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