What Are Atoms?
Everything in the universe is made of incredibly tiny particles called atoms. An atom is the smallest unit of an element that still has the chemical properties of that element. Atoms are so small that millions of them could fit on the full stop at the end of this sentence.
Each atom has a central part called the nucleus, which contains positively charged particles called protons and neutral particles called neutrons. Surrounding the nucleus are negatively charged particles called electrons, which move in regions called electron shells or orbitals. The number of protons in an atom determines which element it is — for example, every atom with 6 protons is a carbon atom.
If an atom were the size of a football stadium, the nucleus would be about the size of a marble at the centre, with the electrons buzzing around the outer stands. Most of an atom is empty space!
What Are Molecules?
A molecule forms when two or more atoms join together by sharing or transferring electrons. This joining is called a chemical bond. Molecules can be made of atoms of the same element — for example, oxygen gas (O₂) is two oxygen atoms bonded together. Or they can contain different elements — water (H₂O) is made of two hydrogen atoms and one oxygen atom bonded together.
The formula of a molecule tells you which atoms it contains and how many of each. For example, carbon dioxide (CO₂) contains one carbon atom and two oxygen atoms. These short formulas are a universal language — scientists all over the world use the same chemical symbols.
Why Do Atoms Bond?
Atoms bond because it makes them more stable and lowers their energy. Most atoms "want" to have a full outer electron shell. By sharing or donating electrons with other atoms, they can achieve this stable state. This drive for stability is what holds molecules — and in turn, all the materials around us — together.
Salt (NaCl) forms when sodium gives an electron to chlorine. Water (H₂O) forms when hydrogen and oxygen share electrons. These bonds explain the properties of the substances they create — why water is liquid at room temperature, why salt dissolves easily, and much more.
Use models or drawings to picture molecules. Building a 3D model of water (H₂O) or carbon dioxide (CO₂) with modelling clay can make the shapes of molecules much easier to remember.
Atoms, Molecules and Matter
All the matter you can see, touch, and feel is made of atoms and molecules. A glass of water contains an almost incomprehensible number of water molecules — roughly 1.67 × 10²⁵, which is 16,700,000,000,000,000,000,000,000 molecules. Yet each one behaves according to the same chemical rules.
The properties of a substance — its colour, hardness, melting point, whether it conducts electricity — all depend on which atoms it contains and how they are bonded. Chemistry is largely the science of understanding and controlling these atomic interactions.
Be careful not to confuse atoms with molecules. An atom is a single particle of one element. A molecule is formed when two or more atoms are bonded together. Oxygen (O) is an atom; oxygen gas (O₂) is a molecule.
Real world: materials science — diamond vs graphite
Both diamond and graphite are made entirely of carbon atoms, yet they have completely different properties. In diamond, each carbon bonds to four others in a tetrahedral lattice (hard, transparent); in graphite, carbon bonds in flat sheets that slide over each other (soft, dark, conducts electricity). Same atoms, different molecular structure — different material. This shows how atomic bonding governs material properties.
Real world: pharmaceutical drug molecules
Ibuprofen has the molecular formula C₁₃H₁₈O₂. Its specific arrangement of atoms — including an acidic group (COOH) — allows it to block enzymes responsible for inflammation and pain. Changing even one atom in the molecule can make it ineffective or toxic. Pharmaceutical chemists design drugs by modelling atomic structures.
Real world: cooking chemistry — why eggs turn solid
Raw egg white is liquid because albumin protein molecules (long chains of amino acids) are loosely coiled. When heated, thermal energy breaks the weak bonds holding the coils together. The unfolded proteins then bond with each other, forming a solid network. A chemical reaction at the molecular level — rearrangement of bonds — makes a cooked egg solid and irreversible.
Practice Problems
What particles are found in the nucleus of an atom?
Show answer
What is the chemical formula for water, and what does it tell you?