Atomic Number and Mass Number

– In this subject, we will discuss the Atomic Number and Mass Number

Mosley’s Determination of Atomic Number

– The discovery that an 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:

– The 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 the 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 Problems on atomic Number and Mass Number

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

Solution

– Atomic Number of uranium = 92

∴ Number of electrons = 92

and Number of protons = 92

– The 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.