Chemistry- atomic structure and the periodic table

• All substances are made of atoms
• Atom


  The smallest part of an element that can exist
• Atoms, Elements and Compounds


  • Atom
  • Element
  • Compound
• An element is a substance that contains only one sort of atom
• There are about 100 different elements
• Elements are displayed in the periodic table
• Chemical symbol


  A different symbol represents the atoms of each element, e.g. sodium = Na, carbon = C and iron = Fe
• Most substances are compounds
• Compound


  Contains atoms of two or more elements, which are chemically combined in fixed proportions
• Chemical formula


  A combination of numbers and chemical symbols that represents a compound
• What chemical formulae show

  1. The different elements in a compound

  2. How many atoms of each element one molecule of the compound contains
• Chemical formulae examples


  • Water, H2O, contains 2 hydrogen (H) atoms and 1 oxygen (O) atom
  • Sulfuric acid, H2SO4, contains 2 hydrogen (H) atoms, 1 sulfur (S) atom and 4 oxygen (O) atoms
• Compounds can only be separated into their component elements by chemical reactions or electrolysis
• Word equation


  Sums up what has happened during a chemical reaction
• Balanced symbol equation

  Sums up what has happened during a chemical reaction
• The reactants (the substances that react) are on the left-hand side of the equation
• The products (the new substances that are formed) are on the right-hand side of the equation
• The total mass of the products of a chemical reaction is always equal to the total mass of the reactants. This is because no atoms are lost or made
• The products of a chemical reaction are made from exactly the same atoms as are found in the reactants
• Mixture


  Two or more elements or compounds, which are not chemically combined
• The components of a mixture retain their own properties
• Physical processes


  Used to separate mixtures, do not involve chemical reactions and do not produce new substances
• Filtration


  1. Used to separate soluble solids from insoluble solids

  2. Example: Mixture of salt (soluble) and sand (insoluble) can be separated by dissolving the salt in water and then filtering the mixture
• Crystallisation


  1. Used to obtain a soluble solid from a solution

  2. Example: Salt crystals can be obtained from a solution of salty water by gently warming the mixture to evaporate the water, leaving behind pure salt crystals
• Simple distillation


  Used to obtain a solvent from a solution
• Required practical: Analysis and purification of water samples from different sources
• Water sample analysis


  1. Use a pH probe or suitable indicator to analyse the pH of the sample

  2. Heat a set volume to 100°C so the water changes from liquid to gas

  3. The water collects in the condenser and changes state from gas to liquid, collect this pure water

  4. Measure the mass of solid that remains to find the amount of dissolved solids present in the sample
• There is a risk of the experimenter burning themselves on hot equipment, so care must be taken during and after the heating process
• Fractional distillation


  1. Used to separate mixtures in which the components have different boiling points

  2. Example: Oxygen and nitrogen can be obtained from liquid air by fractional distillation because they have different boiling points
• Chromatography


  1. Used to separate the different soluble, coloured components of a mixture

  2. Example: The different colours added to a fizzy drink can be separated by chromatography
• Scientific Models of the Atom


  • In early models, atoms were thought to be tiny spheres that could not be divided into simpler particles
  • In 1898, Thomson discovered electrons and the representation of the atom had to be changed
  • Overall, an atom is neutral, i.e. it has no charge
  • Thomson thought atoms contained tiny, negative electrons surrounded by a sea of positive charge. This was the 'plum-pudding' model
  • Later, Geiger and Marsden carried out an experiment in which they bombarded a thin sheet of gold with alpha particles. Although most of the positively charged alpha particles passed straight through the atoms, a tiny number were deflected back towards the source
  • Rutherford looked at these results and concluded that the positive charge in an atom must be concentrated in a very small area. This area was named the 'nucleus' and the resulting model became known as the 'nuclear' model of the atom
  • Bohr deduced that electrons must orbit the nucleus at specific distances, otherwise they would spiral inwards
  • Later experiments showed that the nucleus is made of smaller particles: some of which have a positive charge and are called protons, some of which have no charge and are called neutrons
• Subatomic Particles


  • Atoms are very, very small and typically have an atomic radius of about 0.1nm or 1 x 10^-10m
  • Atoms contain three types of subatomic particle: proton, neutron, electron
  • Almost all of the mass of an atom is in the nucleus
  • However, the radius of the nucleus is less than 1/10000 of the atomic radius of the atom, or 1 x 10^-14m
  • Atoms have no overall charge because they contain an equal number of protons and electrons
  • All atoms of a particular element have the same number of protons
• Subatomic Particles


  • Proton
  • Neutron
  • Electron
• Proton


  • Relative Mass: 1
  • Relative Charge: +1
• Neutron


  • Relative Mass: 1
  • Relative Charge: 0
• Electron


  • Relative Mass: very small
  • Relative Charge: -1
• Atomic number


  The number of protons in an atom
• Mass number


  The sum of the protons and neutrons in an atom
• In the modern periodic table, elements are arranged in order of increasing atomic number
• Number of neutrons


  Mass number - atomic number