The Atom and Atomic ModelsAQA GCSE Physics: Revision notes

An atom has a tiny, positively charged nucleus made of protons and neutrons, surrounded by negatively charged electrons. The radius of the nucleus is less than 1/10,000 of the radius of the atom - almost all of an atom's volume is empty space occupied by the electrons.

Electrons are arranged at different energy levels (shells) around the nucleus. When an atom absorbs electromagnetic radiation, an electron can move to a higher energy level; when an electron moves to a lower energy level, electromagnetic radiation is emitted.

The number of electrons in an atom always equals the number of protons, so atoms have no overall electrical charge.

• Atomic number = number of protons in an atom
• Mass number = total number of protons and neutrons

Isotopes are atoms of the same element with different numbers of neutrons (same atomic number, different mass number). If an atom loses one or more outer electrons, it becomes a positively charged ion.

The model of the atom has changed over time as new evidence emerged:

1. Indivisible sphere - the earliest model; atoms thought to be solid spheres with no internal structure
2. Plum pudding model - after the discovery of the electron, atoms seen as a ball of positive charge with negative electrons embedded in it
3. Nuclear model - the alpha particle scattering experiment showed most of an atom's mass is concentrated in a tiny, charged nucleus, with the rest mostly empty space
4. Bohr model - electrons proposed to orbit the nucleus at fixed, specific distances (energy levels)
5. Current model - includes protons and neutrons in the nucleus, with electrons in defined energy levels

The scattering experiment showed that most alpha particles passed straight through the gold foil, but a small number were deflected back - this could only be explained if the atom's mass and positive charge were concentrated in a small, dense nucleus, disproving the plum pudding model.