Chemistry Toolkit Revision Resource


Atom Structure

  • All substances are made from atoms.
  • Each atom is made from a nucleus - containing protons and neutrons - which is orbited by electrons.
  • The atomic number is the number of protons in the atom. The elements in the periodic table are ordered by atomic number.
  • The atomic mass is the total number of protons and neutrons.
  • An atom is 1 single thing.
  • A compound is 2 or more atoms bonded together - the atoms must be different.
  • A molecule is 2 or more of any atom bonded together - the atoms can be the same
  • In a atom, the number of protons equals the number of electrons.
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Protons, Neutrons and Electrons

  • At the centre of every atom is a nucleus containing protons and neutrons. 
  • Electrons are contained in shells around the nucleus.
  • The number of electrons is always the same as the number of protons in the nucleus.
  • Each proton has an electrical charge of +1
  • Each electron has an electrical charge of -1
  • Each neutron has no charge meaning it is neutral.
  • An atom has the same number of protons as electrons meaning the overall charge is 0 (neutral)
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Counting Atoms

  • The little numbers apply only to the letter before them.                                       eg; CH4 = 1 carbon and 4 hydrogen.
  • The big numbers apply to the whole of that part of the equation after them. eg; 2H2O = 4 hydrogens and 1 oxygen
  • The atoms are the amount of each element and the elements are the amount of letters there are. 
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Balancing Equations

  • A balanced equation si a symbolic representation showing the kind and amount of the starting materials and products of a reaction. There is the same amount of each element on each side of the equation. eg;  2Na + Cl2= 2NaCl  is a balanced equation. 
  • To balance an equation you can only change the big numbers not the little ones. eg; Na + Cl = 2NaCl would become 2Na +2Cl = 2NaCl. 
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