPETGCUGU CETQUU C RGTKQF KP IQKPI HTQO
NGHV VQ TKIJV
6JG OGVCNU G I CNMCNK OGVCNU YJKEJ NKG CV VJG NGHV GPF QH VJG RGTKQFKE
VCDNG JCXG NQY XCNWGU QH GNGEVTQP CHſPKV[ 6JG PQPOGVCNU G I JCNQIGPU
which lie at the right end of the periodic table have high values of electron
CHſPKV[ 6JG GNGEVTQP CHſPKV[ XCNWGU HQT UGEQPF RGTKQF GNGOGPVU CTG IKXGP
below:

Element Li Be B C N O F Ne
'NGEVTQP CHſPKV[ Ō 0 –23 –122 Ō –141 –328 0
–1
(kJ mol ) (+780)*
6JKU KU VJG UGEQPF GNGEVTQP CHſPKV[
Explanation: On moving from left to right in a period, the size of the
CVQO FGETGCUGU CPF VJG PWENGCT EJCTIG KPETGCUGU $QVJ VJGUG HCEVQTU KPETGCUG
VJG HQTEG GZGTVGF D[ VJG PWENGWU QP VJG GNGEVTQPU #U C TGUWNV VJG CVQO JCU
C ITGCVGT VGPFGPE[ VQ ICKP CP GNGEVTQP 6JGTGHQTG OCIPKVWFG QH GNGEVTQP
CHſPKV[ KPETGCUGU KP IQKPI HTQO NGHV VQ TKIJV *QYGXGT UQOG GNGOGPVU KP
GCEJ RGTKQF UJQY GZEGRVKQP VQ UWEJ RGTKQFKEKV[ (QT GZCORNG $G /I 0 2
CPF PQDNG ICUGU UJQY XGT[ NQY QT GXGP \GTQ GNGEVTQP CHſPKV[
The reason for these exceptions will be taught to you in your higher
ENCUUGU

Electronegativity

What is meant by the electronegativity of an element
Electronegativity of an element is described as follows: dŚĞ ĞůĞĐƚƌŽŶĞŐĂƟǀŝƚLJ ŝƐ ŶŽƚ ƚŚĞ
“The relative tendency of an atom in a molecule to attract a shared ƉƌŽƉĞƌƚLJ ŽĨ ĂŶ ŝƐŽůĂƚĞĚ ĂƚŽŵ͘
pair of electrons towards itself is termed as its electronegativity.”