PRESENTED BY: DAVE LESTER C. GALANG

Writing Chemical Formulas

is a way of expressing information about the proportions of atoms that constitute a particular chemical compound, using a single line ofchemical element symbols, numbers, and sometimes also other symbols, such as parentheses, dashes, brackets, commas and plus (+) and minus (−) signs.

The chemical formula of a covalent molecular compound tells us the number of atoms of each element present in the compound. E.g. Water H2O -The formula tells us that there are 2 Hydrogen atoms and 1 oxygen atom present in each water molecule.

The chemical formula of a covalent network tells us the ratio of the elements present. E.g. Silicon dioxide, SiO2 structure-The formula tells us there are 2 silicon atoms present for every 1 oxygen atom.

The chemical formula of an ionic compound tells us the ratio of the elements present. E.g. Sodium chloride, NaCl – The formula tells us there is one ion of sodium for every one ion of chlorine

Chloride ion Sodium ion

Sodium chloride, NaCl, structure

1. CHEMICAL FORMULA FOR ELEMENTS

The chemical formula of an element is simply its’ symbol

e.g. Magnesium Mg Iron Fe Phosphorus P Argon Ar

Whenever we write the chemical formula of these elements we put a subscript ‘2’ beside them

e.g. Iodine I2

I n Oxygen O2

O ctober Chlorine Cl2

C hildren Bromine Br2

B uy Nitrogen N2

N asty Hydrogen H2

H alloween Fluorine F2

F oods

2.USING VALENCY

The valency of an atom or ion is the number of electrons it shares, loses or gains in a chemical reaction to become stable i.e. the number of bonds it forms with other atoms

Table: Group number 1 2 3 4 5 6 7 0 Valency 1 2 3 4 3 2 1 0

We can follow a set of simple rules to write chemical formula:

1. Write symbols of elements present in compound

2. Put valency above each symbol

3. Cross valency over (swap and drop)

4. Cancel down ratio if necessary 5. Write correct chemical

formula

Example

1. Sodium chloride 1. Na Cl 2. Na1 Cl1

3. Na1 Cl1 4. (Not needed) 5. NaCl

Example 2. Calcium bromide 1. Ca Br 2. Ca2 Br1

3. Ca1 Br2 4. (Not needed) 5. CaBr2

3. ROMAN NUMERALS

Some elements, particularly the transition metals in the centre block of the periodic table can have more than one valency.

Roman Numeral Valency I 1 II 2 III 3 IV 4 V 5 VI 6

Example 1. Copper(II) chloride 1. Cu Cl 2. Cu2Cl1

3. Cu1 Cl2 4. CuCl2

Example 2. Iron(III)fluoride 1. Fe F 2. Fe3 F1

3. Fe1 F3 4. FeF3

4. PREFIXES

In the names of some compounds the ratio of atoms present can be indicated by prefixes.

Prefix Meaning Mono 1 Di 2 Tri 3 Tetra 4 Penta 5 Hexa 6

Examples Carbon monoxide CO

Nitrogen dioxide NO2

Phosphorus trichloride PCl3

Phosphorus pentachloride PCl5

Dinitrogen tetroxide N2O4

5.Polyatomic Ions

Ions formed by more than one type of element

Atoms of different elements held together by covalent bonds

Atoms always stay together and collectively have a single charge

Do not always have “-ide” endingex: NH4

1- = ammonium ion CO3

2- = carbonate

Example

1.Ammonium Nitride2. Sodium Carbonate3. Ammonium Phosphate4. Magnesium Phosphate

6. IONIC FORMULA

The ionic formula shows the charges of the ions present in an ionic substance.

GROUP NUMBER

12345670

VALENCY

1234321

0

CHARGE ON ION

1+ 2+ 3+ 4 3- 2- 1-

0

Example

1 Lithium Oxide2. Potassium Nitride3. Sodium Chloride4. Aluminum Oxide

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