Atomic structure

• Atoms consist of a nucleus containing protons and neutrons, surrounded by electrons in shells.
• The numbers of subatomic particles in an atom can be calculated from its atomic number and mass number.
• John Dalton published his ideas about atoms in 1803, imagining them as tiny spheres that could not be divided.
• Nearly 100 years later, J J Thomson carried out experiments and discovered the electron, a subatomic particle with a negative charge and a negligible mass relative to protons and neutrons.
• This led him to suggest the plum pudding model, an early model of the atom in which an atom is a ball of positive charge with negative electrons embedded in it.
• In 1909 Ernest Rutherford designed an experiment to test the plum pudding model, firing positively charged alpha particles at thin gold foil.
• Most alpha particles went straight through the foil, but a few were scattered in different directions, leading Rutherford to suggest a new model for the atom, called the nuclear model.
• In the nuclear model, the mass of an atom is concentrated at its centre, the nucleus, which contains protons and neutrons and has most of the mass of the atom.
• The plural of nucleus is nuclei, and the nucleus is positively charged.