Concept of atomic number, mass number, fractional atomic mass, isotopes, isobars

The nuclei of atoms is made up of protons and neutrons. These two components of the nucleus are referred to as nucleons. The electrons occupy the space outside the nucleus. Since an atom is electrically neutral, the number of protons in the nucleus is exactly equal to the number of electrons. This number is the atomic number given by the symbol Z.

Atomic number represents the number of protons in an atom. As atoms are electrically neutral, an atom contains as many electrons as it has protons.

Atomic mass, mass number or nucleon number

The total number of protons and neutrons present in one atom of an element is known as its mass number.

Mass number = number of protons + number of neutrons

It can also be said that:

Mass number = Atomic number + number of neutrons

Atomic number and mass number can also be represented on the symbol of an element as follows:

Let's take an example:

  • Represent the atom of sodium whose atomic mass is 23 and atomic number is 11. Calculate the number of protons, electrons and neutrons.


Atomic number Z =11

Atomic mass A = 23

No. of protons = Z = 11

No. of electrons = 11

No. of neutrons = A - Z

23 - 11 = 12

Study the table carefully

Lithium - 7 Silicon - 28 Copper - 65 Dysprosium - 164 Uranium - 238
Proton Number Z 3 14 29 66 92
Nucleon number A 7 28 65 164 238
Number of neutrons N (A-Z) 4 14 36 98 146
7  Li 3
28   Si 14
65   Cu 29
164    Dy  66
238    U  92

Attempt the following:

  • The atomic number of an element is 12. How many protons and electrons are there in the atom?


Atomic number = 12

Number of protons = Number of electrons = Atomic number = 12
  • The nucleus of an atom of an element contains 11 protons and 12 neutrons. Determine the atomic number and mass number of the element.


Atomic number = number of protons = 11

Mass number = number of protons + number of neutrons

= 11 + 12 = 23

Fractional atomic mass

It is interesting to note that atoms of a given atomic number can have different number of neutrons.

Some examples are listed below:


Hydrogen atom (Z=1) has no neutrons.

Number of protons = 1

Number of electrons = 1

Number of neutrons = 0

It has been reported that the hydrogen element has atoms with mass number 2 and 3 also i.e.,

Atoms of elements having the same atomic number with different mass numbers are called isotopes.

isotope depiction

Nomenclature Protium Deuterium Tritium

Protium Deuterium Tritium

At.mass = 1 At.mass = 2 At.mass = 3

isotopes chlorine

Nuclear composition of isotopes of chlorine:

carbon isotopes

Nuclear composition of isotopes of carbon:

Characteristics of isotopes

  • All isotopes of an element have the same number of valence electrons thus have identical chemical properties.
  • The physical properties of the isotopes are different due to the difference in the number of neutrons in their nuclei. The densities, melting points and boiling points etc., are slightly different.

Reason for fractional atomic masses of elements

Atomic masses of many elements are in fractions not in whole numbers.


Cl - 35.5

Cu = 63.5

The fractional atomic masses of elements are due to the existence of isotopes having different masses.


Natural chlorine consists of two isotopes:

Calculate the average atomic mass of chlorine.

Try it out!

  • A naturally occurring sample of Lithium contains 7.42% of and 92.58% of . The relative mass of is 6.015 and that of is 7.016. Calculate the atomic mass of a naturally occurring sample of lithium.


  • Which two of the following nuclei are isotopes of each other?

The two isotopes are:


These are the elements having same atomic number but different mass number. They have the same atomic number because the number of protons inside their nuclei remains the same. The difference in their mass number is due to the difference in their number of neutrons.

Since they are neutral isotopes are elements having same number of electrons, which make them to possess identical chemical properties. Let us see some examples 1H1, 1H2, 1H3 are all isotopes of hydrogen. They all have their atomic number to be unity but the number of neutrons are 0, 1, 2 and z respectively. 17Cl37, 17Cl35 are isotopes of chlorine. They have 17 protons in the nucleus but have number of neutrons equal to 20 and 18 respectively. Practically every element consists of a mixture of several isotopes. The relative abundance of different isotopes differs from element to element. For example chlorine is composed of two isotopes of masses 34.98U and 36.98U, which are nearly integral multiples of the mass of hydrogen atom. Their relative abundances are 75.4 and 24.6 percent respectively. Mass of natural chlorine atom can be found as

= 35.47.

The isotope can occur either naturally or can be produced artificially in the laboratory.


Isotopes are chemically same and physically different. But the converse is true in isobars. That is isobars are elements, which are chemically different but physically same. So, isobars are atoms of different elements having the same atomic mass but different atomic number. Since their number of electrons is different, their chemical properties are different. The light nuclei have unstable isobars. Heavy nuclei have stable isobars and these occur in pairs. Suppose the number of protons of one isobar matches with that of another they are called as mirror-nuclides of each other.

Examples of isobars are

examples for isobars

Since isobars are different elements they appear in different places in the periodic table.


Isotones are elements having the same number of neutrons. Examples of isotones are Chlorine - 37 and Potassium - 39. Both have 20 neutrons in their nuclei.