The nuclear model of the atom: protons, neutrons and electrons, the size of the atom and nucleus, the relative masses and charges, and electron energy levels.

What this dot point is asking

Edexcel statements 6.1 to 6.8 want you to describe the atom as a positive nucleus of protons and neutrons surrounded by electrons, to recall the typical sizes of atoms and nuclei, the relative masses and charges of the particles, and that electrons change orbit (energy level) when they absorb or emit electromagnetic radiation.

The structure of the atom

The whole atom is neutral because the number of protons equals the number of electrons, so the positive and negative charges balance. The nucleus is held together despite the repulsion between its protons, and the electrons are held in their orbits by the attraction to the positive nucleus. Almost all the mass is in the nucleus because protons and neutrons are far more massive than electrons.

The sizes of atoms and nuclei

The huge difference in scale is a favourite exam point. If the nucleus were the size of a marble, the atom would be the size of a sports stadium. This emptiness, with a dense central nucleus, was the key discovery of the alpha-scattering experiment that led to the nuclear model replacing the earlier "plum pudding" model.

Relative masses and charges

These relative values are what you use to work out the charge and mass of an atom or ion. Because the electron's mass is negligible, the mass (nucleon) number counts only protons and neutrons. The positron appears later in beta-plus decay and has the same tiny mass as an electron but the opposite charge.

Electron energy levels

This idea links atomic structure to the electromagnetic spectrum. When an electron absorbs energy (for example from a photon) it jumps to a higher level; when it drops back it releases that energy as electromagnetic radiation. The fixed levels mean only certain energies are absorbed or emitted.

How Edexcel examines this

This dot point is examined on both tiers, usually as a "describe the structure of the atom" question (rewarding the nuclear arrangement and the relative charges and masses of all three particles) and a scale question comparing the atom and nucleus radii. Mark schemes expect the relative values learned exactly: proton and neutron mass $1$, electron mass negligible; proton $+1$, neutron $0$, electron $-1$. For the scale question, examiners reward calculating that the nucleus is about $10^4$ times smaller than the atom and concluding that the atom is mostly empty space with its mass in the nucleus, ideas that come from the alpha-scattering evidence. The electron energy-level statement is tested as a short explanation that electrons absorb electromagnetic radiation to move up a level and emit it to move down. A common error is giving the electron a relative mass of $1$, which would wrongly imply electrons contribute to the mass number, so keep the electron mass negligible.

Try this

Q1. State the relative charge of a proton, a neutron and an electron. [2 marks]

• Cue. Proton $+1$, neutron $0$, electron $-1$.

Q2. State where almost all the mass of an atom is located. [1 mark]

• Cue. In the nucleus.