Materials and their structure

In Grade 6 you met the particle model — the idea that all matter is built from tiny particles. This year we look more closely at the smallest of those particles: the atom.

An atom is the smallest particle of a substance that still behaves like that substance. Atoms are far too small to see, even with most microscopes — millions would fit across a single grain of sand.

The exciting part is how few kinds of atom there are. There are only about 100 different kinds of atom, yet they build every material in the universe — your body, the oceans, the stars.

Atoms rarely sit alone. They join together in two main ways:

• Identical atoms can join — like two oxygen atoms joining to make an oxygen molecule.
• Different atoms can join — and this builds a brand-new substance called a compound.

Think of atoms like letters of the alphabet. Just 26 letters spell millions of words. In the same way, about 100 kinds of atom build every substance there is. The rest of this guide is about how chemists sort and name them.

An element is a pure substance made of only one kind of atom. Because it contains only one kind of atom, an element cannot be broken down into any simpler substance by chemical means.

Gold is an element — every atom in a gold ring is a gold atom. Oxygen is an element — every particle in pure oxygen gas is built from oxygen atoms. Iron, carbon, copper and hydrogen are all elements too.

There are about 100 elements, and each one has its own chemical symbol — a short code of one or two letters. You will meet symbols properly later in this guide.

Some elements are made of single atoms; others are made of molecules of identical atoms joined together. Either way, only one kind of atom is present, so it is still an element.

The key test: if a substance contains only one kind of atom, it is an element. The moment a second kind of atom appears, you no longer have an element — you have a compound or a mixture.

A compound is a substance made when two or more elements are chemically joined together. Joining happens in a chemical reaction, and the result is a brand-new substance with its own properties.

The most important idea about a compound is that it has new properties — properties that are nothing like the elements that made it:

• Water is a compound of hydrogen and oxygen. Both are gases, yet water is a liquid you can drink.
• Sodium chloride (table salt) is a compound of sodium (a soft, dangerous metal) and chlorine (a poisonous gas) — yet salt is safe to eat.

A compound is made in a chemical reaction, and it is hard to undo — you usually need another chemical reaction to split it back into its elements.

In Grade 6 you learned that a mixture is two or more substances mixed together but not chemically joined. This year you compare a mixture directly with a compound — and the difference is one of the most tested ideas in chemistry.

	Mixture	Compound
Joined?	Not chemically joined	Chemically joined
How made?	Just mixed together	Made in a chemical reaction
Proportions	Any amounts	Fixed amounts
Properties	Each part keeps its own	New properties
Separating	Easy — filtering, evaporating	Hard — needs a reaction

Here is a classic example. If you mix iron filings and sulfur powder, you get a mixture — the iron is still magnetic and a magnet pulls it straight out. But if you heat that mixture, a reaction happens and you get iron sulfide, a compound — a new grey solid that is no longer magnetic and cannot be separated with a magnet.

So the test is simple: ask are the substances chemically joined? If yes, it is a compound. If no, it is a mixture.

With about 100 elements to keep track of, chemists needed a sensible way to organise them. The answer is the periodic table — an organised list of every element.

The table is not random — it is organised by properties:

• Each element has its own box.
• The horizontal rows are called periods.
• The vertical columns are called groups — elements in the same group have similar properties.
• Metals are on the left and in the middle; non-metals are gathered on the right.

Because the table is organised, it lets chemists predict. If you know how one element in a group behaves, you can make a sensible guess about the others.

Chemists write substances using a quick shorthand instead of words.

A chemical symbol is a short code for an element — one or two letters. The first letter is always a capital; the second, if there is one, is always lower case.

• O = oxygen, H = hydrogen, C = carbon, N = nitrogen.
• Na = sodium, Fe = iron, Cl = chlorine, Mg = magnesium.

Some symbols come from older names — Na for sodium comes from the Latin natrium, and Fe for iron from ferrum.

A chemical formula uses symbols to show which atoms are in a compound, and how many of each:

• H2O = water — 2 hydrogen atoms joined to 1 oxygen atom.
• CO2 = carbon dioxide — 1 carbon atom joined to 2 oxygen atoms.
• NaCl = sodium chloride — 1 sodium atom joined to 1 chlorine atom.

The small number after a symbol tells you how many of that atom there are. No number means just one. Reading a formula tells you exactly what a compound is built from — a tiny code packed with information.

Sorting substances into elements, compounds and mixtures sets you up for the rest of chemistry:

• Properties of materials — you will use the periodic table to compare metals and non-metals.
• Chemical reactions — reactions join elements into compounds, exactly as you have seen here.
• Word equations — you will write reactions using the names and symbols from this topic.
• Separating mixtures — knowing the difference between a mixture and a compound tells you whether separation is even possible.

Whenever you meet a new substance, ask: is this an element, a compound or a mixture? That single question is the doorway into understanding what it will do.