Important Questions for Class 9 Chapter 4 - Structure of The Atom

Important questions based on NCERT syllabus for Chapter 4 - Structure of The Atom:

Question-1: State the two laws of chemical combination.

Solution: The two laws of chemical combination are

1. Law of conservation of mass:
The law of conservation of mass states that mass in an isolated system is neither created nor destroyed by chemical reactions or physical transformations. According to the law of conservation of mass, the mass of the products in a chemical reaction must equal the mass of the reactants.
2. Law of definite proportions:
The law of definite proportions, also called Proust's law or the law of definite composition, or law of constant composition states that a given chemical compound always contains its component elements in fixed ratio (by weight) and does not depend on its source and method of preparation.

Question-2: What is Variable Valency? Do elements exhibit variable valencies? Give examples.

Solution: The valency is the number of the electrons that an atom gains , loses or even shares during a chemical reaction; that is the valency is the number of electrons that an atom needs to gain or lose in order to achieve noble gas electronic configuration.
Yes, sometimes the same element exhibits one valency in one compound and another valency in another compound. This property is called variable valency.
Some elements show variable valency because of the different electronic configurations. An atom of an element can sometimes lose more electrons than that are present in its valence shell i.e. loss from the penultimate shell and hence exhibit more than 1 or variable valency.
Examples of elements with variable valencies: Copper, Iron, Lead etc.

Question-3: Why was mole concept introduced in writing a chemical reaction equation? Does the number of grams differ from one mole to another mole?

Solution: A chemical reaction equation indicates the number of atoms or molecules directly taking part in the reaction. Therefore, it is more convenient to refer to the quantity of a substance in terms of the number of its molecules or atoms, rather than their masses. So, the mole concept was introduced.
Yes, the number of grams in a mole differs from one substance to another substance.