Lesson 1 of 4 · Grade 8 Integrated Science
Atoms, elements and compounds
18 min
The atom
Everything is made of tiny particles called atoms. An atom has:
- A nucleus at the centre containing protons (positive charge) and neutrons (no charge).
- Electrons (negative charge) moving around the nucleus in energy levels (shells).
The atomic number is the number of protons. In a neutral atom, protons = electrons.
Elements
An element is made of one kind of atom. Each has a symbol:
- Hydrogen H, Oxygen O, Carbon C, Nitrogen N, Sodium Na, Iron Fe, Copper Cu, Calcium Ca, Chlorine Cl.
Compounds
A compound forms when two or more elements are chemically combined in fixed proportions.
- Water: H₂O (hydrogen + oxygen)
- Carbon dioxide: CO₂
- Sodium chloride (common salt): NaCl
A compound has properties different from its elements: sodium is a reactive metal and chlorine a poisonous gas, but sodium chloride is table salt.
Mixtures vs compounds
Mixture components are not chemically joined and can be separated physically; compounds can only be separated chemically.
Key points to remember
- Protons (+) and neutrons in the nucleus; electrons (-) around it
- Atomic number = number of protons
- Fe is iron; Na is sodium
- H₂O and NaCl are compounds
Test yourself
