Atom
A tiny unit of matter. It has a nucleus containing protons and neutrons, with electrons in the space around it.
1
Zoom in on matter
Matter is anything that has mass and takes up space. Keep zooming in and you eventually reach atoms.
Element
A pure substance made from one kind of atom. Gold, oxygen, carbon, and iron are elements.
Molecule
Two or more atoms bonded together. A water molecule joins two hydrogen atoms with one oxygen atom.
A useful mental model
Atoms are like a construction set
Elements are different kinds of pieces. Chemical bonds connect them. The same pieces can be rearranged to create completely different substances.
But atoms are not literally hard toy bricks—they are astonishingly small systems governed by electric forces and quantum physics.
2H₂ + O₂ → 2H₂O
Hydrogen and oxygen atoms rearrange to form water. The number of each kind of atom stays the same.
2
Your first periodic table
The full periodic table has 118 known elements. It is arranged by atomic number and repeating properties. Tap the first 20 to meet them.
Hydrogen (H)
Atomic number 1. Hydrogen is the most abundant element in the universe and is part of every water molecule.
3
Decode everyday formulas
A chemical formula names the elements and counts the atoms. A small lower number, called a subscript, tells you how many.
4
What happens in a reaction?
During a chemical reaction, atoms rearrange into new combinations. New substances can have new colours, smells, temperatures, or other properties.
Atoms are conserved
Ordinary chemical reactions do not create or destroy atoms. They reorganize them, which is why equations must balance.
Energy can move
Some reactions release energy as heat or light. Others absorb energy from their surroundings.
Safety is science
Never taste chemicals or mix household products. Experiments need adult supervision, eye protection, and clear instructions.
5
States, mixtures, and changes
Chemists carefully distinguish how particles are arranged from what kinds of particles are present.
Solid
Particles stay close to fixed neighbours and vibrate. A solid keeps its own shape and volume.
Liquid
Particles remain close but move past one another. A liquid keeps its volume but takes the shape of its container.
Gas
Particles are far apart and move freely. A gas spreads to fill the available container.
Physical change
Melting, freezing, cutting, and dissolving can change form without creating a different chemical substance.
Chemical change
Atoms rearrange into different substances. Gas production, lasting colour change, or unexpected heating can be clues—but not perfect proof alone.
Mixture
Substances are together without all becoming one new substance. Filtering, evaporation, magnets, or chromatography can separate some mixtures.
6
Acids, bases, and the pH scale
pH describes how acidic or basic a water-based solution is. The scale commonly runs from 0 to 14.
Reading pH
Values below 7 are acidic, 7 is neutral, and values above 7 are basic or alkaline. Each whole pH step represents a tenfold change in hydrogen ion concentration.
Strong and concentrated do not mean the same thing. Chemical strength describes ion behaviour; concentration describes how much is present.
acid + base → salt + water
This is the general pattern of neutralization. Real reactions and products depend on the exact substances and amounts.
7
Chemistry skills and safe investigations
Good laboratory habits make observations more useful and people safer.
Record units, read liquid levels at eye height, and change only one variable when comparing results.
Never use an unlabelled substance. Keep food and drinks away from experiments.
Describe colour, temperature, state, bubbles, or safe wafted odours only when instructions allow it.
An observation is what you detect; an inference is an explanation you build from evidence.
Do not mix household cleaners or unknown chemicals. Even familiar substances can make dangerous products together.
Follow instructions for cleanup. Chemicals should not automatically be poured into a sink or thrown in ordinary rubbish.
8
Quick knowledge check
Make a prediction before revealing each answer.
Are all molecules compounds?
No. O₂ is a molecule made from one element. A compound contains atoms of at least two different elements chemically joined.
Why must a chemical equation balance?
Because ordinary chemical reactions rearrange atoms rather than creating or destroying them.
Is dissolving sugar in water always a chemical reaction?
No. The sugar molecules can remain chemically unchanged and can be recovered by evaporating the water.
Does “natural” automatically mean safe?
No. Safety depends on the substance, dose, exposure, and situation—not whether something is natural or manufactured.