Elements and Compounds

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Element: A substance that cannot be broken down chemically into anything simpler

Atom: The smallest posssible particle of an element

Compound: A substance composed of two or more elements chemically bonded together

Molecule: An arrangement of atoms covalently bonded together

Covalent Bond: A shared pair of electrons holding atoms together in a molecule

Ion:  A particle formed from an atom of group of atoms by loss of gain of electrons. Losing electrons forms a positive ion/gaining electrons forms a negative ion

Ionic Bond: Electrostatic attraction between oppositely charged ions

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Elements and Compounds


  • Can be divided into metals and non-metals
  • The elements are arranged in vertivle groups in the periodic table numbered 0 to 7. There are some elements in the middle of the table e.g. transition metals that do not belong to any group
  • The group number of an element tells you the number of electrons it has in it's outer shell of each atom of the element


  • For the group one and group two elements, the only ions formed by the elements in normal chemical compounds are those in which the outermost electron or electrons have been lost

Ionic Compounds

  • Compounds containing oppositely charged ions which are strongly attracted to each other by electrostatic attraction
  • This is called ionic bonding
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Elements and Compounds

Covalent Compounds

  • Contain atoms bonded to each other by covalent bonds
  • E.g. the element Oxygen exists as molecules with the formula O2. There are 2 atoms of oxygen in each molecule

Compounds made up only of atoms of non metals have covalent bonding

Compounds made up of a metal and a non-metal have ionic bonding

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Chemical Formulae

Rules for writing formulae

1. Metals

  • The formula of a metal is always written just using the chemical symbol for an atom of the element

2. Common Molecules

  • Must show the correct number of atoms that are bonded to each other

Diatomic Molecules

  • Oxygen: O2
  • Nitrogen: N2
  • Hydrogen: H2
  • Chlorine: Cl2
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Chemical Formulae

Other molecules

  • Ammonia: NH3
  • Methane: CH4
  • Carbon Dioxide: CO2

Common Acids

  • Hydrochloric Acid: HCl
  • Sulphuric Acid: H2SO4
  • Nitric Acid: HNO3

3. Ionic Compounds

  • The formula of an ionic compound shows the ratio of the number of ions in the compound
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Positive Ions

Positive ions

  • Metals generally lose electrons to form positive ions, when they form compounds
  • For the main group metals, the number of the group tells you the number of positive charges on the ion

Group 1: Li+, Na+, K+

Group 2: Mg2+, Ca2+

Aluminium: Al3+

  • For transition metals, roman numerals are used as many of these elements form more than one positive ion with different charges
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Negative Ions

Negative Ions

  • Non-metals tend to gain electrons from metals they react with to form negative ions
  • For simple ions, you can work out the number of negative charges on the ion by using the rule that they will gain electrons until they have 8 in the outer shell

Group 5: N3- (Nitride)

Group 6: O2- (Oxide), S2- (Sulphide)

Group 7: F- (Fluoride), Cl-, Br-, I-

Compound Ions

  • Hydroxide: OH-
  • Carbonate: CO3 2-
  • Sulphate: SO4 2-
  • Nitrate: NO3-

The total positive charge in a compound must be the same as the total negative charge

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