CBSE – 9th Standard Science

Class 9 Question Bank Chapter 3 – Atoms and molecules

Q1. Who established laws of chemical combination?

Ans. Antoine L. Lavoisier laid the foundation of chemical sciences by establishing two important laws of chemical combination.

Q2. What is law of Conservation of Mass?

Ans. Law of conservation of mass states that mass can neither be created nor destroyed in a chemical reaction.

Q3. What is law of constant proportion?

Ans. In a chemical substance the elements are always present in definite proportions by mass

Q4. What is an Atom according to Dalton’s theory?

Ans. All matter, whether an element, a compound or a mixture is composed of small particles called atoms.

Q5. What is an Atom?

Ans. The building blocks of all matter are atoms.

Q6. What is IUPAC?

Ans. International Union of Pure and Applied Chemistry

Q7. What is a molecule?

Ans. A molecule is a group of two or more atoms chemically bonded together.

Q8. What are molecules of element?

Ans. The molecules of an element are constituted by the same type of atoms.

Q9. What is Diatomic Molecule?

Ans. A molecule consisting of two atoms of any atom is known as a diatomic molecule. Ex. O2

Q10. What do you mean by Atomicity?

Ans. The number of atoms constituting a molecule is known as its atomicity.

Q11. What is an Ion?

Ans. The charged species of compounds are known as ions.

Q12. What is an Anion?

Ans. A negatively charged ion is called an ‘anion’.

Q13. What is a Cation?

Ans. The positively charged ion,is called as ‘cation’.

Q14. What is polyatomic ion?

Ans. A group of atoms carrying a charge is known as a polyatomic ion.

Q15. What do you mean by Valency?

Ans. The combining power of an element is known as its valency.

Q16. What is a Binary Compound?

Ans. The simplest compounds, which are made up of two different elements are called binary compounds.

Q17. What is Formula Unit mass?

Ans. The formula unit mass of a substance is a sum of the atomic masses of all atoms in a formula unit of a compound.

Q18. Define One Mole?

Ans. One mole of any species is that quantity in number having a mass equal to its atomic or molecular mass in grams.

Q19. What is an Avogadro constant or Avogadro Number?

Ans. The number of particles present in 1 mole of any substance is fixed, with a value of 6.022 x 1023. This number is called the Avogadro Constant or Avogadro Number.

Q20. What is Molar mass?

Ans. Mass of 1 mole of a substance is called its molar mass.

Try these :-

Calculate the molecular masses of H2, O2, Cl2, CO2, CH4, C2H6,,C2H4, NH3, CH3OH.

 Calculate the formula unit masses of ZnO, Na2O, K2CO3,

given atomic masses of Zn = 65 u,Na = 23 u, K = 39 u, C = 12 u,and O = 16 u

Define the atomic mass unit.

 Why is it not possible to see an atom with naked eyes?

How many atoms are present in a H2S molecule

Calculate the number of moles for the following:

(i) 52 g of He (finding mole from mass)

(ii) 12.044 × 1023 number of He atoms (finding mole from number of particle)

Calculate the number of particles in each of the following:

(i) 46 g of Na atoms (number from mass)

(ii) 8 g O2 molecules (number of molecules from mass)

(iii) 0.1 mole of carbon atoms (number from given moles)