Describe the structure of the atom in terms of protons, neutrons and electrons, and use proton number and nucleon number to write nuclide notation and identify isotopes

What this dot point is asking

SEAB wants you to describe how an atom is built from protons, neutrons and electrons, to know where each particle sits, and to use the proton number and the nucleon number to write an atom in nuclide notation and to spot isotopes. The big idea is that all matter is made of atoms, and each atom is mostly empty space with a tiny, dense centre called the nucleus.

The answer

The three particles in an atom

An atom is made of three smaller particles:

• The proton, which has a positive charge ($+$).
• The neutron, which has no charge (it is neutral).
• The electron, which has a negative charge ($-$).

Protons and neutrons are heavy and sit together in the centre. Electrons are very light and move around the outside.

The nucleus and the electrons

The protons and neutrons are packed tightly into the centre of the atom. This centre is called the nucleus. The nucleus is tiny compared with the whole atom, but it holds almost all of the mass.

The electrons move around the nucleus in the space outside it. Most of the atom is empty space. A neutral atom has the same number of electrons as protons, so the positive and negative charges cancel and the atom has no overall charge.

Proton number and nucleon number

Two numbers describe the nucleus:

• The proton number (also called the atomic number), $Z$, is the number of protons in the nucleus. This number decides which element the atom is. Every carbon atom has $6$ protons; every sodium atom has $11$ protons.
• The nucleon number (also called the mass number), $A$, is the total number of protons and neutrons. "Nucleon" is the name for any particle in the nucleus, so a nucleon is a proton or a neutron.

To find the number of neutrons, subtract:

Nuclide notation

We write an atom in a short form called nuclide notation. The symbol of the element is written with the nucleon number on the top left and the proton number on the bottom left:

For example, $^{23}_{11}\text{Na}$ is a sodium atom with nucleon number $23$ and proton number $11$. It has $11$ protons, $11$ electrons, and $23 - 11 = 12$ neutrons.

Isotopes

Atoms of the same element always have the same number of protons, but they can have different numbers of neutrons. Atoms that have the same proton number but a different nucleon number are called isotopes.

For example, $^{12}_{6}\text{C}$ and $^{14}_{6}\text{C}$ are both carbon because both have $6$ protons. They are isotopes because one has $6$ neutrons and the other has $8$ neutrons. Isotopes behave the same in chemical reactions but can have different nuclear properties, which matters in the next dot points on radioactivity.

Examples in context

Example 1. The same element, different mass. Hydrogen has an isotope called deuterium, $^{2}_{1}\text{H}$, which has one proton and one neutron, while ordinary hydrogen $^{1}_{1}\text{H}$ has one proton and no neutrons. Both are hydrogen because both have one proton, but deuterium is twice as heavy. This is a clear example of isotopes of the same element.

Example 2. Why atoms are neutral. A sodium atom, $^{23}_{11}\text{Na}$, has $11$ protons and $11$ electrons, so the $11$ positive charges and $11$ negative charges cancel exactly. If the atom loses one electron it becomes a charged ion, which is the link to the static electricity dot point where rubbing transfers electrons.

Try this

• Cue. State the charge of a proton, a neutron and an electron. [2 marks] A proton is positive ($+$), a neutron is neutral (no charge), and an electron is negative ($-$).

• Cue. An atom is $^{27}_{13}\text{Al}$. Find its number of protons, neutrons and electrons. [3 marks] Protons $= 13$; neutrons $= 27 - 13 = 14$; electrons $= 13$ because the atom is neutral.

• Cue. Explain why $^{16}_{8}\text{O}$ and $^{18}_{8}\text{O}$ are called isotopes. [2 marks] Both have the same proton number $8$ (so both are oxygen), but they have different nucleon numbers, meaning different numbers of neutrons.