Class 9 Science Chapter 3 Question Answers - Atoms and Molecules

Q1: What is the building block of all matter?
Ans: Atom is the building block of all matter.

Q2: Give the derivation source of the symbol of sodium (Na).
Ans: The symbol of ‘Na’ for sodium is derived from its Latin name ‘Natrium’.

Q3: What is the measuring unit of atomic radius?
Ans: Nanometer (nm) is the measuring unit of atomic radius.

Q4: Who stated the Law of Constant Proportion?
Ans: Joseph Louis Proust stated the Law of Constant Proportion.

Q5: What is the full form of IUPAC?
Ans: International Union of Pure and Applied Chemistry.

Q6: Name the instrument which produces an image of the surface of an element that shows atoms.
Ans: Scanning tunnelling microscope.

Q7: Who was the first scientist to give the concept of the formation of compounds?
Ans: Antoine L. Lavoisier gave the concept of the formation of compounds.

Q8: Give the full form of amu?
Ans: The full form of amu is atomic mass unit.

Q9: State the law of conservation of mass.
Ans: This law states that mass can neither be created nor destroyed in a chemical reaction. That is, the mass of reactants is always equal to the mass of products.

Q10: Who introduced the word ‘Mole’?
Ans: ‘Wilhelm Ostwald’ introduced the word ‘Mole’.

Q11: What is Avogadro's Constant?
Ans: The number of particles present in one mole of any substance is fixed with a value of 6.022 × 10²³.

Q12: Give two examples of triatomic molecules.
Ans: Carbon dioxide (CO₂) and water (H₂O).

Q13: Define valency.
Ans: The combining power of an element to attain the noble gas configuration is called valency. Or, it is defined as the number of electrons lost or gained by an atom to acquire noble gas configuration.

Q14: Name any two monatomic atoms.
Ans: Sodium, Aluminium.

Q15: What is the latest short form of the atomic mass unit?
Ans: The latest short form of the atomic mass unit is u, according to IUPAC.

Q16: What do we get if 3 atoms of oxygen unite into a molecule, instead of the usual 2?
Ans: We get (O₃) ozone.

Q17: How does an atom exist?
Ans: Atom exists in the form of atom, molecule, or ions.

Q18: Give difference between 2H and H₂.
Ans: 2H indicates 2 atoms of hydrogen, and H₂ indicates one molecule of hydrogen.

Q19: How did the scientist lay the foundation of chemical sciences? Name the scientist.
Ans: Antoine Laurent Lavoisier laid the foundation of chemical science by establishing two important laws of chemical combination.

Q20: Define law of conservation of mass.
Ans: Law of conservation of mass states that, ‘Mass is neither created nor destroyed in a chemical reaction.’

Q21: Define law of constant proportion.
Ans: Law of constant proportion states that, ‘In a pure chemical substance, the elements are always present in definite proportions by mass.’

Q22: Which organization approves the names of elements all over the world?
Ans: International Union of Pure and Applied Chemistry (IUPAC).

Q23: Write the symbols of tungsten and iron.
Ans: (i) Tungsten (W) and (ii) Iron (Fe)

Q24: Name two atoms which exist as independent atoms.
Ans: Noble gases such as argon (Ar) and helium (He) exist as independent atoms.

Q25: Define atomicity.
Ans: The number of atoms present in one molecule of an element or a compound is known as its atomicity.