The development of the model of the atom

Early ideas about atoms

  • New experimental evidence may lead to a scientific model being changed or replaced over time. In 1804, John Dalton presented his atomic theory that all matter was made up of tiny hard particles or spheres called atoms that could not be broken up.

  • In 1897, J J Thomson proposed that the atom looked like a plum pudding.

  • J J Thomson discovered that atoms contained even smaller negatively charged particle called electrons which could be removed from atoms.

  • He said that the atoms were spheres of positive charge with a lot of negative electrons stuck in it like fruit in a pudding.

  • In 1909 Rutherford and his assistants had proven the existence of the nucleus.

  • Rutherford performed alpha scattering experiment in which a beam of alpha particles was fired at a gold foil.

  • They realised that most of the mass of the atom must be concentrated at the centre in a tiny nucleus because a few alpha particles were deflected back and the electrons orbiting in paths around the nucleus.

  • In 1913, Niels Bohr adapted Ernest Rutherford's nuclear model. Niels Bohr suggested that the electrons orbited the nucleus in different energy levels or certain distances from the nucleus.

  • In 1932 James Chadwick found evidence for the existence of neutrons within the nucleus.

Structure of the atom

Atoms are very small, having a radius of about 0.1 nm. An atom has a central nucleus. The nucleus is tiny compared to the atom.

Atoms are made up of three particles: protons, neutrons and electrons. In an atom, the number of electrons is equal to the number of protons in the nucleus.

Almost all of the mass of an atom is concentrated in its nucleus. Atoms have no overall electrical charge. The nuclei of all atoms contain subatomic particles called protons and neutrons.

The number of protons in an atom is called the atomic number.

The total number of protons and neutrons in an atom is called its mass number.

The table below gives information about the relative mass and charge of protons, neutrons and electrons.

Particle Relative mass Relative charge
Proton 1 +1
Electron very small -1
Neutron 1 0

Atoms can be represented as shown in this examples.


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

Most elements have one or two stable isotopes and other isotopes are unstable.


An ion is a charged atom. When an atom is attracted to another atom because it has an unequal number of electrons and protons, the atom is called an ION.

Atoms can lose or gain electrons due to collisions or other interactions. When they do this they form charged particles called ions.

  • Positive Ion - happens when an atom loses one or more electrons.

  • Negative Ion - happens when an atom gains one or more electrons.