What is the periodic table? GCSE Chemistry

The periodic table is the chart that organises every known chemical element in order of increasing atomic number. Elements with similar chemical properties end up in the same vertical column, called a group, and the horizontal rows are called periods. This pattern was first published by Dmitri Mendeleev in 1869 and is still the single most important reference tool in chemistry.

This guide covers how the periodic table is structured, what groups and periods tell you, the headline groups you need to know for GCSE Chemistry (1, 7 and 0), and the trends examiners reward you for spotting.

The periodic table is arranged in order of increasing atomic number from left to right and top to bottom. Each box contains one element, labelled with its chemical symbol (e.g. Na for sodium), its atomic number, and its relative atomic mass. The shape of the table comes from how electrons fill the energy levels around the atom.

There are 118 confirmed elements at GCSE level. The first 20 are the ones you are most likely to be tested on by name, and you should know their symbols, group and period without looking.

A group is a vertical column in the periodic table. Elements in the same group have the same number of electrons in their outer shell, which is why they react in similar ways. The group number (1 to 7, plus 0) tells you the number of outer electrons directly.

Group 1 elements have 1 outer electron, group 7 have 7, and group 0 (the noble gases) have a full outer shell. The transition metals sit in the central block and do not follow this neat rule, which is why GCSE tends to focus on groups 1, 7 and 0.

A period is a horizontal row in the periodic table. The period number tells you how many electron shells the atoms in that row have. Period 1 has 2 elements (hydrogen and helium), period 2 has 8, and period 3 has 8 again.

Across a period, you move from highly reactive metals on the left to highly reactive non-metals on the right, ending at the unreactive noble gases. This shift from metal to non-metal is a key trend examiners ask about.

Group 1 contains lithium, sodium, potassium, rubidium, caesium and francium. They are soft metals that can be cut with a knife, and they all react vigorously with water to produce hydrogen gas and an alkaline metal hydroxide.

Reactivity increases as you go down the group. This is because the outer electron is further from the nucleus, less strongly held, and easier to lose. Caesium reacts explosively with water, while lithium fizzes gently.

Group 7 contains fluorine, chlorine, bromine, iodine and astatine. They exist as diatomic molecules (Cl₂, Br₂, I₂) and form salts when they react with metals. The state at room temperature changes down the group: Fluorine and chlorine are gases, bromine is a liquid, iodine is a solid.

Reactivity decreases as you go down the group. This is the opposite of group 1, because halogens react by gaining an electron, and the further the outer shell is from the nucleus, the harder that electron is to attract.

Group 0 contains helium, neon, argon, krypton, xenon and radon. They are all colourless, odourless and almost completely unreactive. The reason is that they already have a full outer shell of electrons, so they do not need to gain, lose or share electrons to be stable.

Noble gases have very low boiling points and are used in lighting (neon signs), as inert atmospheres for welding (argon), and in balloons (helium). Their unreactivity is the main thing to remember for the exam.

The transition metals fill the wide central block between groups 2 and 3. Common examples include iron, copper, zinc, nickel and chromium. They differ from group 1 metals in four key ways that examiners often ask about.

They are harder and denser, have much higher melting points, often form coloured compounds (like blue copper sulfate), and many are useful as catalysts (iron is used in the Haber process). Their compounds can also have more than one charge, so iron exists as Fe²⁺ and Fe³⁺.

Two big trends examiners reward you for stating clearly. First, metallic character decreases from left to right across a period: You start with reactive metals and end with reactive non-metals. Second, reactivity changes as you move down each group, but the direction depends on the group: Group 1 metals get more reactive as you go down, while Group 7 non-metals get less reactive as you go down.