Question

(a) What is the difference between a cation and an anion ? Explain with examples.
(b) The valencies (or charges) of some of the ions are given below :

Ion	Valency (Charge)	Ion	Valency (Charge)
Sodium ion	1+	Nitrate ion	1–
Copper ion	2+	Sulphide ion	2–

Using this information, write down the formulae of :
(i) Sodium sulphide
(ii) Copper nitrate

Answer 1

Cation	Anion
1. Cation is a positively charged ion.	1. Anion is a negatively charged ion.
2. When a neutral atom loses one or more electrons, it develops an overall positive charge and becomes a cation.	2. When a neutral atom gains one or more electrons, it develops an overall negative charge and becomes an anion.
3. A cation has fewer electrons than the neutral (normal) atom of that element.	3. An anion has more electrons than the neutral (normal) atom of that element.
4. Example: Sodium atom readily loses an electron to form a positively charged sodium ion. Na → Na+ + e−	4. Example: Chlorine atom accepts an electron to become a negatively charged chloride ion. Cl + e− → Cl−
Similarly, calcium atom loses two electrons to become a positively charged calcium ion. Thus, the cation formed has two units of positive charge. Ca → Ca2++ 2e−	Oxygen atom readily accepts two electrons to become a negatively charged oxygen ion. Thus, the oxygen anion formed has two units of negative charge. O + 2e− → O2−

(b) Formula of the listed compounds can be derived by crossing the individual valencies over the ions/elements. Therefore, the formula for sodium sulphide and copper nitrate are:
(i) Na₂S
(ii) Cu(NO₃)₂