Atomic structure and the periodic table

• An atom is the smallest part of an element
• Atoms of each element are represented by a chemical symbol
• Compounds are formed from elements by chemical reactions
• Chemical reactions always involve the formation of one or more new substances, and often involve a detectable energy change
• Compounds contain two or more elements chemically combined
• Compounds can only be separated into elements by chemical reactions.
• Mixtures are substances containing two or more different elements that are NOT chemically combined together
• Mixtures can be seperated by:

  • filtration
  • crystalisation
  • fraction distillation
  • simple distillation
  • chromatography
• filtration is a physical way of seperating mixtures and seperates insoluble solids from liquids
• Insoluble means it doesnt dissolve in water
• Filtration practical:

  • EQUIPTMENT: filter funnel, filter paper and conical flask
  • place filter funnel on conical flask
  • place filter paper around the inside of the filter funnel
  • pour the mixture into the filter paper
  • the liquid (filtrate) will pass through the tiny pores in the filter paper
  • the solid cant pass through the filter paper so it is trapped
• simple distillation separates a liquid from a solution
• simple distillation practical:

  • the flask is connected to a glass tube
  • glass tube is surrounded by condenser
  • cold water from tap flows through condener
  • thermometer placed in flask
  • first, heat solution using bunsen burner
  • solution starts to evapourate, turns into vapour
  • this vapour rises up the glass tube
  • as vapour passes thermometer, the thermometer reading increases
  • heat the solution until it boils
  • vapour passes into condenser
  • cold water in condenser causes vapour to turn back into liquid
  • liquid is collected in beaker
• simple distillation is used when producing potable water from sea water
• fractional distillation separates a mixture of liquids with different boiling points into two or more fractions
• Fractional distillation practical:

  • put mixture in flask and put fractionating column on top, then heat it
  • the different liquids have different boiling points so evaporate at different temperatures, lower boiling point will evapourate more easily
  • when vapours reach the fractionating column, they condense
  • this condensed vapour drips back into flask and liquid evapourates again
  • warm vapour passes up the column and reaches thermometer
  • temperature on thermometer rises
  • vapour passes into condenser and turns back into liquid
• fractional distillation is used when purifying crude oil
• chromatography seperates different dyes in ink
• Chromatography practical:

  • draw line at the bottom of chromatography paper in pencil (pen ink can be dissolved by the solvent)
  • draw a dot in first colour on the pencil line
  • draw a dot in second colour on the pencil line
  • place bottom of paper in a solvent
  • the solvent goes up the paper, dissolves ink in the coloured dots
  • ink is carried up the paper
• atoms were thought to be tiny spheres that couldnt be divided before the discovery of the electron
• the discovery of the electron led to the plum pudding model, which suggested that the atom is a ball of positive charge with negative electrons in it.
• the alpha particle scattering experiment led to conclusion that atoms contain a nucleus and that the nucleus was charged. This replaced plum pudding model
• Rutherford invented the alpha particle scattering experiment to test the nuclear model
• Niels Bohr discovered that electrons orbit around the nucleus
• James Chadwick discovered that the nucleus contained neutrons
• JJ Thomson discovered the plum pudding model
• the alpha particle scattering experiment prove the plum pudding wrong because the plum pudding model contained electrons so some particles were expected to pass straight through the sheet of gold. However, some were deflected backwards
• • nuclear model has a nucleus whilst plum pudding doesnt
  • nuclear model has electrons in a shell, whilst plum pudding had electrons randomly placed
  • nuclear model has electrons in an orbid, plum pudding doesnt
  • plum pudding was a solid sphere, nuclear model wasnt
• atoms dont have an overall charge because the number of protons and number of electrons are the same
• atomic number is the number of protons in an atom
• the mass number is the sum of protons and neutrons
• top number is atomic number and bottom number is mass number
• isotopes are atoms of the same element with different numbers of neutrons
• relative atomic mass is an average mass of isotopes making up an element and how much of each element there is
• elements are arranged on the periodic table by increasing atomic number, which is the number of protons.
• elements were arranged on the first periodic table in order of their atomic weights
• Mendeleev changed the periodic table
• Mendeleev left gaps for undiscovered elements and changed the order of atomic weights
• metals are elements that react to form positive ions
• non metals are elements that dont react to form positive ions