Dalton's ideas about the atom are summed up in his Atomic Theory:
1. All matter is made of tiny indivisible and indestructible particles called atoms.
2. All atoms of a given element are identical in size, mass and properties.
3. Atoms combine in simple whole-number ratios to form compounds.
4. Atoms are rearranged in chemical reactions but are not destroyed.
Dalton's ideas were based on there being no knowledge of atomic structure in his day. He envisioned atoms as hard spheres that were the same throughout.
Two parts of his atomic theory that were contradicted by later discoveries are:
- Atoms of a given element are identical - we now know that isotopes of elements exist. These are atoms of the same element that have different masses.
- Atoms are indivisible - we now know that atoms are made of smaller particles: protons, electrons and neutrons. However, an atom is the smallest particle of a given element that has the characteristics of that element.