Atoms and isotopes · GCSE Physics

The atom

Atoms are very small, with a radius of about 1 × 10⁻¹⁰ m.
An atom has a 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, but most of the mass of the atom is in the nucleus.
Electrons are arranged at different distances from the nucleus (different energy levels).

Electron arrangements can change when electromagnetic radiation is absorbed or emitted.
If an electron absorbs radiation, it moves further from the nucleus to a higher energy level.
If it emits radiation, it moves closer to the nucleus to a lower energy level.
If an atom loses one or more outer electrons, it becomes a positive ion.

Atoms have no overall charge: the number of electrons equals the number of protons.
All atoms of an element have the same number of protons: the atomic number.
The mass number is the total number of protons and neutrons.
Isotopes are atoms of the same element with different numbers of neutrons, so they have different mass numbers.
Notation: in ²³₁₁Na, 23 is the mass number and 11 is the atomic number, so there are 23 − 11 = 12 neutrons.

The plum pudding model (a ball of positive charge with electrons embedded) was replaced after the alpha scattering experiment, which showed that the mass is concentrated in a small, positive nucleus.
Bohr suggested electrons orbit at specific distances. Later, protons were found in the nucleus, and James Chadwick found evidence for neutrons.
New evidence can lead to a scientific model being changed or replaced.

Nucleus: The tiny, positively charged centre of an atom, containing protons and neutrons.
Proton: A positive particle in the nucleus with relative mass 1.
Neutron: A neutral particle in the nucleus with relative mass 1.
Electron: A negative particle with very small mass, found in energy levels around the nucleus.
Atomic number: The number of protons in an atom.
Mass number: The number of protons plus neutrons.
Isotope: An atom of an element with a different number of neutrons.
Ion: An atom that has lost or gained electrons, so it has a charge.
Energy level: A fixed distance from the nucleus at which electrons are found.