CCEA GCSE Chemistry

## How the model of the atom changed The idea of the atom has been revised several times as new evidence arrived. In the plum pudding model, an atom was a ball of positive charge with negative electrons embedded in it, rather like fruit in a pudding. Rutherford's model replaced this after experiments showed that most of an atom's mass and all of its positive charge are packed into a tiny central nucleus, with the electrons orbiting around it at a distance and the rest of the atom mostly empty space. That model could not account for all of an atom's mass, because protons alone were too light. Chadwick's discovery of the neutron, a particle in the nucleus with no charge, filled the gap and led to the model used today. ## Particles in the atom An atom has a central nucleus containing protons and neutrons, which together hold almost all of its mass, and electrons that occupy shells around the nucleus. | Particle | Relative charge | Relative mass | |---|---|---| | Proton | +1 | 1 | | Neutron | 0 | 1 | | Electron | -1 | about $\frac{1}{1840}$ (very small) | An atom has no overall charge because it has equal numbers of protons and electrons. The **atomic number** is the number of protons in an atom, and it identifies the element. The **mass number** is the total number of protons and neutrons. The number of neutrons is therefore the mass number minus the atomic number. Atoms are very small, with a radius of about 0.1 nm, which is $1 \times 10^{-10}$ m. The nucleus is less than one…