Why atomic mass is an average vàlue

There are different isotopes for different elements

Explanation:

The mass of each of the isotopes of an element are added with their percentage each then they are divided by 100

Example boron 11 is 80 % boron 10 is 20%

Total mass of boron 10 is 20*10

Total mass of boron 11 is 80*11

Atomic mass is (20*10) + (80*11) / 100

Which is 10.8 and it is rounded off as 11 in the periodic table

The atomic mass is the average number of protons and neutrons for all natural isotopes of an element. It is a decimal number.

Explanation:

Atomic Mass and Mass Number Example:

Hydrogen has three natural isotopes: 1H, 2H, and 3H. Each isotope has a different mass number.

1H has 1 proton. Its mass number is 1. 2H has 1 proton and 1 neutron. Its mass number is 2. 3H has 1 proton and 2 neutrons. Its mass number is 3. 99.98% of all hydrogen is 1H 0.018% of all hydrogen is 2H 0.002% of all hydrogen is 3H Together, they give a value of atomic mass of hydrogen equal to 1.0079 g/mol.