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• New experimental evidence


  May lead to a scientific model being changed or replaced
• Atoms (before the discovery of the electron)


  Tiny spheres that could not be divided
• Plum pudding model of the atom

  The atom is a ball of positive charge with negative electrons embedded in it
• Alpha particle scattering experiment


  1. Led to the conclusion that the mass of an atom was concentrated at the centre (nucleus) and that the nucleus was charged

  2. Replaced the plum pudding model
• Nuclear model


  The mass of an atom is concentrated at the centre (nucleus) and the nucleus is charged
• Bohr's model


  Electrons orbit the nucleus at specific distances
• Bohr's theoretical calculations


  Agreed with experimental observations
• Proton


  A smaller particle with the same amount of positive charge as the nucleus
• Neutron


  Exists within the nucleus
• The experimental work of James Chadwick provided the evidence to show the existence of neutrons within the nucleus, about 20 years after the nucleus became an accepted scientific idea
• Atom


  The smallest part of an element that can exist
• Chemical symbol


  Represents an atom of an element, e.g. O for oxygen, Na for sodium
• Elements


  • There are about 100 different elements
• Elements are shown in the periodic table
• Compound


  Formed from elements by chemical reactions
• Chemical reactions


  • Always involve the formation of one or more new substances
  • Often involve a detectable energy change
• Compound formula


  Represents the elements and their proportions in a compound
• Compounds can only be separated into elements by chemical reactions
• Word equation


  Represents a chemical reaction using words
• Symbolic equation


  Represents a chemical reaction using symbols and formulae
• Mixture


  Two or more elements or compounds not chemically combined together
• The chemical properties of each substance in a mixture are unchanged
• Separation techniques for mixtures

  • Filtration
  • Crystallisation
  • Simple distillation
  • Fractional distillation
  • Chromatography
• Physical processes used to separate mixtures do not involve chemical reactions and no new substances are made
• In an atom, the number of electrons is equal to the number of protons in the nucleus
• Atoms have no overall electrical charge
• Atomic number


  The number of protons in an atom of an element
• All atoms of a particular element have the same number of protons
• Atoms of different elements have different numbers of protons
• Before the discovery of protons, neutrons and electrons, scientists attempted to classify the elements by arranging them in order of their atomic weights
• The early periodic tables were incomplete and some elements were placed in inappropriate groups if the strict order of atomic weights was followed
• Mendeleev overcame some of the problems

  1. Leaving gaps for elements that he thought had not been discovered

  2. Changing the order based on atomic weights in some places
• Elements with properties predicted by Mendeleev were discovered and filled the gaps
• Knowledge of isotopes made it possible to explain why the order based on atomic weights was not always correct
• Noble gases


  Elements in Group 0 of the periodic table
• Noble gases


  • Unreactive
  • Do not easily form molecules
  • Atoms have stable arrangements of electrons
  • Have eight electrons in their outer shell, except for helium which has two
• Halogens


  Elements in Group 7 of the periodic table
• Halogens


  • Have similar reactions
  • Have seven electrons in their outer shell
  • Non-metals
  • Consist of molecules made of pairs of atoms
• Students should be able to describe the nature of the compounds formed when chlorine, bromine and iodine react with metals and non-metals
• In Group 7, the further down the group an element is

  The higher its relative molecular mass, melting point and boiling point