(a) What is an ion ? How is an ion formed ? Explain with the help of two examples of different ions.(b) The valencies (or charges) of some of the ions are given below : Ion Valency (Charge) Ion Valency (Charge)Sodium ion 1+ Bromide ion 1–Ammonium ion 1+ Hydroxide ion 1–Calcium ion 2+ Sulphate ion 2–Lead ion 2+ Phosphate ion 3–Using this information, write down the formulae of the following compounds :(i) Sodium phosphate(ii) Ammonium sulphate(iii) Calcium hydroxide(iv) Lead bromide

Academic Chemistry NCERT Class 9

(a) An ion is an electrically-charged atom or molecule in which the total number of electrons is not equal to the total number of protons.

It is formed when a neutral atom loses or gains electrons. This loss or gain of electrons imparts a charge (positive or negative) to the neutral atom making it an ion.

There are two different types of ions:

If a neutral atom loses an electron, an overall positive charge is imparted to the atom and it becomes a positively-charged ion or cation. It has less number of electrons than the total number of protons as compared to the neutral atom.

Example: Sodium readily loses an electron to become a positively-charged sodium ion (Na⁺).

If a neutral atom gains an electron, an overall negative charge is imparted to the atom and it becomes a negatively-charged ion or anion. It has more no of electrons than the number of protons compared to the neutral atom.

Example: Chlorine readily gains an electron to become a negatively-charged chloride ion (Cl^-).

(b) The formula of the given compounds are given below:

(i) Sodium phosphate	Na₃PO₄
(ii) Ammonium sulphate	(NH₄)₂SO₄
(iii) Calcium hydroxide	Ca(OH)₂
(iv) Lead bromide	PbBr₂

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Updated on: 10-Oct-2022

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