Atomic structure

Abstract image of an atom

Aims of this section

After studying this section, you should be able to:

  • describe how the atomic model has changed over time
  • recall atomic structure in terms of the positions and properties of subatomic particles
  • calculate the numbers of subatomic particles in isotopes from atomic numbers and mass numbers
  • calculate relative atomic masses.


Development of the atomic model

Our understanding of atomic structure has developed over time. The diagrams show some important atomic models.

Image of a grey sphere.
John Dalton's atomic model (circa 1808)
J.J. Thomson's plum pudding model (1904)
Ernest Rutherford's nuclear model (1911)
Rutherford's planetary model (1911)
Niels Bohr's model (1913)
Bohr model with protons and neutrons in the nucleus

Atoms consist of subatomic particles. The table shows their locations in atoms, relative masses and relative charges.

Locationnucleusnucleusshells (energy levels)
Relative charge+10–1
Relative mass1111836

Protons, neutrons and electrons are called subatomic particles because they are smaller than atoms.

The relative mass of an electron may be described as zero or negligible in some GCSE specifications – check yours.

Atomic number, mass number and isotopes

Atomic number is the number of protons in the nucleus.

Mass number is the total number of protons and neutrons in the nucleus.

You can calculate the number of particles in an atom from these two numbers:

  • number of protons = atomic number
  • number of neutrons = (mass number) – (atomic number)
  • number of electrons = number of protons
In these symbols, mass number is the top number and atomic number is the bottom number

Isotopes of an element are different atoms with:

  • the same number of protons but different numbers of neutrons in their nuclei
  • the same atomic number but different mass numbers.

Isotopes of an element have identical chemical properties. The existence of isotopes explains why relative atomic masses are not whole numbers (unless they are rounded up or down).

Three isotopes of hydrogen

Calculating relative atomic mass

The relative atomic mass of an element is the weighted mean mass of its atoms compared to 112th the mass of a 12C atom. It is given the symbol Ar.

You can often find Ar values on periodic tables, but take care – they are not mass numbers.

You can calculate the relative atomic mass of an element if you know:

  • the mass numbers of its isotopes
  • the percentage abundance of each isotope.

Worked example

A sample of chlorine contains two different isotopes, as shown in the table. Calculate the relative atomic mass of chlorine.

IsotopeMass numberPercentage abundance


\displaystyle\footnotesize\textsf{relative atomic mass = }\frac{(35\times75)+(37\times25)}{(75+25)}


Chlorine is used to kill harmful bacteria in drinking water and in swimming pools

The periodic table shows that chlorine has an Ar of 35.5, rather than 35 or 37. The value is closer to 35 than to 37 because most chlorine atoms are 35Cl atoms rather than 37Cl atoms.