Classwork Series and Exercises {Chemistry- SS1}: Radicals

Introduction

A radical is a group of atoms which behave as one unit. They do not exist separately outside a compound. Radicals are divided into two parts namely cations and anions.

1. The cations are the positive radicals and they are known as the metallic radicals.
2. The anions are the negative radicals and they are non-metallic radicals. Some non-metallic radicals contain more than one atom e.g. tetraoxosulphate (VI) ion (SO₄^2-).

Their oxidation numbers act as valency when combining with metallic radicals to form a compound.

Common Radicals and Their Oxidation State (Valency)

Names of Cations             Symbols         Oxidation State        Valency

Aluminium                             Al³⁺                     +3                                 3
Calcium ion                           Ca²⁺                     +2                                 2
Copper(II) ion                       Cu²⁺                     +2                                 2
Iron(II) ion                             Fe²⁺                     +2                                 2
Iron(III) ion                            Fe³⁺                     +3                                 3
Magnesium ion                    Mg²⁺                    +2                                2
Potassium ion                       K⁺                        +1                                 1
Silver ion                               Ag⁺                      +1                                 1
Sodium ion                           Na⁺                      +1                                 1
Ammonium ion                   NH₄⁺                    +1                                  1
Zinc ion                                 Zn²⁺                     +2                                  2

Names of Anions                   Symbols             Oxidation State        Valency
Chloride ion                                     Cl⁻                              -1                                 1
Fluoride ion                                     F⁻                               -1                                  1
Trioxosulphate (IV) ion             SO₃^2-                         -2                                 2
Trioxocarbonate (IV) ion          CO₃^2-                         -2                                2
Hydroxide ion                                OH⁻                           -1                                  1
Trioxonitrate (V)                          NO₃^–                           -1                                 1
Tetraoxophosphate (V) ion         PO₄^3-                      -3                                 3
Tetraoxosulphate (VI) ion           SO₄^2-                      -2                                 2
Tetraoxomanganate (VII) ion      MnO₄^–                   -1                                1
Trioxochlorate (V) ion                 ClO₃^–                      -1                                1

FORMULAE OF COMPOUNDS

The molecules are particles of elements and compounds and they are represented by chemical formulae.

A chemical formula is the representation of molecules of elements and compounds by symbols. More importantly, it denotes the number of atoms of each element present in the compound. For example, the formula for Ferric oxide or Iron [III] oxide is Fe₂O₃, which implies that 2 atoms of Fe and 3 atoms of O are present in an electrically-neutral molecule of the compound.

Molecules of Elements and Atomicity

A molecule of an element may contain one atom or more.

• Molecule of element containing one atom of the element in the molecule is called a monoatomic molecule. The atomicity is 1. Examples are Helium (He), Neon (Ne), etc.
• Molecule of element containing two atoms of the element in the molecule is called a diatomic molecule. The atomicity is 2. Examples are Hydrogen (H₂), Oxygen (O₂), Nitrogen (N₂), Chlorine (Cl₂), etc.
• Molecule of element containing three atoms of the element in the molecule is called a triatomic molecule. The atomicity is 3. Example is Ozone (O₃)

Atomicity is the number of atoms in each molecule of an element.

Molecules of Compounds

When elements combine compounds are formed. To form compounds:

1. A metallic element can combine with a non-metallic element.
2. A non-metallic element can combine with a non-metallic element.

It must be noted that, a metallic element cannot combine with another metallic element but can be mixed to form alloy.

Compound may be classified as:

1. Binary Compounds: These compounds contain two elements only. Examples are NaCl, MgCl₂, Fe₂O₃, etc
2. Trinary Compounds: These compounds contain three elements only. Trinary compounds are derived from trinary acid. Examples are HNO₃, H₂SO₄, H₃PO₄, etc.

How to write a chemical formula

To write a chemical formula, one must know the symbols and valencies of the elements / radicals.

Example 1: Write the chemical formula for Calcium Tetraoxophosphate (V).
Valency of Calcium (Ca) = 2 ; Valency of Tetraoxophosphate (V) (PO₄) = 3.
Interchanging their valencies and writing as subscripts,
Formula for Calcium Tetraoxophosphate (V) is Ca₃(PO₄)₂.
Note that 3 calcium ion [Ca²⁺] and 2 tetraoxophosphate ions [PO₄^3-] are present in an electrically-neutral molecule of calcium tetraoxophosphate (V) [Ca₃(PO₄)₂].

Example 2: Write the chemical formula for Zinc trioxocarbonate (IV).
Valency of Zinc (Zn) = 2 ; Valency of Trioxocarbonate (IV) (CO₃) = 2.
Interchanging their valencies and simplifying (on dividing by 2),
formula for Zinc carbonate is ZnCO₃.
Note that 1 zinc ion [Zn²⁺] and 1 carbonate ion [CO₃^2-] are present in an electrically-neutral molecule of zinc carbonate [ZnCO₃].

EXERCISES

Lets see how much you’ve learnt, attach the following answers to the comment below

1. Atomicity of Ozone is _________ (a) 1 (b) 2 (c) 3 (d) 4
2. The major difference between [i] SO₃ and [ii] SO₃^2- is that (a) is a molecule while [ii] is an atom (b) is a radical while [ii] is a molecule (c) is a molecule while [ii] is a radical (d) is a radical while [ii] is an atom
3. The atomicity in chlorine gas, Cl₂ is (a) 3 (b) 4 (c) 2 (d) 1
4. The formula of the compound formed in a reaction between a trivalent metal M and a tetravalent non-metal X is (a) MX (b) M₃X₄ (c) M₄X₃ (d) M₃X₂
5. Compounds contain two elements only are said to be (a) molecule (b) binary compound (c) trinary compound (d) mixture

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