This document discusses how to write chemical formulas. It explains that a chemical formula tells us the types and numbers of atoms in a compound. It then outlines five steps for writing chemical formulas: 1) Write the symbols of the ions side by side in order of positive then negative, 2) Write the valency of each ion, 3) Transfer the valencies to offset them, 4) Omit valencies that are the same, and include different valencies in the formula, 5) For radicals like sulfate and phosphate, the net charge indicates the radical's valency and it is written in parentheses.

1. CHEMISTRY
TOPIC:
HOW TO WRITE A CHEMICAL FORMULA
ZAHRA FAZAL

2. CHEMICAL FORMULA
 Compounds are represented by chemical
formula
 It tells us types and number of atoms of an
element present in a compound.
 For example:
 sodium chloride (NaCl)
 water (H2O)
 Calcium oxide (CaO)
 Copper sulphate ( CuSO4 )

3. STEPS OF WRITING
CHEMICAL FORMULA
STEP I
• Symbols of two elements are
written side by side , in order of
positive ion first and negative ion
later.
• For example:
Na+ Cl-
Cu2+ O 2-
Ca 2+ SO4
2-

4. STEP II
 The valency of each ion is written on the
right top corner of its symbol.
 For example:
 Na+
 Cl-
 Ca2+
 O2-

5. STEP III
 The valency of each ion is brought to the
lower right corner of other ion by ‘ cross-
exchange’ method.

6. STEP IV
If the valencies are
same they are offset
and not written in the
chemical formula. In
case of sodium chloride
both the valencies are
offset and formula is
written as NaCl.
if the valencies are
different , they are
indicated as such at the
same position. For
example in case of
calcium chloride both
valencies are not same
so its formula written as
CaCl2

7. STEP V
 In case of radicals like sulphate (SO4
2-) and
phosphate (PO4
3-) , the net charge represent the
valency of a radical.
 Write negative radical within the parenthesis ( )
 For example:
 Al3+ SO4
2- Al2(SO4)3
 Ca2+ PO4
3- Ca3(PO4)2