S. Podder
Formulae, equations and amounts of substance
Writing chemical formula
Chemical formula has two parts, positive and negative.
For elements on their own, formula and their charges can be found from the periodic table. Metals have positive charge, and non-metals have negative charge. Charge of a metal ion, which is positive, is decided by the group it belongs to, and charge of the non-metal ion, which is negative, is decided by subtracting eight from the group number.
Sodium belongs to group one, and hence its ion is Na+1, and aluminium belongs to group three, and hence its ion is Al+3.
Nitrogen belongs to group five, and hence its ion is N-3, and chlorine belongs to group seven, and hence its ion is Cl-1.
Some negative ions have more than one elements and their formula and charges cannot be found in the periodic table. We must know the formulas of the acids from where these ions are formed.
When a formula ends with ide, (hydroxide is an exception) the formula has two elements only. Sodium nitride has two elements, sodium and nitrogen. Aluminium chloride has two elements, aluminium and chlorine.
In Copper (II) oxide, copper has a charge of +2. In Iron (III) oxide, iron has a charge of +3.
When a formula ends with ate, or ite, the formula has three elements. Sodium nitrite has three elements, sodium, nitrogen and oxygen. Potassium sulphate has three elements, potassium, sulphur and oxygen.
Please note ate has one oxygen atom more than ite. Sulphate is SO4 and sulphite is SO3 (charges are same for both, -2), nitrate is NO3 and nitrite is NO2(charges are same for both, -1). The charges can be deduced by looking at the formula of the corresponding acid.
Sulphate comes from sulphuric acid, H2SO4 and hence the formula of sulphate is SO4-2 (one sulphate is joined to two hydrogen atoms).
Nitrate comes from nitric acid, HNO3 and hence the formula of nitrate is NO3-1(one nitrate is joined to one hydrogen atom).
Hydroxide comes from water, H(OH) and hence the formula of hydroxide is OH -1 (one hydroxide is joined to one hydrogen atom).
Sulphate and sulphite has the same charge. Nitrate and nitrite has the same charge.
Ammonia (NH3) is a compound and ammonium (NH4+) is an ion and it is like sodium.
Carbonate comes from carbonic acid, H2CO3 and hence the formula of carbonate is CO3-2 (one carbonate is joined to two hydrogen atoms).
Hydrogen carbonate comes from carbonic acid, H(HCO3)and hence the formula of hydrogen carbonate is HCO3-1(one hydrogen carbonate is joined to one hydrogen atom).
There are some metals thatdo not belong to any group. Some simple rules would be useful. Silver (also ammonium) is like sodium, and zinc is like magnesium. This means silver (also ammonium) has the same charge as sodium, and zinc has the same charge as magnesium.
How do we write the formula of aluminium sulphate?
Aluminium is a metal and we can get its formula and charge from the periodic table, Al +3. Sulphate comes from sulphuric acid and hence its formula is SO4-2. We swap the charges of the ions, and hence formula of aluminium sulphate is Al2(SO4)3.
How do we write the formula of silver nitrate?
Silver is like sodium, silver has the same charge as sodium and its formula is Ag +1. Nitrate comes from nitric acid and hence its formula is NO3-1. We swap the charges of the ions, and hence formula of silver nitrate is Ag(NO3).
How do we write the formula of calcium hydrogen carbonate?
Calcium is a metal and we can get its formula and charge from the periodic table, Ca+2. Hydrogen carbonate comes from carbonic acid, H(HCO3)and hence the formula of hydrogen carbonate is HCO3-1. We swap the charges of the ions, and hence formula of calcium hydrogen carbonate is Ca(HCO3)2.
Remember
Silver is like sodium. Both has a charge of +1. So is ammonium NH4 .
Zinc is like Magnesium. Both has a charge of +2.
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