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Atomic Number - Mass Number

Mosley’s Determination of Atomic number

The discovery that atom has a nucleus that carries a positive charge raised the question : What is the magnitude of the positive charge? This question was answered by Henry Mosley in 1913.

Hitherto atomic number was designated as the ‘position number’ of a particular element in the Periodic Table.

Mosley found that when cathode rays struck different elements used as anode targets in the discharge tube, characteristic X-rays were emitted. The wavelength of these X-rays decreases in a regular manner in passing from one element to the next one in order in the Periodic Table.

Mosley plotted the atomic number against the square root of the frequency of the X-rays emitted and obtained a straight line which indicated that atomic number was not a mere ‘position number’ but a fundamental property of the atom.

He further made a remarkable suggestion that the wavelength (or frequency) of the emitted X-rays was related to the number of positive charges or protons in the nucleus. The wavelength changed regularly as the element that came next in the Periodic Table had one proton (one unit atomic mass) more than the previous one.

Mosley calculated the number of units of positive charge on the nuclei of several atoms and established that :

Atomic Number of an element: is equal to the number of protons in the nucleus of the atom of that element.

Since the atom as a whole is electrically neutral, the atomic number (Z) is also equal to the number of extranuclear electrons. Thus hydrogen (H) which occupies first position in the Periodic Table has atomic number 1. This implies that it has a nucleus containing one proton (+1) and one extranuclear electron (– 1).

Now the term Atomic Number is often referred to as the Proton Number.

What Is Mass Number ?

 Mass Number, A, of the atom: is The total number of protons and neutrons in the nucleus of an atom.

In situations where it is unnecessary to differentiate between protons and neutrons, these elementary particles are collectively referred to as nucleons. Thus mass number of an atom is equal to the total number of nucleons in the nucleus of an atom.

Obviously, the mass number of an atom is a whole number. Since electrons have practically no mass, the entire atomic mass is due to protons and neutrons, each of which has a mass almost exactly one unit. Therefore, the mass number of an atom can be obtained by rounding off the experimental value of atomic mass (or atomic weight) to the nearest whole number.

For example, the atomic mass of sodium and fluorine obtained by experiment is 22.9898 and 26.9815 amu respectively. Thus their mass numbers are 23 for sodium and 27 for fluorine.

Each different variety of atom, as determined by the composition of its nucleus, is called a nuclide.

Composition of The Nucleus

Knowing the atomic number (Z) and mass number (A) of an atom, we can tell the number of protons and neutrons contained in the nucleus. By definition :

Atomic Number, Z = Number of protons

Mass Number, A = Number of protons + Number of neutrons

The number of neutrons is given by the expression:
N = A – Z

Solved Problem

Uranium has atomic number 92 and atomic weight 238.029. Give the number of electrons, protons and neutrons in its atom.


Atomic Number of uranium = 92
Number of electrons = 92
and Number of protons = 92

Number of neutrons (N) is given by the expression:

N = A – Z

Mass Number (A) is obtained by rounding off the atomic weight:

= 238.029 = 238
N = 238 – 92 = 146

Thus uranium atom has 92 electrons, 92 protons and 146 neutrons.

The composition of nuclei of some atoms is given in the following  table:

Reference: Essentials of Physical Chemistry /Arun Bahl, B.S Bahl and G.D. Tuli / multicolour edition.

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