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wise otter.Chemistry
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Bonding and Structure

Topic 1 of 5

Particles and substances

Distinguish atoms, molecules and ions, then classify elements, compounds and mixtures.

O-Level 2026 SEC G3 2027

First, identify the particles

An element can contain molecules. A compound can contain ions.

An atom is a single particle of an element. A molecule is a discrete group of atoms joined by covalent bonds. An ion is a charged particle. These describe particles; element, compound and mixture describe what a substance or sample contains.

Atom

One helium atomA single circle labelled He represents one neutral helium atom. The circle is a particle symbol, not a diagram of its electrons.He
One atom of an element. Helium exists as separate atoms.

Molecule

One water moleculeOne oxygen atom joined to two hydrogen atoms by covalent bonds. The three atoms together form one neutral water molecule. The bent drawing is not to scale.OHH
In water, one oxygen and two hydrogen atoms form a discrete H2O molecule.

Ion

A positive sodium ion and a negative chloride ionSeparate particle symbols labelled Na plus and Cl minus. An ion has a net charge because the numbers of protons and electrons are unequal.Na+Cl-
Charged particles. Na+ has lost an electron; Cl- has gained one.

Particle symbols, not shell diagrams. Sizes and spacing are not to scale.

Read the labels and connections: a molecule contains joined atoms; an ion has an overall charge.
Classifying a sample
SampleWhat makes it different?Example
ElementOnly one type of atom; cannot be chemically broken into simpler substances.O2: pairs of oxygen atoms.
CompoundDifferent elements chemically combined in a fixed proportion; separation requires chemical change.H2O: two hydrogen atoms per oxygen atom.
MixtureSubstances together without being chemically combined; proportions can vary and physical separation is possible.Air: several gases mixed together.
Check your understandingA diagram contains only identical pairs of oxygen atoms. Element, compound or mixture?Think it through, then reveal the answer
An element. Each pair is a molecule, but every atom is oxygen. Two atoms do not necessarily mean two elements.