Describe the structure of the atom in terms of protons, neutrons and electrons, work out proton number and nucleon number, and explain how elements are arranged in the periodic table

What this dot point is asking

SEAB wants you to describe what an atom is made of, to work out the numbers of protons, neutrons and electrons from the symbol of an element, and to explain how the periodic table arranges the elements into groups and periods. The central idea is that the structure of an atom, especially its outer electrons, decides how an element behaves.

The answer

Inside the atom

Every atom has a tiny central nucleus surrounded by electrons. The three subatomic particles are:

• protons: positive charge, found in the nucleus,
• neutrons: no charge, found in the nucleus,
• electrons: negative charge, moving around the nucleus in shells.

In a neutral atom the number of protons equals the number of electrons, so the positive and negative charges cancel.

Proton number and nucleon number

Two numbers describe an atom:

• the proton number (atomic number) is the number of protons, which identifies the element,
• the nucleon number (mass number) is the total number of protons and neutrons.

So the number of neutrons is the nucleon number minus the proton number:

Electron shells

Electrons fill shells around the nucleus, starting from the one nearest the nucleus:

• the first shell holds up to $2$ electrons,
• the second shell holds up to $8$ electrons,
• the third shell holds up to $8$ electrons (at this level).

The electron arrangement of sodium ($11$ electrons) is written $2, 8, 1$.

The periodic table

The periodic table arranges all the elements in order of increasing proton number. Its layout is meaningful:

• a group is a vertical column. Elements in the same group have the same number of outer electrons and similar chemical properties.
• a period is a horizontal row. Going across a period, each element has one more electron than the last.

Metals are on the left, non-metals on the right, and the unreactive noble gases are in Group 0 on the far right because they have full outer shells.

Examples in context

Example 1. Why sodium and potassium react similarly. Sodium ($2, 8, 1$) and potassium ($2, 8, 8, 1$) both have one outer electron, so they sit in the same group (Group 1) and react in similar ways, both forming compounds by losing that single outer electron.

Example 2. Using the periodic table to predict a charge. An element in Group 2 has two outer electrons, so it tends to lose them and form an ion with a $2+$ charge. Reading the group number tells you how an element is likely to bond, without memorising every element separately.

Try this

• Cue. An atom is $^{40}_{20}\text{Ca}$. Find its number of neutrons: $40 - 20 = 20$.
• Cue. State what elements in the same group share. The same number of outer-shell electrons and similar chemical properties.
• Cue. Write the electron arrangement of an atom with $12$ electrons: $2, 8, 2$.