The Periodic Table Explained: KS3 Chemistry Guide

The periodic table is a systematic arrangement of all known chemical elements, ordered by atomic number, in which elements with similar chemical properties appear in the same vertical column (group). It was first published by Dmitri Mendeleev in 1869. Understanding how it is organised is a key KS3 chemistry skill covered in Year 8 and Year 9.

How is the periodic table organised?

Every element in the periodic table is arranged by its atomic number — the number of protons in the nucleus of one atom of that element. As you read left to right and top to bottom across the table, atomic number increases by one at each step.

Two structural features organise the table:

Periods — the horizontal rows, numbered 1 to 7. All elements in the same period have the same number of electron shells. Period 1 has 1 electron shell (hydrogen and helium); Period 2 has 2 shells; Period 3 has 3 shells, and so on.

Groups — the vertical columns, numbered 1 to 7 (and group 0 for the noble gases, sometimes called Group 8). Elements in the same group have the same number of electrons in their outermost shell, which gives them similar chemical properties.

Key table

Feature | What it tells you
Atomic number (top) | Number of protons (= electrons in a neutral atom)
Symbol | Chemical shorthand for the element (e.g., Na for sodium)
Relative atomic mass (bottom) | Average mass of one atom relative to carbon-12
Period (row) | Number of electron shells
Group (column) | Number of outer-shell electrons (for groups 1–7)

Key groups to know at KS3

Group 1 — Alkali metals (Li, Na, K, Rb, Cs, Fr)

The alkali metals are soft, silvery-grey metals that are stored under oil because they react vigorously with water and air. When they react with water, they produce a metal hydroxide and hydrogen gas:

metal + water → metal hydroxide + hydrogen

For example:

sodium + water → sodium hydroxide + hydrogen 2Na + 2H₂O → 2NaOH + H₂

Reactivity increases down the group. Lithium reacts gently; sodium reacts rapidly (fizzing and moving on the water surface); potassium reacts so vigorously it ignites the hydrogen, producing a lilac flame. This is because atoms get larger down the group — the outermost electron is further from the nucleus and less strongly attracted to it, so it is given up more easily.

All Group 1 metals form ions with a 1+ charge (e.g., Na⁺, K⁺).

Group 7 — Halogens (F, Cl, Br, I, At)

The halogens are non-metals that form coloured diatomic molecules (e.g., Cl₂, Br₂). Their appearance at room temperature changes down the group:

Halogen | Symbol | State at room temperature | Colour
Fluorine | F | Gas | Pale yellow
Chlorine | Cl | Gas | Yellow-green
Bromine | Br | Liquid | Orange-red
Iodine | I | Solid | Grey/purple-black

Reactivity decreases down the group. A more reactive halogen can displace a less reactive halogen from a solution of its salt. For example, chlorine water added to potassium bromide solution displaces bromine, turning the solution orange:

Cl₂(aq) + 2KBr(aq) → 2KCl(aq) + Br₂(aq)

This is called a displacement reaction and is used as evidence of reactivity order.

All halogens form ions with a 1− charge (e.g., Cl⁻, Br⁻).

Group 0 — Noble gases (He, Ne, Ar, Kr, Xe, Rn)

The noble gases are colourless, odourless gases at room temperature. They are extremely unreactive because their outermost electron shells are full (helium has 2; all others have 8), giving them no tendency to gain or lose electrons. This is why they were not discovered until the 1890s — they form virtually no compounds. Uses include: helium in balloons (low density, non-flammable), argon in electric light bulbs (inert atmosphere prevents the filament reacting), neon in advertising signs.

Metals, non-metals and metalloids

The periodic table is divided by a stepped line (sometimes called the "staircase") running from below boron (B) to between polonium (Po) and astatine (At):

Metals (left of the line) — good conductors of heat and electricity, shiny, malleable (can be hammered into shape), ductile (can be drawn into wire). Most elements are metals.

Non-metals (right of the line) — generally poor conductors, dull, brittle as solids. Include oxygen, nitrogen, carbon, sulphur and the halogens.

Metalloids/semi-metals (along the staircase, e.g., silicon, germanium) — have properties intermediate between metals and non-metals. Silicon is a semiconductor used in computer chips.

Trends in the periodic table

At KS3, two main trends are required:

Reactivity in Group 1 increases going down the group (easier to lose outer electron as atom size increases).

Reactivity in Group 7 decreases going down the group (harder to gain an electron as the outer shell gets further from the nucleus).

At GCSE these trends are explained using electronic structure in more detail, but understanding the direction of the trend and a simple reason is sufficient for KS3.

Dmitri Mendeleev and the history of the periodic table

Mendeleev published his first periodic table in 1869, arranging 63 known elements by atomic mass and grouping elements with similar properties in the same columns. Crucially, he left gaps for elements he predicted had not yet been discovered — for example, he predicted the existence and properties of gallium (which he called "eka-aluminium") and germanium ("eka-silicon"). When these elements were discovered in 1875 and 1886 respectively, their measured properties closely matched Mendeleev's predictions, which was powerful evidence that his table was correct.

The modern table arranges elements by atomic number (proton number) rather than atomic mass. The change was needed because a few elements (such as tellurium and iodine) are in the wrong order if sorted by atomic mass alone but fall correctly into their groups when sorted by atomic number.

The Department for Education's Science Programmes of Study for Key Stage 3 requires pupils to be taught about the periodic table as a way of classifying elements, trends within groups, and the properties of alkali metals, halogens and noble gases.

BBC Bitesize KS3 Chemistry covers the periodic table, groups and periods, the alkali metals, halogens and noble gases as key Year 8 and Year 9 content (see bbc.co.uk/bitesize/subjects/zrkw2hv). The Royal Society of Chemistry's interactive periodic table provides element data and history at rsc.org/periodic-table.

Frequently asked questions

What is the difference between a group and a period in the periodic table?

A group is a vertical column of elements; a period is a horizontal row. Elements in the same group have the same number of outer-shell electrons and share similar chemical properties. Elements in the same period have the same number of electron shells. For example, lithium (Li), sodium (Na) and potassium (K) are all in Group 1 — they all have one outer-shell electron and are all reactive metals that produce hydrogen when added to water.

Why are Group 1 metals stored under oil?

The alkali metals react vigorously with both water (in the air) and oxygen. If left exposed to air, they would react with the moisture in the atmosphere and with oxygen. Storage under oil prevents contact with air and moisture, keeping the metals safe to handle. Even when stored under oil, they must be handled with care — sodium and potassium should only be cut with a dry knife in small quantities, as the freshly cut surface reacts immediately.

What is a displacement reaction?

A displacement reaction occurs when a more reactive element takes the place of a less reactive element in a compound. In the halogens, a more reactive halogen (higher up Group 7) will displace a less reactive one from a solution of its salt. For example, chlorine displaces bromine: Cl₂(aq) + 2KBr(aq) → 2KCl(aq) + Br₂(aq). The solution turns orange-brown as bromine is released. This can be confirmed by adding cyclohexane — bromine dissolves in cyclohexane and turns it orange.

How did Mendeleev predict undiscovered elements?

Mendeleev noticed gaps in his table where no known element fitted the pattern of properties for that position. He used the properties of neighbouring elements in the same group and period to predict the atomic mass, density, melting point and chemical behaviour of the missing element. When gallium was discovered in 1875, its measured density (5.9 g/cm³) almost exactly matched Mendeleev's prediction of 5.9 g/cm³. This predictive power demonstrated that the periodic table reveals genuine underlying patterns in the properties of matter.

Explore the patterns of the periodic table with a Socratic chemistry tutor who builds understanding from particles up — visit aitutors.me.