Topic 1: Atomic structure and the periodic table

• Atom

  Smallest part of an element that can exist
• Element

  Substance that only contains one type of atom
• The periodic table contains approximately 100 elements
• Compound

  Substance that contains at least two different elements that are chemically combined in fixed proportions
• Formation of compounds

  1. Formed from elements by chemical reactions

  2. Can only be separated into elements by chemical reactions
• Molecule

  Two or more atoms joined together by sharing their electrons
• Mixture

  Two or more elements or compounds that are 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 separation techniques cannot be used to separate elements in a compound
• All physical separation techniques are physical processes, so they do not involve chemical reactions and no new substances are made
• Filtration

  Separates an insoluble solid from a liquid, using filter paper and a funnel
• Filtration process

  1. Insoluble solid gets caught in the filter paper

  2. Filtrate passes through the tiny pores in the filter paper
• Crystallisation

  Separates a soluble solid from a liquid
• Crystallisation process

  1. Liquid evaporates, leaving behind crystals of the solid

  2. Can be carried out faster by gently heating the solution
• Simple distillation

  Separates liquid from a solution, the liquid boils off and condenses in the condenser
• Simple distillation process

  1. Evaporate the solution in the flask by heating until it boils

  2. Vapour rises up the glass tube and passes through the condenser

  3. Vapour condenses back to a liquid by cooling
• Simple distillation requires a lot of energy
• Simple distillation is commonly used to separate ethanol from water
• Fractional distillation

  Separates a mixture of different liquids with different boiling points
• Fractional distillation process

  1. Flask containing mixture of liquids

  2. Attached to a fractionating column with glass beads

  3. Substances with high boiling points condense at the bottom

  4. Substances with low boiling points condense at the top
• If liquids have very similar boiling points, it is much harder to separate them using fractional distillation
• Paper chromatography

  Separates substances based on their different solubilities
• Paper chromatography process

  1. Spot of mixture placed near bottom of chromatography paper

  2. Paper placed upright in a solvent

  3. Different components move at different rates
• Before the discovery of the electron, atoms were thought to be tiny spheres that could not be divided
• In 1897 scientists discovered that atoms have an internal structure and contain tiny negative particles called electrons
• Plum pudding model

  Atom as a ball of positive charge with negative electrons embedded in it
• Alpha particle scattering experiment

  Firing tiny positive alpha particles at a sheet of very thin gold foil
• The alpha particle scattering experiment showed that atoms are mainly empty space because many alpha particles passed straight through the gold foil
• The alpha particle scattering experiment showed that the centre of the atom (nucleus) must have a positive charge because some alpha particles were deflected
• The alpha particle scattering experiment showed that most of the mass is concentrated at the centre of the atom (nucleus) because some alpha particles bounced straight back
• The results of the alpha particle scattering experiment meant that the nuclear model replaced the plum pudding model of the atom
• Nuclear model

  Most of the atom is empty space, there is a tiny positive nucleus in the centre of the atom containing most of the mass of the atom
• Bohr's model

  Negative electrons orbit the nucleus at specific distances in energy levels
• After Bohr's discovery, Chadwick discovered that the nucleus also contains neutral neutrons
• Nucleus of an atom

  Contains protons and neutrons
• Number of protons

  Determines the amount of positive charge in the nucleus
• The radius of an atom is about 0.1 nanometres or 1 x 10^(-10) m
• The radius of a nucleus is less than 1/10 000 of that of the atom, about 1 x 10^(-14) m
• Almost all of the mass of an atom is in the nucleus