valency

Question 1 Define atomic number?

Question 2 Define mass number?

Question 3 Define valency?

Question 4 What are valence electrons?

Question 5 What are noble gases?

Question 6 An element has atomic number 12.How many electrons are present in K,L,M shell?

Question 7 Write the distribution of electrons in an atom of element whose atomic number is 18?

Atomic Number
The number of protons in an atom of an element is called atomic number.

Sodium
Number of protons = 11 Atomic Number=11

Carbon
Number of protons = 6 Atomic Number = 6

It is represented by Z.
Atomic Number of an element=Number of electrons in an atom

Mass Number
Mass number of an atom is due to protons and neutrons.
The total number of protons and neutrons present in an atom is its mass number.

Mass Number=No. of protons+No. of neutrons
A=P+N

Mass Number of sodium is 23
Mass Number of carbon is 12

Mass number= Atomic number+Number of Neutrons

Valency
The outermost electron shell of an atom is called valence shell.
The electrons present in outermost shell of an atom are called as valence electrons.
The valence electron of an atom take part in a chemical reaction because they have more energy than all the inner electrons.

For Ex:

1) Sodium(Z=11)
The electronic configuration of sodium is
K L M
2 8 1
Valence Electrons=1

2)Chlorine(Z=17)
The electronic configuration of chlorine is
K L M
2 8 7
Valence electrons=7

3)Magnesium(Z=12)
The electronic configuration of magnesium is
K L M
2 8 2
Valence Electrons=2

Name of ElementSymbolAtomic

Number

ProtonsNeutronsElectronsElectronic configuration

K L M N

HydrogenH11011
HeliumHe22222
LithiumLi33432 1
BerylliumBe44542 2
BoronB55652 3
CarbonC66662 4
NitrogenN77772 5
OxygenO88882 6
FluorineF991092 7
NeonNe101010102 8
SodiumNa111112112 8 1
MagnesiumMg121212122 8 2
AluminiumAl131314132 8 3
SiliconSi141414142 8 4
PhosphorusP151516152 8 5
SulphurS161616162 8 6
ChlorineCl171718172 8 7
ArgonAr181822182 8 8
PotassiumK191920192 8 8 1
CalciumCa202020202 8 8 2

The combining capacity of an atom of an element to form chemical bond is called its valency.

The valency of an element is
1)Equal to the number of valence electrons
2)Equal to the number of electrons required to complete eight electrons in valence shell.

Valency of a metal=No. of Valence electrons
Valency Of a non-metal=8-No. of valence electrons

For Ex:

1)Sodium(Z=11)
Electronic Configuration=2,8,1
Valency=1

2)Magnesium(Z=2)
Electronic Configuration=2,8,2
Valency=2

3)Chlorine(Z=17)
Electronic Configuration=2,8,7
Valency=8-7=1

4)Oxygen=16
Electronic Configuration=2,8,6
Valency=8-6=2

There are some elements which do not combine with other elements.They are known as noble gases or inert gases.

For Ex:1)Helium(Z=2)
Electronic Configuration=2
Valency=2

2)Neon(Z=10)
Electronic Configuration=2,8
Valency=8

3)Argon(Z=18)
Electronic Configuration=2,8,8
Valency=8

4)Krypton(Z=36)
Electronic Configuration=2,8,18,8
Valency=8

All the noble gases have completely filled outermost shells.
1)Atoms having 8 electrons in their outermost shell are very stable and chemically unreactive.

2) Electrons in outermost shell is stable only when the atom has just one shell,k,shell.

The atoms combine with each other to achieve the inert gas electron arrangement and become more stable.

An atom can achieve the inert gas configuration by:
1)Losing one or more electrons(Valence electron=1,2,3)
2)Gaining one or more electrons(valence electron=5,6,7)
3)Sharing one or more electrons(Valence electrons=4)