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2 Chemical Bonding
The smallest unit of an element which takes part in a chemical reaction
CONTENTS is an atom. There are in all 118 elements known to us at present—only 92
of these occur in nature, the rest are man-made. The atoms of different
ͻ ŚĞŵŝĐĂů ƌĞĂĐƟǀŝƚLJ ĂŶĚ ĞůĞĐƚƌŽŶŝĐ
ĐŽŶĮŐƵƌĂƟŽŶ elements differ from each other. So, there are in all 118 different types
ͻ ůĞĐƚƌŽŶ ĚŽƚ ƐƚƌƵĐƚƵƌĞƐ of atoms. If atoms could not combine together, there would be only 118
ͻ KĐƚĞƚ ƌƵůĞ different substances in the world. But as we know, the world around us
ͻ ƚŽŵƐ ĂŶĚ ŝŽŶƐ consists of millions of different substances with different properties. This
ͻ KdžŝĚĂƟŽŶ ĂŶĚ ƌĞĚƵĐƟŽŶ could become possible because atoms are able to combine with each other
in many ways.
ͻ ŚĞŵŝĐĂů ďŽŶĚ
ͻ ůĞĐƚƌŽǀĂůĞŶƚ ;Žƌ ŝŽŶŝĐͿ ďŽŶĚ Most elements do not exist in the free atomic state. For example, gases
ͻ ^ƚƌƵĐƚƵƌĞƐ ŽĨ ƐŽŵĞ ĞůĞĐƚƌŽǀĂůĞŶƚ like hydrogen, oxygen etc., occur as diatomic molecules. Elements combine
ĐŽŵƉŽƵŶĚƐ with each other to form compounds. Such compounds occur as molecules.
ͻ ůĞĐƚƌŽǀĂůĞŶƚ ;Žƌ ŝŽŶŝĐͿ ĐŽŵƉŽƵŶĚƐ The questions which may arise in your mind are,
ͻ ŽǀĂůĞŶƚ ďŽŶĚ Why do atoms readily combine to form molecules?
ͻ ^ƚƌƵĐƚƵƌĞƐ ŽĨ ƐŽŵĞ ĐŽǀĂůĞŶƚ What is the nature of forces which keep the atoms together in a
ĐŽŵƉŽƵŶĚƐ ĂŶĚ ŵŽůĞĐƵůĞƐ molecule?
ͻ ŽǀĂůĞŶƚ ĐŽŵƉŽƵŶĚƐ 9J[ FQ GNGOGPVU EQODKPG KP EGTVCKP ſZGF TCVKQU QPN[!
ͻ īĞĐƚ ŽĨ ĞůĞĐƚƌŝĐŝƚLJ ŽŶ ĞůĞĐƚƌŽǀĂůĞŶƚ
ĂŶĚ ĐŽǀĂůĞŶƚ ĐŽŵƉŽƵŶĚƐ In this chapter, we would answer these questions.
ͻ WŽůĂƌ ĂŶĚ ŶŽŶƉŽůĂƌ ĐŽǀĂůĞŶƚ ďŽŶĚƐ
ͻ ŽŽƌĚŝŶĂƚĞ ďŽŶĚ Chemical Reactivity and Electronic Configuration
Why do Atoms Combine
The chemical behaviour of any element depends upon the position of the
element in the periodic table. The position of an element in the periodic
VCDNG KU TGNCVGF VQ KVU GNGEVTQPKE EQPſIWTCVKQP 5Q the chemical reactivity of
dŚĞ ŽƵƚĞƌŵŽƐƚ Žƌďŝƚ ŝŶ ĞĂĐŚ ĐĂƐĞ ŚĂƐ
ĞŝŐŚƚ ĞůĞĐƚƌŽŶƐ͕ ĞdžĐĞƉƚ ŝŶ ƚŚĞ ĐĂƐĞ ŽĨ CP GNGOGPV FGRGPFU WRQP KVU GNGEVTQPKE EQPſIWTCVKQP.
ŚĞůŝƵŵ ǁŚŝĐŚ ŚĂƐ ŽŶůLJ ƚǁŽ ĞůĞĐƚƌŽŶƐ The noble gases, viz., Helium, Neon, Argon, Krypton, Xenon and
ŝŶ ŝƚƐ ŽƵƚĞƌŵŽƐƚ Žƌďŝƚ͘ Radon do not show any chemical reactivity.
This nonreactivity of noble gases is due to their stable electronic
EQPſIWTCVKQPU
6JG GNGEVTQPKE EQPſIWTCVKQPU QH PQDNG ICUGU CTG IKXGP DGNQY
yĞŶŽŶ͕ ŚŽǁĞǀĞƌ͕ ĨŽƌŵƐ ŇƵŽƌŝĚĞƐ Table 2.1 Electronic configurations of noble gases
ĂŶĚ ŽdžLJŇƵŽƌŝĚĞƐ ƵŶĚĞƌ ĐŽŶƚƌŽůůĞĚ
ĐŽŶĚŝƟŽŶƐ͘ Atomic Electronic No. of electrons
Name Symbol EQPſIWTCVKQP
Number in outermost shell
K L M N O P
Helium He 2 2 2
Neon Ne 10 2 8 8
Argon Ar 18 2 8 8 8
Krypton Kr 36 2 8 18 8 8
Xenon Xe 54 2 8 18 18 8 8
Radon Rn 86 2 8 18 32 18 8 8
