Page 16 - Chemistry ICSE Class IX

4 ICSE Chemistry – 9
These are,
ƒ the symbol stands for the name of the element.
ƒ the symbol stands for one atom of the element.
ƒ the symbol represents quantity of the element equal in mass to its
atomic mass or gram-atomic mass.
ƒ the symbol also represents mass of the element which contains one
Avogadro’s number of atoms of the element.
For example, the symbol O stands for
Š the element oxygen.
Š one atom of oxygen.
Š the mass of oxygen equal to its gram-atomic mass, K G , O represents
16 g of oxygen.
Š the mass of oxygen which contains one Avogadro’s number
23
(= 6.023 × 10 ) of oxygen atoms.
Valence Electrons

In an atom, electrons revolve around a positively charged core called
nucleus. These electrons are distributed in various shells around the
nucleus.

What are valence electrons
The electrons present in the outermost shell (generally termed as valence
dŚĞ ĂƚŽŵƐ ŽĨ ŵĞƚĂůƐ ƚĞŶĚ ƚŽ ůŽƐĞ shell) are called valence electrons. The number of valence electrons in the
ĞůĞĐƚƌŽŶƐ ƚŽ ĨŽƌŵ ƉŽƐŝƟǀĞ ŝŽŶƐ ;Žƌ
ĐĂƟŽŶƐ) atoms of some common elements are given in Table 1.4.
ͻ Table 1.4 Number of valence electrons in the atoms of some elements
dŚĞ ĂƚŽŵƐ ŽĨ ŶŽŶŵĞƚĂůƐ ƚĞŶĚ ƚŽ ŐĂŝŶ
ĞůĞĐƚƌŽŶƐ ƚŽ ĨŽƌŵ ŶĞŐĂƟǀĞ ŝŽŶƐ ;Žƌ 'NGEVTQPKE EQPſIWTCVKQP No. of electrons in No. of valence
ĂŶŝŽŶƐͿ͘ Element K L M N the outermost shell electrons
ͻ
dŚĞ ŶƵŵďĞƌ ŽĨ ƉŽƐŝƟǀĞ ĐŚĂƌŐĞƐ ŽŶ H 1 1 1
Ă ĐĂƟŽŶ ŝƐ ĞƋƵĂů ƚŽ ƚŚĞ ŶƵŵďĞƌ ŽĨ
ĞůĞĐƚƌŽŶƐ ůŽƐƚ ďLJ ƚŚĞ ƉĂƌĞŶƚ ĂƚŽŵ͘ C 2 4 4 4
ͻ
dŚĞ ŶƵŵďĞƌ ŽĨ ŶĞŐĂƟǀĞ ĐŚĂƌŐĞƐ ŽŶ O 2 6 6 6
ĂŶ ĂŶŝŽŶ ŝƐ ĞƋƵĂů ƚŽ ƚŚĞ ŶƵŵďĞƌ ŽĨ Cl 2 8 7 7 7
ĞůĞĐƚƌŽŶƐ ŐĂŝŶĞĚ ďLJ ƚŚĞ ƉĂƌĞŶƚ ĂƚŽŵ͘
ͻ K 2 8 8 1 1 1
ƚŽŵƐ ŽĨ Ăůů ĞůĞŵĞŶƚƐ ĞdžĐĞƉƚ ŶŽďůĞ
ŐĂƐĞƐ ĐĂŶ ůŽƐĞ Žƌ ŐĂŝŶ ĞůĞĐƚƌŽŶƐ͘
Valency
The valency of any element can be de ned in two ways:
ƒ on the basis of electronic theory
ƒ on the basis of combining capacity during compound formation

What is the electronic concept of valency
According to the electronic concept of valency, it is de ned as follows:
The number of electrons which an atom loses, gains or shares with
QVJGT CVQOU VQ CVVCKP VJG PGCTGUV PQDNG ICU EQPſIWTCVKQP KU VGTOGF CU KVU
valency.
For example, during the formation of sodium chloride (NaCl), sodium
atom loses one electron and chlorine atom gains one electron.