Mass number and isotopes.

Mass number and isotopes.

1. Not all atoms of an element have the same of neutrons. Atoms of the same element with different masses are called isotopes.

2. The proportion of each isotope of an element is fixed, and it is the same in both the pure element and in the compounds containing that element.

3. Because the isotopes have the same number of electrons, they all react in exactly the same way.

4. How Science Works. The relative atomic mass (Ar) of an element is the average mass of an atom. You work out using the percentage or number of atoms of each different isotope and its mass. Potassium-39; the other 10 per cent are of potassium-40. The A, of potassium is (90 times 39)+(10 times 40) divied 100 = 39.1.