Glossary / History of physics
Atomic nucleus
The tiny, heavy core at the centre of an atom, made of protons and neutrons and carrying a positive charge.
An atom is about a ten-millionth of a millimetre across, and its nucleus is only about a hundred-thousandth of that. Yet almost all of the atom's mass is in the nucleus, with the light electrons spread around it.
In 1911 Ernest Rutherford concluded from the way alpha particles fired at gold foil were scattered that an atom's charge is concentrated in a tiny region at its centre[1]. That paper, however, speaks of a "central charge"; the word nucleus does not appear.
That the nucleus is made of positively charged protons and uncharged neutrons became clear only after James Chadwick discovered the neutron in 1932[2].
Related terms
- Alpha particleA heavy, positively charged particle made of two protons and two neutrons: the same thing as a helium nucleus. It is thrown out by radioactive substances.
- Bohr modelThe 1913 model in which electrons circle only on certain orbits and emit light of definite energy when jumping between them.
- Binding energyThe energy needed to pull something that is held together apart into its pieces. The larger it is, the more tightly the parts are bound.