The Nucleus

The nucleus is the tiny, dense centre of an atom. It holds almost all the atom’s mass and is positively charged. Experiments with alpha particles bouncing off thin metal showed that most particles passed straight through (so atoms are mostly empty space), but a few were deflected strongly (showing a small, heavy, positive nucleus).

What is inside the nucleus?

• Protons: positively charged particles (charge +1).
• Neutrons: no charge (0), but similar mass to protons.
• Electrons are not in the nucleus; they move around it and have charge −1.
• Almost all the atom’s mass is in the nucleus; the rest of the atom is mostly empty space.

Key numbers and notation

Atoms are written in nuclide notation, for example: $^{14}_{6}\text{C}$.

• Proton (atomic) number $Z$: number of protons. This decides the element.
• Nucleon (mass) number $A$: total number of protons + neutrons.
• Neutron number $N$: $$N = A - Z$$
• Relative charge of a nucleus: $$+Ze$$
• Relative mass of a nucleus is about $A$ (in atomic mass units).
• Isotopes: same $Z$, different $A$ (so different numbers of neutrons).

Why we believe in the nucleus

• Most alpha particles went straight through thin metal: atoms are mostly empty space.
• A few were deflected or bounced back: a tiny, dense, positive nucleus holds most of the mass.

Common misconceptions

• Neutrons have mass even though they have no charge.
• Electrons are not inside the nucleus.
• Ions form by gaining or losing electrons, not by changing the nucleus.
• Isotopes are the same element (same $Z$), not different elements.

Real-world connections

• Medical tracers use isotopes (same $Z$, different $A$).
• Smoke detectors and nuclear energy involve changes in nuclei, not electron orbits.