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Kamis, 05 April 2012

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-Fe2O, 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
Ax   yB  à  ABx
e.g. C4  1H  à C1H= CH4
e.g. P3  1Br  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): 

(Ax+)(By-)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-





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