The relative mass of an electron is sometimes quoted as zero as its weight is very nearly zero especially compared to the weight of the proton and neutron.

The protons are found in the centre of the atom in the nucleus. Whereas the electrons orbit in shells around the nucleus

The atomic number of an element is the number of protons I the nucleus. It has the symbol Z.

Element

Atomic Number

Mass

Number

Number of

Protons

Number of

Electrons

Number of

Neutrons

Hydrogen

1

1

1

1

0

Helium

2

4

2

2

2

Lithium

3

7

3

3

4

Beryllium

4

9

4

4

5

Boron

5

11

5

5

6

Carbon

6

12

6

6

6

Nitrogen

7

14

7

7

7

Oxygen

8

16

8

8

8

Fluorine

9

19

9

9

10

Neon

10

20

10

10

10

Sodium

11

23

11

11

12

Magnesium

12

24

12

12

12

Aluminium

13

27

13

13

14

Silicon

14

28

14

14

14

Phosphorus

15

31

15

15

16

Sulphur

16

32

16

16

16

Chlorine

17

35.5

17

17

17

Argon

18

40

18

18

22

Potassium

19

39

19

19

29

Calcium

20

40

20

20

20

Isotopes

Isotopes are atoms of the same elements with the same element with the same atomic number but different mass numbers.

Number of neutrons = Mass Number – Atomic Number

Isotopes

Protons

Neutrons

Electrons

Chlorine-35

17

18

17

Chlorine-37

17

20

17

Silicon-28

14

14

14

Silicon-29

14

15

14

Silicon-30

14

16

14

Examples

  1. About 75% of naturally occurring chlorine is Chlorine-35 and 25% of chlorine-37

Average= (75×35) x (25×37) =35.5

100

  1. There are 2 isotopes of Bromine Br-79 has an abundance of 50.69%. Br-81 has an abundance of 49.31%. What is its mass?

(50.69 x79) x (49.31 x 81) = 79.9862

100