The Atom

• An atom is the smallest part of an element that can exist.
• Dalton's model of the atom was a solid sphere that could not be divided into smaller parts.
• The plum pudding model is a sphere of positive charge with negative electrons embedded in it.
• In the alpha scattering experiment, some alpha particles were deflected by the gold foil which shows that an atom's mass and positive charge must be concentrated in one small space (the nucleus).
• The nuclear model of the atom is a dense nucleus with electrons orbiting it.
• Niels Bohr discovered that electrons orbit in fixed energy levels/shells.
• James Chadwick discovered the uncharged particle called the neutron.
• Protons and neutrons make up the nucleus.
• The relative mass of each sub-atomic particle:

  • Proton: 1
  • Neutron: 1
  • Electron: Very small
• The relative charge of each sub-atomic particle:

  • Proton: +1
  • Neutron: 0
  • Electron: -1
• The amount of protons is the atomic number of the element on the periodic table.
• Mass number - atomic number = neutrons
• Atoms have no overall charge because there are equal numbers of protons and electrons.
• There can be up to 2 electrons in the first shell then up to 8 in the rest of the shells.
• An element is a substance made of one type of atom.
• A compound is a substance made of more than one type of atom chemically joined together.
• A mixture is two or more substances not chemically combined.
• Isotopes are atoms of the same element with the same number of protons but different number of neutrons.
• Filtration, crystallisation, simple or fractional distillation and chromatography are the four physical processes used to separate mixtures.
• Relative mass is the average mass of all the atoms of an element.