Valency MCQs Quiz | Class 9

Test your understanding of Class IX Science (Code 086) Unit I: Matter – Its Nature and Behaviour with this quiz on Valency. Topics include definition, combining capacity, examples, and formula writing. Submit your answers to view your score and download the solution PDF.

Understanding Valency

Valency is a fundamental concept in chemistry that explains how atoms interact to form compounds. It is defined as the combining capacity of an atom of an element. In simpler terms, it determines how many hands an atom has to hold onto other atoms.

  • Definition: The number of electrons an atom needs to gain, lose, or share to achieve a stable octet (8 electrons in the outermost shell) or duplet (for Hydrogen/Helium).
  • Valence Electrons: The electrons present in the outermost shell of an atom are called valence electrons. These determine the valency.

How to Determine Valency

The valency of an element depends on the number of valence electrons:

  • 1 to 4 Valence Electrons: Valency equals the number of valence electrons. (e.g., Sodium has 1 valence electron, so valency is 1).
  • 5 to 7 Valence Electrons: Valency equals 8 minus the number of valence electrons. (e.g., Oxygen has 6 valence electrons, so valency is 8 – 6 = 2).
  • 8 Valence Electrons: Valency is 0 (Noble gases).

Common Valencies (Reference Table)

Element Symbol Valency
Hydrogen H 1
Oxygen O 2
Carbon C 4
Sodium Na 1
Magnesium Mg 2
Chlorine Cl 1

Writing Chemical Formulas

To write the formula of a compound, we use the method of “crossing over” valencies:

  1. Write the symbols of the constituent elements side by side.
  2. Write their valencies below the respective symbols.
  3. Cross over the valencies to become the subscript of the other element.
  4. Example: Magnesium Chloride. Symbols: Mg, Cl. Valencies: Mg (2), Cl (1). Cross over -> Mg1Cl2 -> MgCl2.

Practice Questions

Try these additional questions for practice:

  • 1. Why does Helium have zero valency? (Ans: It has a stable duplet shell).
  • 2. What is the formula for Aluminium Oxide? (Ans: Al2O3).
  • 3. Which element shows variable valency of 2 and 3? (Ans: Iron).
  • 4. Find the valency of Nitrogen (Atomic number 7). (Ans: 3).
  • 5. What is the valency of the sulphate radical? (Ans: 2).