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 Example:

(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 Element	Symbol	Atomic Number	Protons	Neutrons	Electrons	Electronic configuration K L M N
Hydrogen	H	1	1	0	1	1
Helium	He	2	2	2	2	2
Lithium	Li	3	3	4	3	2 1
Beryllium	Be	4	4	5	4	2 2
Boron	B	5	5	6	5	2 3
Carbon	C	6	6	6	6	2 4
Nitrogen	N	7	7	7	7	2 5
Oxygen	O	8	8	8	8	2 6
Fluorine	F	9	9	10	9	2 7
Neon	Ne	10	10	10	10	2 8
Sodium	Na	11	11	12	11	2 8 1
Magnesium	Mg	12	12	12	12	2 8 2
Aluminium	Al	13	13	14	13	2 8 3
Silicon	Si	14	14	14	14	2 8 4
Phosphorus	P	15	15	16	15	2 8 5
Sulphur	S	16	16	16	16	2 8 6
Chlorine	Cl	17	17	18	17	2 8 7
Argon	Ar	18	18	22	18	2 8 8
Potassium	K	19	19	20	19	2 8 8 1
Calcium	Ca	20	20	20	20	2 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 Example :

(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 (Z=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 Example:

(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)