Nucleus: definition in chemistry
Nucleus: The tiny, dense, positively charged center of an atom, made of protons and neutrons. It holds almost all of the atom's mass.
A nucleus is roughly 10,000 to 100,000 times smaller in diameter than the whole atom, so most of an atom is empty space occupied by electrons. Ernest Rutherford proposed the nucleus in 1911 to explain why a few alpha particles fired at thin gold foil bounced almost straight back. The protons in a nucleus repel each other, but the strong nuclear force, which acts only over very short distances, holds the protons and neutrons together.
Examples
- A hydrogen-1 nucleus is a single proton
- A helium-4 nucleus has 2 protons and 2 neutrons, the same as an alpha particle
- Scaled up to a football stadium, an atom's nucleus would be about pea-sized
- Over 99.9% of an atom's mass is in its nucleus
On the periodic table
The atomic number in each box gives the number of protons in that element's nucleus, and so the size of its positive nuclear charge.