The document discusses chemical formulas and how they are used to express information about the proportions of atoms in compounds. It covers: - Chemical formulas for covalent molecular compounds, covalent networks, and ionic compounds show the number or ratio of elements present - Formulas are written using element symbols and subscripts to indicate atom counts - Valence and Roman numerals are used to indicate an element's bonding tendency which helps write formulas - Prefixes like mono-, di-, tri- are also used to show atom ratios - Polyatomic ions, ions formed of multiple elements, are also represented in formulas - Ionic formulas show the charges of ions present in ionic substances

1. PRESENTED BY:
DAVE LESTER C. GALANG
Writing Chemical
Formulas

2.  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.

3. 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.

4. 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.

5. 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

6. 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

7.  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

8. 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

9.  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

10. Example
1. Sodium chloride
 1. Na Cl
 2. Na1 Cl1
 3. Na1 Cl1
 4. (Not needed)
 5. NaCl

11. Example 2. Calcium bromide
 1. Ca Br
 2. Ca2 Br1
 3. Ca1 Br2
 4. (Not needed)
 5. CaBr2

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

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

14. Example 1. Copper(II) chloride
 1. Cu Cl
 2. Cu2Cl1
 3. Cu1 Cl2
4. CuCl2

15. Example 2. Iron(III)fluoride
 1. Fe F
 2. Fe3 F1
 3. Fe1 F3
 4. FeF3

16. 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

17. Examples
 Carbon monoxide CO
 Nitrogen dioxide NO2
 Phosphorus trichloride PCl3
 Phosphorus pentachloride PCl5
 Dinitrogen tetroxide N2O4

18. 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” ending
ex: NH4
1- = ammonium ion
CO3
2- = carbonate

19. Example
1. Ammonium Nitride
2. Sodium Carbonate
3. Ammonium Phosphate
4. Magnesium Phosphate

20. 6. IONIC FORMULA
 The ionic formula shows the
charges of the ions present in
an ionic substance.

21. GROUP NUMBER
1
2
3
4
5
6
7
0
VALENCY
1
2
3
4
3
2
1
0
CHARGE ON ION
1+
2+
3+
4
3-
2-
1-
0

22. Example
1 Lithium Oxide
2. Potassium Nitride
3. Sodium Chloride
4. Aluminum Oxide

23. QOUTES FOR TODAY
THE CHEMICALCOMPOSITION
OF YOUR TEARS CHANGE,
DEPENDING ON YOUR
EMOTION

24. THANKYOU