The sub-atomic particles and their relative masses and charges, the nucleus and electrons, atomic number and mass number, isotopes, relative atomic mass, and the size and scale of atoms.

What this topic is asking

OCR wants you to describe the structure of the atom: the nucleus of protons and neutrons surrounded by electrons, the relative masses and charges of these particles, and what the atomic number and mass number tell you. You also need to define isotopes and calculate relative atomic mass from isotopic abundances, and have a sense of the tiny size of atoms.

The sub-atomic particles

The relative masses and charges are:

• Proton: relative mass $1$, relative charge $+1$, found in the nucleus.
• Neutron: relative mass $1$, relative charge $0$ (neutral), found in the nucleus.
• Electron: relative mass very small (about $\frac{1}{1836}$, taken as negligible), relative charge $-1$, found in shells around the nucleus.

Almost all the mass of an atom is concentrated in the nucleus, because electrons have negligible mass. An atom is electrically neutral overall because it has equal numbers of protons and electrons, so the positive and negative charges cancel.

Atomic number and mass number

For example, an atom of sodium has atomic number $11$ and mass number $23$. It therefore has $11$ protons, $11$ electrons and $23 - 11 = 12$ neutrons. The atomic number is written at the bottom left and the mass number at the top left of the chemical symbol.

Isotopes

Because isotopes have the same number of protons and electrons, they have identical chemical properties (chemistry depends on the electrons). They differ slightly in physical properties such as density and rate of diffusion because they have different masses. For example, chlorine-35 and chlorine-37 both have $17$ protons and $17$ electrons, but chlorine-35 has $18$ neutrons while chlorine-37 has $20$ neutrons.

Relative atomic mass

The relative atomic mass ($A_r$) of an element is the weighted mean mass of its atoms, taking into account the masses of all the isotopes and how common each one is (its abundance). The formula is:

Because it is an average, the relative atomic mass is usually not a whole number (chlorine is $35.5$, not $35$ or $37$). Relative masses are compared to carbon-12, so they have no units.

The size of atoms

Atoms are extremely small, with radii of about $0.1$ nanometres ($1 \times 10^{-10}\ \text{m}$). The nucleus is far smaller still, around $1 \times 10^{-14}\ \text{m}$ across, so most of the atom is empty space through which the electrons move. If an atom were the size of a sports stadium, the nucleus would be about the size of a pea at its centre.