Materials and their structure

Last year you treated the atom as the smallest building block. This year you discover something surprising: the atom itself is built from three even smaller particles, called subatomic particles.

These three particles are the proton, the neutron and the electron. They are not scattered randomly — they sit in a very particular arrangement.

• Protons and neutrons are packed together in a tiny central core called the nucleus.
• Electrons move around the nucleus in layers called shells.

The nucleus is incredibly small compared with the whole atom. If an atom were the size of a sports stadium, the nucleus would be a marble in the centre — almost all of an atom is empty space.

The three subatomic particles are not the same. Each one has its own electric charge and its own mass.

Particle	Charge	Where it is	Mass
Proton	Positive (+)	In the nucleus	Heavy
Neutron	None (neutral)	In the nucleus	Heavy
Electron	Negative (−)	In the shells	Almost nothing

Here is the key idea. In any atom, the number of protons equals the number of electrons. The positive charges and the negative charges balance exactly, so the atom as a whole has no overall charge — we say it is neutral.

For example, a carbon atom has 6 protons and 6 electrons. Six positives cancel six negatives, leaving zero. That balance is true for every neutral atom.

Almost all of an atom's mass is in the nucleus, because protons and neutrons are heavy while electrons weigh almost nothing. So if you want to know how heavy an atom is, you count the protons and neutrons — the electrons barely add anything at all.

Electrons do not crowd together randomly — they fill shells in a strict order, from the inside out.

The rule is simple: the first shell holds up to 2 electrons, and the next shells hold up to 8 each. A shell only starts filling once the shell inside it is full.

So for a sodium atom with 11 electrons: 2 go in the first shell, 8 fill the second, and the last 1 goes into the third. We write this electron arrangement as 2, 8, 1.

• Oxygen has 8 electrons → 2, 6.
• Magnesium has 12 electrons → 2, 8, 2.
• Argon has 18 electrons → 2, 8, 8.

The outer shell — the one furthest from the nucleus — is the most important. It is the outer electrons that decide how an atom behaves and how it joins to other atoms.

An atom with a full outer shell is very stable and unreactive. An atom with a nearly full or nearly empty outer shell is reactive, because it can easily gain, lose or share electrons to become stable. Keep that idea in mind — it explains the whole periodic table.

In Grade 7 you met the periodic table as an organised list. This year you discover why it has the shape it does — it mirrors the structure of the atom.

• The vertical columns are called groups. Elements in the same group have the same number of electrons in their outer shell — that is why they behave so alike.
• The horizontal rows are called periods. As you move along a period, each atom gains one more electron, and the outer shell gradually fills up.

This is the powerful part: the group number tells you the outer electrons. Group 1 elements have 1 outer electron; Group 2 have 2; Group 7 have 7. The unreactive Group 8 (the noble gases) have a full outer shell.

Because the table follows atomic structure, you can read an element's behaviour straight from its position.

The periodic table also sorts elements into two great families: metals and non-metals. A step-like line runs down the right-hand side, with metals on one side and non-metals on the other.

• Metals fill the left and centre of the table. Most elements are metals. They are usually shiny, good conductors of heat and electricity, strong and bendy (malleable). Iron, copper, sodium and gold are metals.
• Non-metals are gathered in the top right. They are usually dull, poor conductors, and brittle if solid. Oxygen, carbon, sulfur and chlorine are non-metals.

There is a link to electron arrangement. Metal atoms tend to have few outer electrons (1, 2 or 3) and lose them easily. Non-metal atoms tend to have many outer electrons (5, 6 or 7) and gain or share them.

This split is one of the most useful tools in chemistry. The moment you know whether an element is a metal or a non-metal, you can already predict a great deal about how it will look, conduct and react — before you have done a single experiment.

Why do elements join together at all? The answer is the outer shell. Atoms react in order to gain a full, stable outer shell — and they do that by losing, gaining or sharing electrons.

• When a metal meets a non-metal, the metal atom loses its few outer electrons and the non-metal gains them. The result is a compound held together by the attraction of opposite charges.
• When two non-metals meet, neither wants to lose electrons, so they share them instead, joining their atoms together.

Either way, atoms of different elements become chemically joined — and that is the definition of a compound you met last year.

The proportions are fixed. Because joining depends on outer electrons, sodium and chlorine always combine in a 1-to-1 ratio to make sodium chloride (NaCl), and hydrogen and oxygen always combine in a 2-to-1 ratio to make water (H2O).

So the hidden structure of the atom — those outer electrons — quietly controls which elements join, how they join, and in what amounts. Atomic structure is the key that unlocks all of it.

Understanding the inside of the atom makes the rest of chemistry far clearer:

• Reactivity of metals — how easily a metal reacts depends on how easily it loses its outer electrons.
• Acids, alkalis and salts — these all involve charged particles formed when atoms gain or lose electrons.
• Chemical reactions — every reaction is atoms rearranging to reach more stable outer shells.
• Predicting behaviour — once you can read the periodic table, you can predict properties of elements you have never even met.

Whenever a chemistry idea feels puzzling later, come back to the outer shell. Take your time with atomic structure now — it is the single most powerful idea in the whole subject.