How to Calculate Valency of Radicals?

To construct the formulas of ionic compounds, we must first understand the valencies of the positive and negative ions that combine to produce the ionic compounds. As a result, we’ll start with ion valencies. The valency of an ion is equal to its charge. If an ion has one unit charge, its valency is one, and it is referred to as a monovalent ion. When an ion contains two units of charge, its valency is two, and it is referred to as a divalent ion. And if an ion has three units of charge, its valency is three, and it is referred to as a trivalent ion. Please keep in mind that a cation has a positive charge and so has positive valency; an anion has a negative charge and thus has negative valency.

What are Radicals?

Radical behaves like a single atom with a positive or a negative charge on it, they are defined as an atom or a group of atoms of the same or different elements that shows the nature of the atom with charge on it. they have their own combining power due to their combining capacity they form chemical formulae.  

For Example:  

NH⁴⁺, Na⁺, NO³⁻, SO₄²⁻ 

Valency of Ions or Radicals

The valency of any element is defined as a measure of its combining capacity and the number of electrons that must be gained or lost by an atom to obtain a stable electronic configuration.

Rules for writing of electrons configuration in different shells for the first 18 elements are formula to calculate the maximum number of electrons present in a shell = 2n², where n = 1, 2, 3 (i.e. orbit number).        

For different shells maximum electrons distributes as,

• K shell n = 1 = 2n² =  2(1)² = 2
• L shell n = 2  = 2n² = 2(2)² = 8
• M shell n = 3  = 2n² = 2(3)² = 18
• N shell n = 4  = 2n² = 2(4)² = 32

Different shells are shown in the magnesium atom

The valency of charged ions or radicals is equal to the amount of charge present on them. On the basis of charge present on radicals or ions, they are divided into three:

1. Monovalent ions –  The ion having valency one or ions having one unit charge, are known as Monovalent ions. For example- Hydrogen ion (H⁺), lithium ion (Li⁺), ammonium ion (NH₄⁺), Bromide ion (Br^–), Hydroxide ion (OH^–) etc.
2. Divalent ions – The ion having valency two or ions having two unit charge, are known as Divalent ions. For example- Magnesium ion (Mg²⁺), Calcium ion (Ca²⁺), Zinc ion (Zn²⁺), Oxide ion (O^2-), sulphide ion (S^2-) etc.
3. Trivalent ions – The ion having valency three or ions having three unit charge, are known as Trivalent ions. For example- Aluminum ion (Al³⁺), Iron ion (Fe³⁺), Nitride ion (N³⁻), Phosphate ion (PO₄^3-) etc.

Valency of Common ions or radicals

Following is the tabular representation of common monoatomic cations:

Valency	Element	Ion of the Element
1+ (Monovalent)	Hydrogen	H+
	Lithium	Li+
	Sodium	Na+
	Potassium	K+
	Cesium	Cs+
	Silver	Ag+
2+ (Divalent)	Beryllium	Be2+
	Magnesium	Mg2+
	Calcium	Ca2+
	Cadmium	Cd2+
	Nickel	Ni2+
	Strontium	Sr2+
	Barium	Ba2+
3+ (Trivalent)	Aluminum	Al3+

Following is the tabular representation of common monoatomic anions:

Valency	Element	Ion of the Element
1- (Monovalent)	Hydride	H–
	Fluoride	F–
	Chloride	Cl–
	Bromide	Br–
	Iodide	I–
2- (Divalent)	Oxide	O2-
	Sulfide	S2-
3- (Trivalent)	Nitride	N3-
	Phosphide	P3-

Following is the tabular representation of common polyatomic radicals:

Valency	Radical	Symbol	Ion
Monovalent [1]	Hypochlorite	ClO	ClO1-
	Chlorite	ClO2	ClO21-
	Chlorate	ClO3	ClO31-
	Perchlorate	ClO4	ClO41-
	Hypobromite	BrO	BrO1-
	Bromite	BrO2	BrO21-
	Bromate	BrO3	BrO31-
	Perbromate	BrO4	BrO41-
	Hypoiodite	IO	IO1-
	Iodite	IO2	IO21-
	Iodate	IO3	IO31-
	Periodate	IO4	IO41-
	Hydroxide	OH	OH1-
	Nitrite	NO2	NO21-
	Nitrate	NO3	NO31-
	Bicarbonate [Hydrogen carbonate]	HCO3	HCO31-
	Bisulfite [Hydrogen sulfite]	HSO3	HSO31-
	Bisulfate [Hydrogen sulfate]	HSO4	HSO41-
	Dihydrogen phosphate	H2PO4	H2PO41-
	Cyanide	CN	CN1-
	Cyanate	CNO	CNO1-
	Thiocyanate	SCN	SCN1-
	Permanganate	MnO4	MnO41-
	Acetate	C2H3O2	C2H3O21-
Divalent [2]	Peroxide	O2	O22-
	Carbonate	CO3	CO32-
	Sulfite	SO3	SO32-
	Sulfate	SO4	SO42-
	Thiosulfate	S2O3	S2O32-
	Biphosphate [Hydrogen phosphate]	HPO4	HPO42-
	Chromate	CrO4	CrO42-
	Dichromate	Cr2O7	Cr2O72-
	Oxalate	C2O4	C2O42-
Trivalent [3]	Phosphite	PO3	PO33-
	Phosphate	PO4	PO43-

Sample Questions

Question 1: Find the number of atoms present in an H₂S molecule and a PO₄^3- ion.

Answer:

In H₂S molecule, there are two atoms of hydrogen (H) and one atom of sulphur (S). Hence three atoms of H and S totally present in H₂S molecule. However, in PO₄^3- ion, there are one atom of phosphorus (P )and four atoms of oxygen (O) hence five atoms of P and O totally present in PO₄^3- ion.

Question 2: Calculate the molecular mass of each species given as,

H₂O, Cl₂, CO₂ and CH₄

Answer:

The molecular masses of the given species are determined as shown below:

• The molecular mass of H₂O =  2 × atomic mass of H + 1 × atomic mass of oxygen = 2 × 1  + 1 × 16  = 2 + 16 = 18 u.
• The molecular mass of Cl₂ =  2 × atoms atomic mass of Cl = 2 × 35.5 = 71 u.
• The molecular mass of CO₂ = 1 × atoms atomic mass of C + 2 × atomic mass of O = 1 × 12 + 2 × 16  = 12 + 32 = 44 u.
• The molecular mass of CH₄ = 1 × atoms atomic mass of C + 4 × atomic mass of H = 1 × 12 + 4 × 1 = 16 u.

Question 3: What are polyatomic ions? Give examples.

Answer:

Polyatomic ions are the ions that contain more than one atom but they are as a single unit. Examples of polyatomic ions are : CO₃^2-, H₂PO^4–.

Question 4: Write the formulae of Sodium oxide and Aluminum chloride.

Answer:

The formula of Sodium oxide can be calculated as,

Valency of Sodium = 1 

Valency of Oxygen  = 2

Hence, the formula of sodium oxide is Na₂O.

And the formula of Aluminum chloride can be calculated as,

Valency of Aluminum = 3

Valency of chloride = 1

Hence, the formula of Aluminum chloride is AlCl₃.

Question 5: If 1 mole of carbon (C) atoms weighs 12 grams, what will be the mass of 1 atom of carbon?

Answer:

Given that, 

One mole of carbon (C) = 12 g weighs

one mole of carbon atoms = 6.022 × 10²³

Molecular mass of carbon atoms = An atom of carbon mass = 12 g

Hence, mass of 1 carbon atom = 12 / 6.022 × 10²³  = 1.99 ×10^-23 g