Writing Formulas (I am so lost)

[lacydacy]

I have no idea how to write these formulas.  I know that it is probably simple, but I don't understand how you decide what number goes where. 

For example:  Copper(I) Sulfate  or Magnesium bicarbonate

Have no idea!!!  Any help would be great right now!  Thanks!!

[kingchemist]

The (I) in copper (I) sulphate is the valency or combining power of copper in this compound.
Sulphate should be written in data as SO₄^2-. The number of charges is its valency.
So SO⁴ has a valency of 2. In writing formulae, the valencies needed to be crossed over.
                 Cu       SO₄ 
Valency        1           2
Formula       Cu₂SO₄

But copper (II) sulphate will be CuSO₄ as both have a valency of 2 giving a ratio of 2:2 in the formula which simplified to 1:1

Magnesium bicarbonate
Magnesium is in Group 2 of the Periodic table so has a valency of 2.
Bicarbonate ( also called hydrogencarbonate) has the formula HCO₃^-. It has 1 charge, so its valency is 1.
                     Mg        HCO₃
Valency           2                1
Formula          Mg(HCO₃)₂

Notice the brackets around HCO₃ and the crossed over number after the brackets.
Without the brackets it would be MgHCO₃₂ which is not the correct formula. There are meant to be 2 x 'bicarbonates' in the formula

As a general rule, elements in groups 1, 2, 3 and 4 of the periodic table have a valency of 1, 2, 3 and 4 respectively.
Elements in groups 5, 6 and 7 have valency number of 3, 2 and 1 respectively

Watch out for calcium hydroxide which is often wrongly written. Ca is in group 2 so has a valency of 2; OH^- has a valency of 1. The correct formula is Ca(OH)₂. Often the bracket is forgotten as CaOH₂ do not look silly like MgHCO₃₂ but is equally wrong.