Chapter 3: Atoms and Molecules
Learning Notes (Set 2) for Class 9 Science

Question 1: How is the molecular mass of a substance calculated?

Answer: By summing the atomic masses of all the atoms in a molecule of that substance.

Question 2: What is a mole?

Answer: A unit of amount of substance, containing 6.022 x 10²³ particles (atoms, molecules, or ions).

Question 3: What is the formula unit mass?

Answer: The sum of the atomic masses of all atoms in a formula unit of an ionic compound.

You are on Chapter 3: Atoms and Molecules (Learning Notes for Class 9 Science) Set 2

Question 4: What is Avogadro's number?

Answer: The number of particles (atoms, molecules, or ions) in one mole of any substance, equal to 6.022 x 10²³.

Question 5: What is the valency of an atom?

Answer: The combining capacity of an atom or the number of electrons an atom can lose, gain, or share to form a bond.

Question 6: What are ions?

Answer: Charged particles formed when atoms gain or lose electrons.

Question 7: What is the difference between a cation and an anion?

Answer: A cation is a positively charged ion, while an anion is a negatively charged ion.

Question 8: What is the molar mass?

Answer: The mass of one mole of a substance, numerically equal to its atomic or molecular mass but expressed in g/mol.

You are on Chapter 3: Atoms and Molecules (Learning Notes for Class 9 Science) Set 2

Question 9: What is the chemical formula for water?

Answer: H2O.

Question 10: What is an atomicity?

Answer: The number of atoms constituting a molecule.