The structure of the atom in terms of protons, neutrons and electrons, atomic number and mass number, isotopes, the size and charge of the nucleus, and how the nuclear model replaced the plum pudding model.

What this topic is asking

OCR wants you to describe the structure of the atom, define atomic number and mass number, explain what isotopes are, state the size and charge of the nucleus, and describe how the nuclear model replaced the earlier plum pudding model. This is the start of topic P6.1 of the OCR Gateway Physics A (J249) specification, examined on the Paper 2 or Paper 4 side.

The structure of the atom

A neutral atom can gain or lose electrons to become a charged ion: losing electrons gives a positive ion, and gaining electrons gives a negative ion. The number of protons (the atomic number) never changes for a given element; it is what defines the element.

Atomic number and mass number

Nuclei are written in the form $^{A}_{Z}\text{X}$, where $A$ is the mass number (top) and $Z$ is the atomic number (bottom). For example, $^{23}_{11}\text{Na}$ has $11$ protons, $11$ electrons and $23 - 11 = 12$ neutrons.

Isotopes

For example, carbon has three common isotopes: $^{12}_{6}\text{C}$, $^{13}_{6}\text{C}$ and $^{14}_{6}\text{C}$, all with $6$ protons but with $6$, $7$ and $8$ neutrons. Carbon-14 is unstable and radioactive, which is the basis of carbon dating.

The size and charge of the nucleus

The nucleus is incredibly small compared with the whole atom. The radius of an atom is about $1 \times 10^{-10}\,\text{m}$, while the radius of the nucleus is about $1 \times 10^{-14}\,\text{m}$, so the nucleus is roughly $10\,000$ times smaller than the atom. Yet it contains almost all the mass and all the positive charge. This means an atom is mostly empty space, with the electrons occupying the volume around a dense, positive nucleus.

How the nuclear model developed

The current model further refined this: electrons orbit the nucleus only in fixed energy levels (shells), and can move to a higher level by absorbing electromagnetic radiation or drop to a lower level by emitting it.

Try this

Q1. State what is meant by an isotope. [2 marks]

• Cue. Atoms of the same element (same number of protons) with different numbers of neutrons (different mass numbers).

Q2. An atom is written $^{16}_{8}\text{O}$. State its number of neutrons. [1 mark]

• Cue. Neutrons $= 16 - 8 = 8$.