Atomic structure and the periodic table

• Atom


  Smallest part of an element that can exist
• Chemical symbol

  Represents an atom of an element
• Elements


  • Substances with only one type of atom
  • Listed in the periodic table
  • About 100 different elements
• Elements


  • Can be classified as metal or non-metal depending on their properties
  • The columns in the periodic table are called groups and contain similar elements
  • The rows in the periodic table are called periods, elements show a gradual change in properties across a period
• Compounds have different properties from the elements they are made from
• Compounds can only be separated into elements by chemical reactions
• Mixture

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

  • Filtration
  • Crystallisation/Evaporation
  • Simple distillation
  • Fractional distillation
  • Chromatography

  
• Simple distillation

  Separates a soluble solid and a liquid (e.g. salt and water)

  
• Crystallisation/Evaporation

  Separates a soluble solid from a (non-flammable) liquid
• Filtration

  Separates an insoluble solid suspended in a liquid (usually water)
• Fractional distillation

  Separates soluble liquids with different boiling points (e.g. crude oil)

  
• The fractionating column has a temperature gradient and is hotter at the bottom than at the top
• The liquid with the lowest boiling point will be the first 'fraction' to collect
• Before the discovery of the electron atoms were thought to be tiny spheres that could not be divided
• Plum-pudding model

  The atom was a ball of positive charge with negative electrons embedded in it
• The Rutherford and Marsden's alpha scattering experiments led to the plum-pudding model being replaced by the nuclear model
• Nuclear model

  The centre of the atom was called the nucleus, all the mass and positive charge of the atom was in the nucleus, electrons orbited the nucleus
• Bohr model

  Electrons orbit the nucleus at specific distances, on energy levels or shells
• Chadwick's experimental work provided evidence for the existence of neutrons within the nucleus
• Atom


  Small central nucleus made up of protons and neutrons, around which there are electrons
• Subatomic particles

  • Proton
  • Neutron
  • Electron
• 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
• Mass number


  The total number of protons and neutrons in an atom
• Isotopes


  Atoms of the same element with different numbers of neutrons
• Isotopes of an element have the same chemical properties because they have the same electronic structure
• Relative atomic mass


  An average value that takes account of the abundance of the isotopes of an element
• Elements in the same group in the periodic table have the same number of electrons in the highest energy levels (outer electrons) and this gives them similar properties
• Noble gases


  The elements in Group 0 of the periodic table, they are unreactive because their atoms have stable arrangements of electrons
• The modern periodic table can be seen as an arrangement of the elements in terms of their electronic structures
• Mendeleev's periodic table


  • Left gaps for elements that had not been discovered
  • Changed the order based on atomic weights in some places
• Knowledge of isotopes made it possible to explain why the order based on atomic weights was not always correct
• Metals


  Elements that react to form positive ions
• Non-metals


  Elements that do not form positive ions
• The majority of elements are metals, they are found to the left and towards the bottom of the periodic table, non-metals are found towards the right and top of the periodic table
• Group 1 (alkali metals)

  • All have 1 electron in their highest energy level
  • Are metals with low density
  • Are stored under oil to prevent them from reacting with oxygen or water
• In group 1, the further down the group an element is


  The more reactive it becomes
• Group 1 metals react vigorously with water, fizzing and moving around on the surface