Models of the atom

• J J Thomson proposed the plum pudding model of the atom after discovering the electron in 1897.
• The plum pudding model suggests that the atom is a sphere of positive charge, with negatively charged electrons in it.
• The plum pudding model is consistent with the evidence available at the time: solids cannot be squashed, therefore the atoms which make them up must be solid throughout.
• The atomic model consists of a nucleus containing protons and neutrons, surrounded by electrons in shells.
• The numbers of particles in an atom can be calculated from its atomic number and mass number.
• Ancient Greek thinkers believed that all matter was made of a combination of earth, air, fire and water, which they called 'elements'.
• These substances were not the same as modern elements.
• Later Greek thinkers suggested that matter could be made up of invisible particles.
• A particle is a general term for a small piece of matter, for example, protons, neutrons, electrons, atoms, ions or molecules.
• John Dalton carried out a series of experiments and concluded that all matter was made of tiny particles called atoms.
• Dalton suggested that an atom was a tiny solid ball.
• Dalton published his ideas in 1803.
• In 1897, Thomson discovered the electron.
• Dalton’s model included the ideas that atoms cannot be broken down into anything simpler, the atoms of a given element are identical to each other, the atoms of different elements are different from one another, and the atoms of different elements join to make compounds.
• Rutherford's model was later refined by Bohr, who showed that it had limitations.
• According to Rutherford's model, electrons move in fixed orbits, called electron shells, around the nucleus.
• The central part of an atom, the nucleus, is made up of two types of subatomic particles: protons and neutrons, and has most of the mass of the atom.
• The plural of nucleus is nuclei.
• Protons are subatomic particles with a positive charge and a relative mass of 1.
• The relative charge of a proton is +1.
• Neutrons are uncharged subatomic particles, with a mass of 1 relative to a proton.
• The relative charge of a neutron is 0.
• Electrons are subatomic particles with a negative charge and a negligible mass relative to protons and neutrons.
• Atoms of an element have identical numbers of protons and electrons, but can have different numbers of neutrons.
• Atoms of different elements have different numbers of protons and electrons.
• Scientists used Dalton’s atomic model to make predictions.
• Sometimes the results of their experiments were a surprise and they did not fit with the existing model.
• Scientists changed Dalton’s model so that it could explain the new evidence.