Topic 2 - balancing equations

Question 1

Chemical Symbols – key points

Answer 1

Atoms of each element are represented by a chemical symbol made of one or two letters
E.g. Oxygen atoms are represented by the letter O
Each element has a capital letter

There are about 100 naturally occurring elements
They are combined to make millions of compounds

Question 2

Chemical Formulae – key points

Answer 2

Chemical symbols for elements can be combined to show what is in a compound
E.g. water has the formula H2O
This means it has 2 hydrogen atoms and 1 oxygen atom

The number AFTER the symbol tells you how many atoms of that element are present

The periodic table can be used to determine whether a compound will be ionic or covalent

Group numbers of the elements tell us how many electrons the atom needs to gain, lose or share to form a bond

Question 3

Writing formulae

Answer 3

If we combine a metal and a non-metal, we get an ionic compound

In an ionic compound, the charges must balance overall
E.g. Magnesium chloride
Mg2+ and Cl- must combine in a 1:2 ratio
This gives us a formula of MgCl2

If we only have non-metals, we get a covalent compound
The formula can be worked out by the number of shared pairs needed

Question 4

Balancing equations

Answer 4

Reactants → Products

Remember when a chemical reaction occurs, the reactants have re-arranged themselves to form the products

No atoms have been lost or gained

So we should have the same amount of atoms of each element on either side of the arrow!!!

Question 5

State symbols

Answer 5

We can also show which physical state the reactants and products are in using state symbols

E.g. CaCO3(s) + HCl(aq) → CaCl2(aq) + CO2(g) + H2O(l)

The symbols are:
Solid (s)
Liquid (l)
Gas (g)
Aqueous (aq) [means dissolved in water]