To get the best deal on Tutoring, call 1-855-666-7440 (Toll Free)


We have learnt how electrons involve in stabilisation of an atom. What’s about Proton? Has there been no role for them? Has there been any way to express the number of protons in an atom? The answer is “YES”. Protons and neutrons are also known as the nucleons. 

The number of protons in the nucleus is known as the atomic/proton number of the particular element. It is conventionally represented by the symbol Z. It is the proton/atomic number (Z) that determines the number of electrons, specific electron structure and the specific identity of an element in terms of its physical and chemical properties. Since an atom having no charge, means the number of electrons must be equal to the number of protons that is the atomic number. Hence, in an atom the atomic number is also equal to the number of electrons.

Symbolic Representation of Isotopes

All atoms have a certain value of mass number which is derived as follows. Hence, the sum of number of proton and neutron is known as mass number. The number of neutrons, N, is known as the number of neutrons in an atom. Thus, 

A = Z + N

The mass of protons and neutrons have approximately same, hence the atomic mass of an atom is equal to A. On the basis of atomic number and mass number, elements can be classified as isotopes, isobars and isotones. Let’s discuss about isotopes with some common examples.


Isotopes Definition

Back to Top
"Different kinds of atoms of the same element which have the same atomic number but different mass numbers or atomic masses (or atomic weights) are called isotopes of that element."
As isotopes have the same atomic number, we may define it as:
"Atoms of the same element with same number of protons, but a different number of neutrons in their respective nuclei."

Characteristics of Isotopes
  • The isotopes of any element will have the same number of valence electrons or valency, resulting in identical chemical properties.
  • The isotopes' physical properties are different mainly due to the neutron number variation, present in the nucleus. Properties such as melting point, boiling point, density etc., which depend upon the atomic mass should be different for different isotopes because the isotopes of an element have different masses.
    • Cl - 35.5
    • Cu = 63.5
Atomic Mass of Isotopes

We know that an atom is made up of neutrons, protons and electrons. The sum of the masses of these constituent particles in a given isotope of an element is known as its atomic mass.

Atomic mass of an isotope = Mass of neutrons + Mass of electrons + Mass of protons.
Since mass of an electron is practically nil, in the range of 0.0005449 a.m.u, the atomic mass of an electron is equal to the sum of the masses of the neutrons and protons present in it.

Atomic mass of an isotope = Mass of neutrons + Mass of protons
Also, we know that the mass number of an isotope is a whole number. Now, since the atomic masses of an isotope of an element are generally a fractional number, the mass number of that isotope which should be a whole number is obtained by rounding off the value of the atomic masses of that isotope to the nearest whole number. 

For example, since two isotopes of chlorine have atomic masses equal to 34.980 and 36.977, the mass numbers are rounded off to 35 and 37 respectively.

Stable Isotopes

Back to Top

Hydrogen atom (Z=1) has no neutrons.


Number of protons = 1

Number of electrons = 1

Number of neutrons = 0

The hydrogen element has atoms with mass number 1, 2 and 3 i.e., $^{1}_{1}H$ , $^{1}_{1}H$ & $^{1}_{1}H$.

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

Hydrogen Atom

Protium Deuterium Tritium

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

Chlorine Atom

$_{17}^{35}\textrm{C1}$ = 17 protons + 18 neutrons
$_{17}^{37}\textrm{C1}$ = 17 protons + 20 neutrons

Carbon Atom
$_{6}^{12}\textrm{C}$ = 6 protons + 6 neutrons
$_{6}^{14}\textrm{C}$ = 6 protons + 8 neutrons

Table of Isotopes

Element Number of Isotopes
Atomic mass
Hydrogen 3 1, 2, and 3
Helium 2 3 and 4
Beryllium 2 8 and 10
Fluorine 3 17, 18 and 19
Magnesium 4 23, 24, 25 and 26

Examples of Isotopes

Back to Top
Since isotopes of an element have the same atomic number, each of these isotopes contains equal numbers of protons in the nucleus and an equal number of electrons revolving in different orbits around the nucleus.

Now, since they have different mass numbers, they have a different number of neutrons in their nuclei. Thus, the number of protons (p), neutrons (n) and electrons (e) in an isotope with atomic number, Z and mass number A is given by these following relations.

Number of protons = p = Z
Number of neutrons = n = A - Z
Number of electrons = e = Z

Many elements exhibit more than one form and so have a lot of isotopes. The table below shows the list of isotopes exhibited by elements.


Atomic number
Mass number
Electrons Protons
16O 8 16
8 8
16 - 8 = 8
17 8
17 - 8 = 9
18O 8
18 - 8 = 10


There are three isotopes of hydrogen with mass numbers 1, 2 and 3 and atomic number 1.

Atomic number Mass number
1H – Protium 1 1 1 1 1 - 1 = 0
2H- Deuterium 1
2 - 1 = 1
3H- Tritium
3 - 1 = 2


Neon has three isotopes with atomic number 10 and mass numbers, 20, 21 and 22.

Isotopes Atomic number Mass number Electrons
Protons Neutrons
20Ne 10 20 10 10 20 - 10 = 10
21 - 10 = 11
22 - 10 = 12


Chlorine is a mixture of two isotopes with atomic number 17 and mass numbers 35 and 37 respectively.

Atomic number Mass number
Protons Neutrons
35Cl 17 35 17 17 35 - 17 = 18
37 - 17 = 20

Radioactive Isotopes

Back to Top
  1. Isotopes of elements like hydrogen, chlorine, neon, oxygen, etc., exist in nature and are stable. But isotopes of some elements are radioactive in nature and are not quite stable.
  2. Example: Isotopes which are radioactive in nature are called radioisotopes.
  3. Uranium has two isotopes, U - 238 is a stable form of Uranium.
  4. When Uranium was bombarded with slow electrons, U - 239 was obtained.
  5. U - 239 is a radioisotope with a half life period of 23 minutes and it disintegrates emitting beta particles to produce a new element, Neptunium, with mass number 239 and atomic number 93.
  6. Thus, this isotope of Uranium has a very small life cycle and so, is unstable.
  7. As much as 20 isotopes of neptunium were formed during their radioactive reactions with mass numbers ranging from 231 to 240.
  8. Similarly, Plutonium, Pu has two isotopes, both radioactive.
  9. They had mass numbers Pu - 241 and Pu - 239. They both disintegrate to give two important trans uranium elements, Americium, Am, with mass number 241 and atomic number 95 and Curium, with atomic number 96 and mass number 239.

Uses of Isotopes

Back to Top
There are several applications of Isotopes. 
  1. Radioisotopes are used as radioactive tracers. Radioactive isotopes have a property by which they can be easily detected and estimated quantitatively.
  2. They are also used in studying the reaction mechanisms of complicated reactions like photosynthesis, hydrolysis of esters, etc.
  3. Radioactive isotopes are used as a tracer to diagnose many diseases. This is a very important use of radioisotope. The presence and location of a brain tumor, to detect the circulation of blood, to check the pumping action of blood, function of thyroid gland, etc. can be found with the help of radioisotopes.
  4. Apart from the radioisotopes, Isotopes of carbon have been used in carbon dating, a phenomenon for detecting the age of wood.
  5. Similarly, isotopes of Hydrogen, Nitrogen and oxygen finds use in biological systems.

Isotope Problems

Back to Top
Below you could see problems.

Solved Examples

Question 1: A naturally occurring sample of Lithium contains 7.42% of  $^{6}Li $ and 92.58% of $^{7}Li $. The relative mass of  $^{6}Li$ is 6.015 and that of $^{7}Li$ is 7.016. Calculate the atomic mass of a naturally occurring sample of lithium.
Atomic mass = $\frac{(6.015 \times 7.42)+(7.016 \times 92.58)}{100}$ = 6.94


Question 2: Which two of the following nuclei are isotopes of each other?
$^{231}_{90}Z$   $^{230}_{88}Z$   $^{230}_{88}Z$   $^{233}_{90}Z$

The two isotopes are $^{231}_{90}Z$  and  $^{233}_{90}Z$


More topics in Isotopes
Stable Isotopes Unstable Isotopes
Applications of Isotopes Radioactive Isotopes
Stable and Unstable Isotopes Nuclear Half Life
Carbon Isotopes Chlorine Isotopes
Isobar Isotopic Mass
Hydrogen Isotopes Copper Isotopes
Half Life of Radioactive Isotopes Isotope Notation
Isotones Isotopes of Oxygen
Deuterium Deuterium Oxide
Tritium Radioactive Iodine
NCERT Solutions
NCERT Solutions NCERT Solutions CLASS 6 NCERT Solutions CLASS 7 NCERT Solutions CLASS 8 NCERT Solutions CLASS 9 NCERT Solutions CLASS 10 NCERT Solutions CLASS 11 NCERT Solutions CLASS 12
Related Topics
Chemistry Help Chemistry Tutor
*AP and SAT are registered trademarks of the College Board.