Relation Between Atomic Number and Atomic Mass

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While studying the periodic table, it will be noticed that with the increase in the atomic number, there is an increase in atomic mass as well (with some exceptions). Usually an element with high atomic number has high atomic mass as well.

If you recall from earlier lessons,

Atomic number (Z) is the number of protons in the nucleus of an atom. It is also equal to the number of electrons since the atom is electrically neutral.

Mass number (A) is the total number of neutrons and protons present in the nucleus of an atom.

The mass number of elements in most of the cases is approximately twice the value of their atomic number. This indicates that the neutron-proton (n/p) ratio of such elements is close to unity. However, it is not so in case of some heavy metals.

Comparative study of n/p ratio of some elements

It is interesting to note that n/p ratio of lighter elements is around one, whereas for heavier metals it is higher. Higher the n/p ratio, more unstable is the element. That is why metals like uranium and polonium are unstable and radioactive.