Describe the composition of the nucleus in terms of protons and neutrons

5.1.2 The nucleus

The nucleus is the tiny, dense centre of an atom. Think of it as a bustling city where the main residents are protons and neutrons 🏙️. These two particles give the nucleus its mass and determine what element the atom is.

Composition of the nucleus

A nucleus contains:

• Protons (p⁺) – positively charged particles that decide the element’s identity.
• Neutrons (n⁰) – neutral particles that add mass and help keep the nucleus stable.

The total number of protons is called the atomic number (Z). The sum of protons and neutrons is the mass number (A). For example, a carbon‑12 nucleus has 6 protons and 6 neutrons (A = 12).

How the nucleus is built

1. Choose the element → set the number of protons (Z).
2. Add neutrons to reach the desired mass number (A). Different numbers of neutrons give isotopes of the same element.
3. Combine the protons and neutrons in a tightly packed core. The strong nuclear force holds them together, overcoming the repulsive electric force between the positively charged protons.

Quick examples

• Hydrogen‑1: 1 p⁺, 0 n⁰ → A = 1 (the simplest nucleus).
• Helium‑4: 2 p⁺, 2 n⁰ → A = 4 (a very stable nucleus).
• Carbon‑13: 6 p⁺, 7 n⁰ → A = 13 (an isotope used in radiocarbon dating).

Remember: Protons give the element its identity (the “name” of the city), while neutrons help keep the city stable (they’re like the city’s infrastructure). Together, they form the heart of every atom. 🔬⚛️