Modelling the atom

Everyday matter is made up of particles called atoms. The Greek word 'atomos' means 'indivisible' - in the past, atoms were thought to be the smallest particles possible.

Developing models

Dalton's model (1803)

John Dalton imagined atoms as tiny solid balls. Dalton's model included these ideas:

  • atoms cannot be broken down into anything simpler
  • the atoms of a given element are identical to each other
  • the atoms of different elements are different from one another
  • during chemical reactions atoms rearrange to make different substances

In Dalton's time, it was not possible to investigate the structure of atoms.

Thomson's model (1897)

In 1897, J J Thomson discovered the electrons, which are negatively charged subatomic particles that are smaller than atoms. Atoms are neutral overall, so in Thomson's 'plum pudding model':

  • atoms are spheres of positive charge
  • electrons are dotted around inside
A large plus symbol is displayed in a circle to represent the area of positive charge. Lots of smaller circles with minus symbols are scattered around to represent negatively charged particles.

Rutherford's model and the Geiger-Marsden experiment (1909 - 1911)

Hans Geiger and Ernest Marsden tested the plum pudding model. They aimed beams of positively charged alpha particles at very thin gold foil. According to the plum pudding model, these particles should have passed straight through. However, many of them changed direction instead.

Alpha raditaion beamed through gold nuclei, with most passing through unaffected, some deflected at small angle and very few rays being almost reflected back.

Ernest Rutherford explained these results in his 'planetary model':

  • atoms have a central, positively charged nucleus with most of the mass
  • electrons orbit the nucleus, like planets around a star

Bohr's model (1913)

There is a problem with Rutherford's model - the electrons would eventually fall into the nucleus because they are negatively charged and so attracted to the positive nucleus. Niels Bohr improved Rutherford's model. Using mathematical ideas, he showed that electrons occupy shells or energy levels around the nucleus.

How big is an atom?

A molecule consists of two or more atoms chemically joined together. The typical size of an atom or simple molecule is: 1 × 10 -10 m (0.1 nm or 0.000,000,000,1 m).


A steel pin head is about 1.5 mm in diameter. How many iron atoms, 3.0 × 10-10 m in diameter, will fit across the pin head?

1.5 mm = 1.5 × 10-3 m

Number of iron atoms = (1.5 × 10-3) ÷ (3.0 × 10-10)

= 5,000,000

  • an atom consists of a positively charged nucleus surrounded by negatively charged electrons
  • the radius of the nucleus is much smaller than the radius of the atom
  • almost all of the mass is in the nucleus