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1.Fundamentals of Chemistry eLearn.Punjab

1. How many amu 1 g of a substance has?
2. Is atomic mass unit a SI unit of an atomic mass?
3. What is the relationship between atomic number and atomic mass?
4. Define relative atomic mass.
5. Why atomic mass of an atom is defined as relative atomic mass?

1.2.4 How to write a Chemical Formula

Compounds are represented by chemical formulae as elements are represented by symbols.
Chemical formulae of compounds are written keeping the following steps in consideration.
i.Symbols of two elements are written side by side, in the order of positive ion first and negative
ion later.
ii. The valency of each ion is written on the right top corner of its symbol, e.g. Na+ , Ca2+ , CI– and
O2–.
iii. This valency of each ion is brought to the lower right corner of other ion by ‘cross-exchange’
method, e.g.

They are written as:

iv. If the valencies are same, they are offset and are not written in the chemical formula. But
if they are different, they are indicated as such at the same position, e.g. in case of sodium
chloride both the valencies are offset and formula is written as NaCl, whereas, calcium chloride
is represented by formula CaCl2

v. If an ion is a combination of two or more atoms which is called radical, bearing a net charge
on it, e.g. SO42– (sulphate) and PO43– (phosphate), then the net charge represents the valency
of the radical.The chemical formula of such compounds is written as explained in (iii) and (iv);
writing the negative radical within the parenthesis. For example, chemical formula of aluminium
sulphate is written as Al2(SO4)3 and that of calcium phosphate as Ca3(PO4)2.

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