Atoms and the nucleus
An atom has a tiny, dense, positively charged nucleus made of protons and neutrons, surrounded by electrons; atoms of the same element with different neutron counts are called isotopes.
Practise this conceptNearly all of an atom's mass is packed into a nucleus so small that if an atom were the size of a football stadium, the nucleus would be roughly a pea at the centre — everything else is mostly empty space occupied by the comparatively feather-light electrons.
That mass imbalance is the opposite of what intuition suggests. Electrons surround the whole atom and are what most of chemistry cares about, but a proton or neutron is nearly 2,000 times more massive than an electron, so the nucleus dominates an atom's mass almost completely despite being a tiny fraction of its volume.
Every atom of a given element has the same number of protons, which is what defines the element in the first place, but the number of neutrons can vary. Atoms of the same element with different neutron counts are called isotopes — carbon-12 and carbon-14 are both carbon, both with six protons, differing only in how many neutrons ride along in the nucleus.
Key ideas
Requires: Physical quantities and units
Unlocks: Quantised energy levels in atoms, Radioactivity and half-life
Common misconceptions
- Most of an atom's mass comes from its electrons, since they surround the entire atom.
- Protons and electrons have the same mass.
- Isotopes of an element have a different number of protons.