Proton Number / Atomic Number 

The number of protons in an atom is called the proton number (also called the atomic number).                                                                                                                Every element has its own proton number.  In the Periodic Table, elements are arranged in the increasing order of their proton number. Atom is electrically neutral; therefore, the proton number is equal to the number of electrons.

In an atom,  Proton number (atomic number) = Number of protons = Number of electrons

Nucleon Number / Mass Number

It is the sum of the numbers of protons and neutrons in an atom. Each proton and neutron carry 1 unit mass; therefore, the mass number of an atom will be the total number of protons and neutrons present in the nucleus. The mass of an atom is because of nucleus only, since electrons are very light particles.

 

Mass Number / Nucleon Number = Number of protons +  Number of neutrons   

Atomic mass (Relative Atomic Mass, Ar)

is the actual mass of an isotope expressed in atomic mass units (amu or simply u). It is to be noted that the masses of both neutron and proton although very close, are not exactly equal to 1 amu. Hence, the atomic mass can be fractional while the mass number is always a whole number.  The mass of an atom is very small. For example, the mass of a hydrogen atom is about 10−24 g. Since actual masses of atoms are so small; chemists do not use them in calculations. They only need to compare the masses of different atoms. They first started comparing the masses of different atoms with the mass of a hydrogen atom. For example, one nitrogen atom is 14 times heavier than a hydrogen atom. Same way, an oxygen atom is 16 times heavier than one hydrogen atom. Therefore, we can say that the relative atomic mass of hydrogen is 14 and the relative atomic mass of oxygen is 16.  Since it was not always convenient to compare the masses of different atoms with the mass of a hydrogen atom. Therefore, in 1961 carbon – 12 (12C) was chosen as a new standard.                       

This is the isotope of carbon with a relative atomic mass of 12 means a carbon atom has 12 times the mass of one hydrogen atom. So, 1/12 th of a carbon atom has the same mass as one hydrogen atom. Carbon consists of more than one isotope, so to be accurate, one particular isotope 12C chosen for comparing the masses of atoms.

“The relative atomic mass of an element is the average mass of one atom of the element when compared with 1/12 th of the mass of an atom”.

Atomic weight

The third term we need to know is atomic weight. It has its origin in the “relative natural abundance” of isotopes. Since there is more than one isotope for any given element, it is obvious that the total number of all the atoms of that particular element will have a certain percentage of each of those. Let’s take the example of Chlorine. It has two stable isotopes namely 35Cl and 37Cl. In the periodic table, however, the mass of a chlorine atom is given as 35.45 u. This results from the relative abundance of 75.76% of chlorine-35 and 24.24% of chlorine-37.

Key Points:

  1. Mass number = No. of neutrons + No. of protons
  2. Atomic Mass = actual mass of an isotope expressed in amu. It is sum of total number of protons, neutrons and electrons in an isotope of atom.
  3. Atomic Weight = Average of the masses of the stable isotopes of an element according to their natural abundance relative to mass carbon-12.

NOTE:

  • The number of neutrons in an atom is approximately the same as the number of protons,  up to 20 protons (i.e., up to proton number 20).

As the number of protons becomes larger, the number of neutrons becomes increasingly greater than the number of protons. For example, an atom of lead has 82 protons but 125 neutrons.

It is important to note that in some cases the number of neutrons may be greater than by one or two, but not by large numbers, which is why the word “approximately” is used.

For example, fluorine (F) has 9 protons, 9 electrons, and 10 neutrons.

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