Atomic structure and the periodic table

• Atom


  The smallest part of an element that can exist
• Metals and non-metals in the periodic table

  Metals are on the left of the stepped line, non-metals are on the right
• Elements that exist as molecules

  • I2 (iodine)
  • Br2 (bromine)
  • Cl2 (chlorine)
  • F2 (fluorine)
  • O2 (oxygen)
  • N2 (nitrogen)
  • H2 (hydrogen)
• Compound


  A substance that contains two or more elements that are chemically combined
• Many compounds exist naturally and can also be formed from their elements in chemical reactions
• Ion


  A charged particle formed when an atom, or a group of atoms, loses or gains electrons
• Reactants


  Substances that react together in a chemical reaction
• Products


  Substances formed in a chemical reaction
• Balanced chemical equation


  Represents a chemical reaction using the formulae of the reactants and products, showing the number of units of each substance involved
• State symbols


  (s) = solid, (l) = liquid, (g) = gas, (aq) = aqueous solution
• Pure substance


  Consists only of one element or one compound
• Mixture


  Consists of two or more different substances, not chemically joined together
• Filtration


  Used to separate an insoluble solid from a liquid, the filter paper has tiny holes or pores that let small molecules and dissolved ions through but not larger undissolved solid particles
• Crystallisation


  Used to produce solid crystals from a solution, when the solution is warmed some of the solvent evaporates leaving crystals behind
• Simple distillation


  Used to separate a solvent from a solution, the dissolved solute has a much higher boiling point than the solvent so when the solution is heated, solvent vapour leaves the solution and is cooled and condensed
• Fractional distillation


  Used to separate different liquids from a mixture of liquids, the different liquids have different boiling points so they condense at different parts of the distillation column
• Paper chromatography


  Used to separate mixtures of soluble substances, the different substances are attracted to the stationary phase (paper) and mobile phase (solvent) in different proportions causing them to move at different rates
• A pure substance produces one spot on a chromatogram, an impure substance or mixture produces two or more spots
• Plum pudding model of the atom

  The atom is a ball of positive charge with negative electrons embedded in it, like currants in a Christmas pudding
• Nuclear model of the atom

  The mass of an atom is concentrated at its centre, the nucleus, which is positively charged
• Bohr model of the atom

  Electrons orbit the nucleus in shells at certain distances from the nucleus
• Subatomic particles


  Protons (positive charge, relative mass 1), neutrons (no charge, relative mass 1), electrons (negative charge, very small mass)
• Atomic number


  The number of protons in an atom of an element
• Mass number


  The total number of protons and neutrons in an atom
• Subatomic particles


  Very tiny
• Proton


  Relative mass 1, Relative charge +1
• Neutron


  Relative mass 1, Relative charge 0
• Electron


  Relative mass very small, Relative charge -1
• Most of the mass of an atom is concentrated in its nucleus
• Protons and electrons have electrical charges that are equal and opposite
• All atoms of a given element have the same number of protons
• Atoms of different elements have different numbers of protons
• An atom contains equal numbers of protons and electrons
• Atoms have no overall electrical charge
• Atoms of different elements usually have different mass numbers, but they can be the same
• Isotopes


  Atoms of the same element with different numbers of neutrons
• Relative atomic mass


  A weighted average of the masses of the atoms of the isotopes, taking account of the abundance of each isotope
• Before discovering protons, neutrons and electrons, scientists tried to classify the elements by arranging them in order of their atomic weights
• Early periodic tables were incomplete and some elements were placed in groups with elements that were not similar to them
• Dmitri Mendeleev


  A Russian chemist who published the first periodic table of the elements in 1869