How to find the chemical formula

A lot of people have trouble finding out the chemical formula of something, so I'll try to explain how I do it.
1. From valencies
2. From names
3. From ionic charges

1. From valencies
You can usually predict the formula of a compound by looking at the valency of the atoms involved. Valency is the combining power of the atom- i.e. how many bonds it can form with another atom. Usually, this is related to its outer shell electron (OSE).

Example:
Sodium has one OSE, so it can lose one electron. It has a valency of one. Chlorine has 7 OSE, so it can gain one electron. Its valency is also one. When chlorine and sodium combine, one chlorine atom combines with one sodium atom-so the formula of sodium chloride is NaCl.

Mg has a valency of 2, Cl has a valency of one, so for every Mg, 2 Cl are needed. The formula for magnesium chloride is MgCl2.

Rules:
For elements, the valency is related to its position in the periodic table:

Group no.	1	2	3	4	5	6	7
Valency	1	2	3	4	3	2	1

The valency of an element from groups 1-4 equals their group number.
For elements in groups 5-7, the valency equals (8-their group number).

Valency of some polyatomic ions:  

Formula	NH4+	NO3-	OH-	HCO3-	SO42-	CO32-	PO43-
Name	Ammonium	Nitrate	Hydroxide	Hydrogencarbonate	Sulphate	Carbonate	Phosphate
Valency	1	1	1	1	2	2	3

Some atoms (mostly transition metals) can have more than one common valency. In this case, the valency is written as roman numerals in the name:

e.g. Iron (II) Chloride- FeCl2  This is called 'iron two chloride'  

Iron (III) oxide-Fe₂O₃ , Iron (II) oxide- FeO

Zinc ions are always Zn²⁺ and silver always Ag⁺

Cross over the valencies to get the formula (and simplify, for e.g. CaO, both have valency of 2, so cross over and simplify so its 1:1)

e.g. Covalent compounds

A,B=non-metals

Cross valencies

A^{x   y}B  à  A_y B_x

e.g. C^4  1H  à C₁H₄ = CH₄

e.g. P^{3  1}Br  à PBr₃

2. From names: 

1. mono-1
2. di-2
3. tri-3
4. tetra-4

e.g. 

carbon monoxide=CO 

Sulphur dioxide= SO₂

Dinitrogen monoxide= N₂O

Carbon tetrabromide= CBr₄

3. From ionic charges (for ionic compounds only): 

(A^x+)_a (B^y-)_b

A=cation-usually metal ion (NH₄⁺ is an exception), B=anion-- non-metal ion

Total positive charge + total negative charge

i.e. xa=yb

A+B-	e.g. NaCl, KF, LiBr
A2+B2-	MgO, CaS, BaO
A3+B3-	AlN, AlP
A2+ (B-)2	CaBr2, BaCl2
(A+)2B2-	Na2O, K2S
(A2+)3(B3-)2	Ca3N2, Ca3P2
(A3+)2(B2-)3	Al2O3, Al2S3

Charges on some common ions, learn!! 

Positive ions/Cations			Negative ions/Anions
Charge	Name of ion	Formula	Charge	Name of ion	Formula
1+	Ammonium Copper (I) Hydrogen Lithium Potassium Silver Sodium	NH4+ Cu+ H+ Li+ K+ Ag+ Na+	1-	Bromide Chloride Hydroxide Fluoride Iodide Nitrate Hydrogencarbonate	Br- Cl- OH- F- I- NO3- HCO3-
2+	Barium Calcium Copper (II) Iron (II) Lead (II) Magnesium Nickel (II) Strontium Zinc	Ba2+ Ca2+ Cu2+ Fe2+ Pb2+ Mg2+ Ni2+ Sr2+ Zn2+	2-	Carbonate Sulphate Sulphite Sulphide Oxide	CO32- SO42- SO32- S2- O2-
3+	Aluminium Iron (III)	Al3+ Fe3+	3-	Nitride Phosphate	N3- PO43-