Making Formula in Chemistry

A helpsheet for making formula in chemistry

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Making Formula
The information you see above is straight from the data sheet provided in the exam. You also get a
copy of the Periodic Table. The Periodic Table allows you to check electron numbers and the valency
or charge of an ion based on the number of electrons in the outer shell of the atom. Remember the
number required to be gained or lost to achieve a full outer shell indicates the charge of the ion.
Remember that a cation is positive and an anion is negative.
Method 1 - The Hook Method for Making a Formula
This is the method that allows students who cannot visualise abstract ideas to put something down
on paper to show the working out.
Every charge on the ion corresponds to a hook e.g. Na+ would have one hook to pair off whereas
Mg2+ would have two hooks. It is the same for anions like Cl-.
E.g. Magnesium reacts with oxygen to form Magnesium oxide. What is the formula?
Mg2+ is the ion, oxygen has a O2- charge (group 6). They each have two hooks and both get cancelled
out giving an equal match up and a formula of MgO.
Now Pick 5 cations and 5 anions and try to match them off giving their formula.
Method 2 ­ The dice game
Make a six sided dice or use existing dice and number your data sheet 1-6 cations and 1-6 anions.
Roll the dice and match up the two ions to see what bonds are formed and therefore the formula of
the new compound. Sometimes you get slightly complicated formula requiring brackets. See me.

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