The Atomic Number, Mass Number, Isotopes and Relative Atomic Mass

                                The Atomic Number (Z) refers to the number of protons of an element in its nucleus. All atoms of an element have the same atomic number and no two elements can have the same atomic number. For example, the atomic number of calcium in the periodic table is 20, thus, the nucleus of calcium contains 20 protons. The Mass Number (A) refers to the mass of the nucleus or sum of the number of protons and neutrons in the nucleus. For example, calcium has 20 protons and 20 neutrons in its nucleus, therefore, the mass number of calcium is 40.

                                The Atomic Number and Mass Number of an element can written with its Atomic symbol.

In determining the number of subatomic particles in an atom, the following rules should be followed:

1.       The atomic number is equal to the number of protons (Z = p⁺)

2.       The mass number is the sum of the number of protons and neutrons in the nucleus.

(A = p⁺ + n⁰      or     A = Z + n⁰)

3.       The number of neutrons is equal to the mass number minus the number of protons.

(n⁰ = A - p⁺       or    n⁰ = A – Z)

4.       In a neutral atom, the number of protons and electrons are the same

If the atom is charged, subtract the charge to the number of protons (or Z).

Isotopes and Relative Atomic Mass

                                As technology progressed, scientists were able to discover that there are atoms of the same element with different mass numbers. They call these atoms as isotopes. These isotopes are atoms of the same element or atomic number but with different number of neutrons. For example, hydrogen has three isotopes, namely:

                              These isotopes of hydrogen have the same number of protons (atomic number) but have different mass numbers due to their difference in the number of neutrons. These isotopes have varying abundance. These percent abundance and mass numbers of isotope are used to get the relative atomic mass of the element. The Relative Atomic Mass of an element is the average atomic mass of all the naturally occurring isotopes of an element. The formula for the Relative Atomic Mass (A_R) is