Elements are the building blocks

An element is a substance made of only one type of atom. It cannot be broken down into anything simpler by a chemical reaction.

• There are about 118 known elements — from hydrogen to the ones made in laboratories.
• Each element has a one- or two-letter chemical symbol, e.g. O (oxygen), Na (sodium), Fe (iron).
• Everything around you is built from these elements, joined together in different ways.

Symbols matter: the first letter is always a capital, the second (if there is one) is lower case. Co is cobalt, but CO means carbon + oxygen joined together — a totally different thing!

Elements have different properties

Every element has its own set of physical and chemical properties:

• Physical properties — melting point, boiling point, density, colour, and whether it conducts electricity.
• Chemical properties — how it reacts with other substances, e.g. with water, oxygen or acids.

These properties vary hugely. Some elements are gases at room temperature, some are liquids, most are solids. Some are shiny and strong; others are dull and brittle.

Key idea: because properties follow patterns, elements with similar properties can be grouped together — which is exactly what the periodic table does.

Spot the element

How Mendeleev built the table

In 1869, the Russian scientist Dmitri Mendeleev arranged the known elements in order of their atomic mass and put ones with similar properties in the same group.

• He left gaps where he thought an element was still undiscovered.
• He even predicted the properties of those missing elements. Gallium (found 1875) and germanium (1886) later filled gaps and matched his predictions closely.
• Sometimes he swapped the order of two elements so each sat with similar elements, e.g. tellurium and iodine.
• His first table had no noble gases. None had yet been discovered on Earth.

Misconception: a common mistake is thinking Mendeleev's table had "no gaps because he knew every element". In fact the gaps were the clever part — they showed he trusted the pattern more than the incomplete list of elements known at the time. (Today's table is ordered by atomic number, not mass, but the idea of patterns is the same.)

Why the gaps?

Groups and periods

The modern periodic table is a grid. The way you read it is important:

• A group is a vertical column (up and down). Elements in the same group have similar chemical properties.
• A period is a horizontal row (left to right).

Misconception: lots of people mix these up. Remember: gROUP = column and peRIOD = row. Elements side by side in a period are not alike: Period 3 runs from sodium, a reactive metal, to argon, an unreactive gas.

Group ↔ property

Tap a group on the left, then tap the property that matches it on the right.

The big split

Most of the periodic table is metals, found on the left and middle. The non-metals sit on the top-right. A "staircase" line roughly divides them.

Misconception: people often think most elements are non-metals or gases. Actually most elements are metals — the non-metals are a small set in the top-right corner (plus hydrogen).

Metal or non-metal?

Tap an element, then tap the box it belongs in.

Metal oxides and non-metal oxides

Burn an element in oxygen and you make an oxide. Add the oxide to water and test with universal indicator.

• Metal oxides are bases: they neutralise acids. Ones that dissolve make alkaline solutions.
• Many non-metal oxides, such as sulfur dioxide and carbon dioxide, dissolve to make acidic solutions.

Misconception: not all oxides are acidic. Copper oxide does not dissolve, so water stays at pH 7 — but it is still a base, because it neutralises acids.

How could Mendeleev describe elements nobody had found?

How can we predict how astatine reacts?

Group 1 — the alkali metals

Group 1 contains lithium, sodium, potassium and more. They are soft, reactive metals that fizz and react with water, making an alkaline solution and hydrogen gas.

The pattern is clear: as you go DOWN Group 1, the metals get MORE reactive.

Store carefully: alkali metals are so reactive with air and water that they are kept under oil.

Reading the trend

Group 7 — the halogens

Group 7 contains fluorine, chlorine, bromine and iodine. They are reactive non-metals that are coloured and often poisonous.

• Chlorine — a pale green gas.
• Bromine — a red-brown liquid.
• Iodine — a dark grey solid.

The trend here is the opposite way to Group 1: as you go DOWN Group 7, the halogens get LESS reactive.

Halogen trend

Group 0 — the noble gases

Group 0 (on the far right) contains helium, neon, argon and more. They are all colourless gases and are famously unreactive (inert).

• Helium is less dense than air — it makes balloons float.
• Neon glows brightly in advertising signs.
• Argon is used to fill some light bulbs: the hot filament does not react with it.

Key idea: the noble gases hardly react at all. That makes them useful wherever you need a gas that won't react.

The unreactive group

Using patterns to predict

The real power of the periodic table is that it lets you predict how an element behaves before you ever test it.

• Elements in the same group react in similar ways.
• Trends down a group let you estimate reactivity: e.g. rubidium (below potassium in Group 1) will be even more reactive.
• Knowing a group's properties tells you what the whole family is likely to do.

Try it: astatine sits below iodine in Group 7. Because reactivity decreases down that group, we can predict astatine is less reactive than iodine — a prediction made straight from the table. Astatine is so rare and radioactive that this is very hard to test.

Make a prediction

How can we predict how astatine reacts?

Read the symbol

Column or row?

The key ideas to know

You've covered the key ideas in the KS3 periodic table topic — element properties, Mendeleev, groups & periods, metals vs non-metals, oxides, and the reactivity patterns. Press Finish to see your score.