2.1.1 - The Atom & Models Of The Atom

• Atoms are made up of protons, neutrons and electrons (subatomic particles)
• electrons have a relative charge of -1
• protons have a relative charge of +1
• neutrons have a relative charge of 0
• electrons move around the nucleus in orbitals
• orbitals take up most of the volume of the atom
• the nucleus is made up of protons and neutrons
• the nucleus takes up most of the mass of the atom
• the relative mass of protons and neutrons is 1
• the relative mass of electrons is 1/2000
• the mass number of an element is the total number of protons and neutrons in the nucleus
• the atomic (proton) number is the number of protons in the nucleus
• ions are atoms that have lost or gained electrons to form a charge
• atoms that have lost electrons become positively charged thus is a positive ion
• atoms that have gained electrons gain a negative charge thus is a negative ion
• the number before the charge in an ion’s formula (e.g. Mg2+) tells you how many electrons have been lost or gained

  for Mg2+, the ‘2’ shows that 2 electrons have been lost leaving a positive ion charge
• isotopes are atoms of the same element with the same number of protons but different number of neutrons
• ancient Greeks believed that atoms were indivisible particles that couldn’t be broken down into smaller particles
• John Dalton said that atoms were solid spheres and that different types of sphere made up different elements
• J.J. Thompson created the Plum Pudding model in 1897
• the Plum Pudding model suggested atoms were balls of positive charge (pudding) with negative electrons (plum) embedded in it
• In 1909 Rutherford, Geiger and Marsden created the Nuclear Model of the atom with their gold foil experiment
• the Gold Foil experiment involved alpha particles (positively charged) being fired at a thin sheet of gold foil.

  the Plum Pudding model suggested that all the alpha particles should be slightly deflected but experiments showed that some alpha particles passed straight through and some were deflected backwards suggesting there was a concentration of positive charge in the centre of the atom (nucleus) causing this
• the Bohr Model suggested that

  • electrons can only exist in fixed orbits or shells
  • each shell has a fixed energy
  • when electrons move between shells electromagnetic radiation is emitted or absorbed