Formulae

Important prefixes

Penta- 5

Hexa- 6

Tetra- 4

Hepta- 7

Tri-3

Octa- 8

Di- 2

Nona- 9

Mono- 1

Deca- 10

Molecules are formed when two or more non-metal atoms are chemically joined, in a fixed ratio

Key information:ions

Atom, molecule or compound?

The atoms can be from the same element , or different elements

E.g a H₂O Molecule- (also a compound as two different elements chemically joines

E.g. an O₂ molecule (not a compound as the two atoms are from the same element

F- atom - it is a single atom, not joined to anything else

F₂- Molecule - 2 x non- metal atoms chemically joined(not a compound as not two different elements)

F₂O - Compound and a molecule-
compound as 2 x different elements joined together
molecule as 2 x non- metal atoms chemically joined

KF- compound- 2 x different elements joined together (not a molecule as K is a metal)

atom , ion, valency

Noble gases

have a full outer shell

The shells are COMPLETELY full

this makes them very stable (unreactive)

When combining with another element (forming a bond) atoms try to achieve a FULL OUTER SHELL like the noble gases

They achieve a full outer shell by gaining, losing, or sharing electrons

Group 4 metals do not tend to form ions, as they would have to lose or gain 4 electrons, and that takes up too much energy

Group 5 non-metals need to gain 3 electrons to have a full outer shell. They form ions with a 3- charge e.g P 3- and N 3-

Group 3 metals need to lose 3 electron to have a full outer shell. they form ions with a 3+ charge, e.g. Al 3+ an Ga 3+

Group 6 non-metals need to gain 2 electrons to have a full outer shell. They form ions with a 2- charge e.g O 2- and S 2-

Group 2 metals need to lose 2 electrons to have a full outer shell. they form ions with a 2+ charge, e.g. Mg 2+ and Ca 2+

Group 7 non-metals need to gain 1 electron to have a full outer shell. They form ions with a 1- charge e.g F- and Cl-

Group 1 metals(and hydrogen) need to lose 1 electron to have a full outer shell. they form ions with a 1+ charge, e.g. Na+ and Li+

Group 8 non-metals do not tend to form ions: they already have a full outer shell of electrons, sop are stable

COMPOUND IONS TO REMEMBER

Nitrate NO₃ (1-)

Ammonium NH₄ (1+)

Carbonate CO₃ (2-)

Hydroxide OH (1-)

Sulphate SO₄ (2-)

Brackets are used when you need MORE THAN ONE of the following in your formula: Nitrate, Ammonium, Carbonate, Hydroxide and sulphate

Writing chemical equations

  1. Count how many atoms of each element are on each side

remember that some formulae are 'known' - e.g. the diatomic molecules (H, N, O, F, Br , Cl, I) - (I Have No Bright or Clever Friends), water (H₂O), ammonia (NH₃), methane (CH₄)

2.Work out the formulae involved either using prefixes (for molecules) or valencies (for ionic compounds)

  1. start off by writing out the word equation.
  1. Use 'big numbers' to balance any atoms if they are not the same