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What is an Isotope ?

In general non-scientific terms, isotopes are slightly different types (or "versions") of atoms of the same element.
If asked "What is an isotope ?" in GCSE Chemistry, a more detailed answer is required using one of the definitions below

Definitions of an isotope:

Isotopes are atoms of the same element that have the same number of protons but different numbers of neutrons.

Another way to say the same thing is:

Isotopes are atoms of the same element that have the same atomic number (whose symbol is "Z") but different mass numbers (whose symbol is "A").

It is useful to remember that:

  • The number of protons in the nucleus of an atom is the Atomic Number of that atom and defines which type of atom it is, that is - which element the atom is a particle of.
  • The number of neutrons in the nucleus of an atom is the Mass Number (or "Atomic Mass") of the atom - the Atomic number of the atom = A-Z.


How do different isotopes of the same element differ:

Different isotopes (of the same element) have different numbers of neutrons in the nuclei of their atoms.

Note: As isotopes are slightly different versions of the same element, it is not always necessary to specify "isotopes of the same element". When discussing the meaning and properties of isotopes in general, as on this page, the "same element" part may often be assumed.

Chemical Properties:
The chemical properties of most isotopes are the same or very similar.
This is because chemical reactions generally involve electrons rather than protons or neutrons.

Physical Properties:
The physical properties of isotopes differ because the atomic masses (mass numbers, symbol "A") of isotopes differ. This affects physical properties such as densities and temperatures of changes of state e.g. boiling points and melting points.


Isotopes of Hydrogen:

GCSE Chemistry requires knowledge about the isotopes of the element hydrogen.
Hydrogen is the only element whose isotopes have different names, i.e. "hydrogen", "deuterium" and "tritium".

number of
number of
number of



















Notice that the general system for naming isotopes is the name of the element followed by a "-" followed by the mass number (=protons+neutrons) of that particular isotope, e.g. hydrogen-2.


More Examples of Isotopes:

There are two common isotopes of the element chlorine. They are chlorine-35 and chlorine-37.

Isotopes and Mass Numbers of Elements

Have you noticed that some versions of the Periodic Table state the mass numbers of the elements as whole numbers only, whereas other versions of the periodic table state mass numbers to two or three decimal places ?
Given that the mass number of an atom is the total number of protons and neutrons in the atom, doesn't it seem strange that mass numbers are not always whole numbers ? (Protons and neutrons are deemed to have the same mass of one atomic mass unit (u) each.)

When the stated mass number of an element is not a whole number that does not mean that there are parts of protons or neutrons present in the atom. It means that that mass number has been calculated to take into consideration the proportions of the different isotopes of that element believed to exist overall.

Chlorine is a good example because some some versions of the periodic table that state mass numbers as whole numbers make an exception for chlorine, giving its mass number the value 35.5. Consider a sample of chlorine that contains the isotopes chlorine-35 and chlorine-37 in the ratio 3:1, meaning that for every one atom of chlorine-37 there are 3 atoms of chlorine-35.

The relative atomic mass of chlorine may be calculated by taking into account the atomic masses and relative proportions of its isotopes:


Isotope 1


Isotope 2







Mass Number





Relative Proportion





35 x 3 = 105


37 x 1 = 37


= 105 + 37 = 142

= 142 / (3+1)

Relative Atomic Mass

= 142 / 4 = 35.5

Examples of Isotopes of other elements

Many other common elements have several isotopes. Some examples are follow below.
There's no need to memorise this list for GCSE Chemistry.

  • Carbon generally* has a mass number of 12 (see Periodic Tables online or in chemistry textbooks), but another isotope of carbon is carbon-14, which includes an extra 2 neutrons. Carbon-14 is used for dating recent archeological artifacts.
  • Oxygen generally* has a mass number of 16 (see Periodic Tables online or in chemistry textbooks), but another isotope of oxygen is oxygen-18, which includes an extra 2 neutrons.
  • Phosphorus generally* has a mass number of 31 (see Periodic Tables online or in chemistry textbooks), but another isotope of phosphorus is phosphorus-32, which includes one extra neutron and is sometimes used as a metabolic and ecological tracer, as well as for studies of nucleotides and nucleic acids.

Elements with only one Isotope:

Most elements have at least two stable (meaning "non-radioactive") isotopes.
A small number of elements exist in the form of only one isotope, for example flourine-19.

For general interest, here are some more examples of elements that have just one stable isotope:

  • Beryllium (Symbol Be, Z=4, A=9)
  • Sodium (Symbol Na, Z=11, A=23)
  • Aluminium (Symbol Al,Z=13, A=27)
  • Phosphorus (Symbol P, Z=15, A=31)
  • Manganese (Symbol Mn, Z=25, A=55)
  • Arsenic (Symbol As, Z=33, A=75)
  • Yttrium (Symbol Y, Z=39, A=89)
  • Niobium (Symbol Nb, Z=41, A=93)
  • Iodine (Symbol I, Z=53, A=127)
  • Gold (Symbol Au, Z=79, A=197)


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