Structure of Atoms, Mass Number and Isotopes

The atom comprises protons, neutrons and electrons. The protons and neutrons are enclosed in a central nucleus that is at the center of the atom, together they are collectively known as nucleons. All of the mass of an atom is said to be in the nucleus because electrons weigh little.

The table below shows some comparisons between protons, neutrons and electrons:

Sub-atomic particle
Relative Mass
Relative Charge


Finding protons, neutrons or mass number:-
Given the values of two of these variables one can transpose the following formula to find the third variable.

     Mass # (A) = # of protons (z) + # of neutrons
Note: The names may vary for e.g. number of protons can sometimes be called atomic number and mass number can be called nucleon number.

For example calculate the number of neutrons in this atom:

If you look at the diagram closely you will see that they have given you two value the mass number and the number of protons (in this example the term atomic number is used instead). Right away one can see that all is needed is to transpose the previous formula to find number of neutrons.

    Mass number = number of protons or atomic number + number of neutrons
             19          =         9     +      N

When done transposing this formula for N you should get:

                      19 – 9 = 9 – 9 + N
                      19 – 9 = 0 + N
                            10 = N
                         =>N = 10

Finding the number of electrons:-

It is relatively easy to find the number of electrons because no calculation is needed. Atoms are neutral the positive charges from the protons will balance the negative charges of the electrons. Therefore the electrons would have to be the same number as that of the protons.
     Number of protons = Number of electrons.

These are atoms of the same element with the same atomic number but different mass numbers. This is due to a difference in the amount of neutrons present in each atom; the protons however do not change. For example there are three isotopes of carbon namely: Carbon-12, Carbon-13 and carbon-14(the numbers represent mass) their protons remain the same being 6 but the neutrons vary being 6, 7 and 8 respectively.

Relative Atomic Mass:
The relative atomic mass of an element can be defined as the ratio of the mass of an atom of the element to one twelfth the mass of carbon-12.

The relative atomic mass can be found using the following formula:

     R.A.M = Mass of one atom of the element   × 12
                     Mass of one atom of Carbon-12

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