Are Isotopes Stable?11 Exhaustive Facts You Should Know

Are isotopes stable? It is very interesting to find out whether all the isotopes are stable or not as Isotopes possess so many applications.

All isotopes are not stable.Atoms present in nature are either stable or unstable isotopes.On the basis of atomic no. (Z) of isotopes and whether the nucleus contains even or odd no. of isotopes we can predict about the stability of isotopes.

Example: Hydrogen has three isotopes Protium, Deuterium and Tritium. Among these three isotopes, Protium and Deuterium are the stable isotopes but tritium is unstable.

A picture containing drawing

Description automatically generatedProtium
A picture containing drawing

Description automatically generatedDeuterium

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Description automatically generatedTritium

Figure: Are Isotopes Stable?

Which isotopes are stable?

The following isotopes are stable –

Isotopes which do not undergo radioactive decay.Isotopes whose atomic no. less than 20 and with n/p ratio close to 1.Isotopes with long half-life or they do not have half-life.Isotopes with atomic number < 82 have one or more than one stable isotope (Except technetium Z= 43 and promethium with Z = 61).

Isotopic nuclei with such number of proton or neutron which make the isotope stable. These magic numbers occur naturally in the isotopes which are stable.

Some isotopes contains of magic nos. for both neutrons and protons. This is called as double magic numbers.

The magic numbers for the number of protons and neutrons are –

proton: 2,8,20,28,50,82,114

neutron: 2,8,20,28,50,82,126,184

Example: Helium having 2 protons and 2 neutrons

                 Lead having 82 protons and 126 neutrons

No. of ProtonNo. of neutronStable isotopes that have these no. either in proton or neutron
    Even     Even        163
    Even    Odd         53
    Odd    Even         50
    Odd    Odd          4
Table: Isotopes that have magic no. of either proton or neutron

 Which isotopes are unstable?

 The following isotopes are unstable –

  • Isotopes with odd number of both protons and neutrons possess higher probability to be unstable (Exception hydrogen-2, lithium-3, boron-10, nitrogen-14).
  • Isotopes with short half-life that can be easily calculated.
  • Isotopes which have atomic number(Z) > 83 are unstable.

Does parent isotope are stable?

No, parent isotopes are never stable isotopes.Radioactive decay starts from the parent isotope itself. Parent isotopes undergo radioactive decay to form daughter isotope. During the decay, parent isotope gives out alpha, beta and gamma particles.

Example- Uranium undergoes alpha decay and form Thorium (daughter isotope).

Are radioactive isotopes stable?

Radioactive isotopes are not stable.The nucleus of radioactive isotope has unstable n/p ratio that is neutron to proton ratio. When the radioactive isotope undergoes radioactive decay, they get converted into different atom. Radioactive isotopes undergo decay spontaneously and release subatomic particles and electromagnetic radiations.

The process of radioactive decay continues till a stable atom is formed and it is no longer radioactive.

Why are some isotopes stable and other unstable?

Stability of nucleus depends on the number of protons and neutrons present in the nucleus.

Some isotopes are stable due to the following reasons-

Strong nuclear forces which hold neutrons and protons together. These forces are stronger than those forces which drive protons and neutrons apart (like the electrostatic repulsion between the positively charged protons).

Their n/p ratio is in the range of 1:1 or 1.5:1

eg: hydrogen has three isotopes

Protium1 protonStable
Deuterium1 proton + 1 neutronStable
Tritium1 proton + 2 neutronsUnstable (Radioactive)
isotopes of Hydrogen

Strong nuclear forces operate over short distances. As the size of nucleus increases, the electrostatic repulsive forces between positively charged protons decreases.

Presence of extra neutrons increases space between the protons and repulsion between protons decreases.

This makes the isotopes stable.

Some isotopes are unstable because of the following reasons-

  • Presence of extra neutrons produces extra energy in the nucleus an excess of energy makes the isotope unstable.

eg: Carbon-14 having 6 protons and 8 neutrons is unstable.

  • The forces between the particles of neutrons and protons are not balanced due to excess presence of either neutrons or protons.
  • Their n/p ratio is either less than 1:1 or greater than 1.5:1
  • Presence of too many or too less neutrons as compared to required optimum ratio.

Stable Isotopes of some elements

stable isotopes of some common elements are as follows –

What are stable isotopes of carbon?

The stable isotopes of carbon are –

stable isotopes of carbon

What are the stable isotopes of nitrogen?

The stable isotopes of nitrogen are –

stable isotopes of nitrogen

What are the stable isotopes of Oxygen?

The stable isotopes of Oxygen are –

stable isotopes of oxygen

What are the stable isotopes of hydrogen?

The stable isotopes of hydrogen are –

Hydrogen-1 (protium)
stable isotopes of hydrogen

What are the stable isotopes of Helium?

The stable isotopes of Helium are –

stable isotopes of Helium

What are the stable isotopes of uranium?

The stable isotopes of Uranium are –

stable isotopes of Uranium

What are the stable isotopes of Beryllium?

The stable isotopes of beryllium are –

stable isotope of Beryllium

What are the stable isotopes of calcium?

The stable isotopes of calcium are –

Stable isotopes of Calcium

What are the stable isotopes of tin?

The stable isotopes of tin are –

stable isotopes of Tin

What are the stable isotopes of lead?

The stable isotopes of lead are-

stable isotopes of Lead


Isotopes can be stable or unstable. All artificial isotopes are unstable and hence radioactive. Isotopes with n/p ratio 1:1 or 1.5:1 are stable the isotope.

Sonali Jham

Hi...Myself Sonali Jham. I have done my Post-Graduate in Chemistry and also completed B. Ed. I am a Teacher and a Dietician by profession. My hobby is reading and painting. Let's connect through LinkedIn-

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