TOPIC 7: FORMULA BONDING AND NOMENCLATURE | CHEMISTRY FORM 2
Valence and Chemical Formulae
The Concept of Valence
Explain the concept of valence
Valency is the capacity of an atom to combine with one or more atoms to form a molecule or compound. Valency also refers the number of electrons that an atom can gain, lose or share in forming a chemical bond with another atom.
The valency (or combining power) depends on the number of electrons in the outermost orbit (or valency shell) involved in the formation of a chemical bond.
The number of electrons in the valency shell is never greater than 7. The outermost electronic configuration is responsible for the variability of the valency.
Some elements exhibit more than one valency, i.e., they have variable valencies. Examples of elements with variable valencies are iron (2 and 3), tin (2 and 4) and copper (1 and 2). The other elements with variable valencies are as shown in table 7.1.
Valency and Oxidation States
There is a strong correlation between valency and oxidation state. The
oxidation state of an element equals its valency or charge carried by
its ion when an element ionizes in solution. An example of this relation
is iron (II) whose oxidation state (or oxidation number) is 2 and its
valency is 2. The same case applies to iron (III).
Other elements with variable valencies such as copper (I) and copper (II) have oxidation state equal to 1 and 2 respectively. The list continues. You will learn more about oxidation states later.
The valencies of the common transition elements should be memorized.
Valencies of the normal elements may be deduced from the group number
they occupy in the Periodic Table. The valencies of elements of group I
to IV are equal to the group numbers they occupy in the periodic table.
The valency of an element in group V to VIII is equal to eight minus the
group number. For example, the valency of chlorine which is in group
VII is 1, i.e. (8 -7) =1. The valency of oxygen in group VI is 2, i.e.
(8-6) =2. Elements in group 0 (or VIII) have zero valency i.e. (8 – 8) =
Table 7.1. Valencies of common metals and non-metals
Simple Formulae of Binary Compounds
Write simple formulae of binary compounds
The Concept of Empirical and Molecular Formulae
Explain the concept of empirical and molecular formulae
empirical formula is the simplest formula of any compound.It expresses
the simplest ratio of all the atoms or ions that makeup a certain
compound. For example, the empirical formula ofthe compound with the
formula, C2H4is CH2. This means thatthe simplest ratio of (C:H) is 1:2.
This ratio also indicates theratio in which carbon and hydrogen atoms
combine to form thecompound C2H4.
molecular formula is the formula which shows the actual number of all
atoms present in a given compound. For example,the molecular formula of
the above compound is C2H4. This means that two atoms of carbon and four
atoms of hydrogen form the compound. Likewise, the molecular formula of
water isH2O meaning that the compound is made up of two atoms of
hydrogen and one atom of oxygen.
Therefore, the empirical and molecular formulae can each bedefined thus:
The empirical formula
of a compound is the simplest formula which shows its composition by
mass, and which shows the ratio of the number of the different atoms
present in the molecule.
The molecular formula of a compound is the one which showsthe actual number of each kind of atom present in its molecule.
empirical formula differs from the molecular formula of the same
compound since only the molecular formula agrees with the molar mass of
Information given by Empirical and Molecular Formulae
The formula for water is H 2 O . From this information, you can see that:
- 2 hydrogen atoms combine with 1 oxygen atom to form one molecule of water.
of hydrogen atoms combine with 1 mole of oxygen atoms. Moles can be
changed to grams using relative atomic masses (RAMs). So we can write:
- grams of hydrogen combines with 16 grams of oxygen. In the same way:
- 1g of hydrogen combines with 8g of oxygen.
- 4g of hydrogen combines with 32 of oxygen.
masses of each substance taking part in the reaction are always in the
same ratio.Therefore, from the molecular formula of a compound you can
- how many moles of different atoms combine;
- how many grams of the different elements combine;
- the number of each kind of atoms of different elements that combine to make up a compound; and
- the percentage of each atom in a compound based on RAMs of each atom And from the empirical formula you can tell:
- the simplest ratio or proportion of the different atoms that combine to form a compound.
empirical formula of ethane C2H4andpropene C3H6with molar masses 28.0g
and 42.0grespectively is CH2(i.e. the same) although the two compounds
possess different molecular formulae and masses.
general, the empirical formula multiplied by a whole number,n, gives
the molar mass of the compound. So long as the value of n is known, then
the molecular mass can be deduced.For example, suppose the molecular
mass of ethene is 28.0g, its molecular formula can be deduced thus:
suppose carbon dioxide has a molar mass of 44g and its empirical
formula is CO2. Its molecular formula can be determined thus:
The Empirical and Molecular Formulae
Calculate the empirical and molecular formulae