Atoms, Elements & Compounds
Explain the differences between atoms, elements, and compounds; describe the simple Bohr model of the atom (nucleus with protons and neutrons, electrons in shells); and write and interpret chemical symbols and simple formulae
What a learner can do afterwards
- Defines atom, element, and compound and distinguishes between them with examples
- Draws a simple Bohr model of an atom labelling nucleus (protons/neutrons) and electron shells
- Reads a chemical formula to identify the elements and number of each atom (e.g. H₂O, CO₂, NaCl)
- Identifies the difference between a mixture and a compound
1 · Read
You already know solids, liquids and gases are made of tiny moving particles. Those particles include atoms. An atom is the smallest unit of an element that still acts like that element. Billions of them would fit on the head of a pin.
An element is a substance made of only one kind of atom. Oxygen gas is an element because every atom in it is an oxygen atom. Scientists know about 118 elements, and each one has its own chemical symbol, like O for oxygen or Na for sodium. In a simple Bohr picture, protons and neutrons pack in the centre, called the nucleus, and electrons move around it in shells.
Water is a compound, not an element. Each molecule links two hydrogen atoms to one oxygen atom. If you saw the formula H₂O on a label, you'd know exactly what's inside: H for hydrogen, O for oxygen, and the small 2 showing two hydrogen atoms for every one oxygen atom.
Don't mix up a compound with a mixture. In a compound like water, the atoms are chemically joined and can't be pulled apart by simple means. In a mixture, like a bag of trail mix, the different bits just sit side by side, so you can still pick them apart.
Atoms make up elements, elements can join to form compounds, and a formula like H₂O tells you exactly which atoms, and how many, are inside.
The rewritten readings were written for this dataset.
More to read
From atoms to compounds
An atom is the smallest unit that keeps an element's chemical identity. Its central nucleus contains positive protons and usually neutral neutrons. Negative electrons occupy surrounding shells. An element contains a single type of atom, such as iron, gold, oxygen, or hydrogen. Chemically joining different elements forms a compound. Hydrogen joined with oxygen makes water (H₂O). Sodium joined with chlorine makes salt (NaCl).
Electron shells and chemical formulas
A shell model groups electrons around the nucleus. Shell capacities begin with 2, then 8, then 18. Among the first 20 elements, the third shell fills to 8 before the fourth starts. Bonds involve gaining, losing, or sharing electrons, often reaching a more stable arrangement. Formulas show elements and counts: H₂O has 2 hydrogen atoms per oxygen; CO₂ has 2 oxygen atoms per carbon. NaCl shows one sodium ion for each chlorine ion. Subscripts give counts; no subscript means 1.
Separating mixtures and compounds
A mixture combines substances without chemically joining them. Physical methods can separate its parts. For example, evaporating salt water leaves the salt behind. A compound contains chemically bonded elements, so separating those elements requires a chemical change. Filtering or evaporating water will not recover hydrogen and oxygen. Electrolysis can split water into them. This distinction tells you which kind of separation is needed.
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2 · Watch
En anglaisTake it off screen
Where it sits
Where this leads
Jobs that lean on this skill. Follow one to see everything it is built on.
8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.