Atoms Molecules and Structure of the Atom (Class 9)

What this worksheet covers

This is an eighteen-question practice worksheet on atoms, molecules and structure of the atom for Class 9, written to the pattern of the SOF National Science Olympiad (NSO). It carries 24 marks and is designed to be attempted in 45 minutes, split across three sections — Scientific Reasoning, Applying What You Know and Achievers Section. Every question is multiple choice with exactly one correct option, and every one comes with a fully worked solution rather than just an answer key — so a child who gets one wrong can see where the reasoning went astray instead of only learning that it did.

Skills your child practises

• Applied problem-solving on atoms, molecules and structure of the atom at Class 9 level
• Working accurately against a time limit, as the real paper demands
• Scientific Reasoning questions in the olympiad format
• Applying What You Know questions in the olympiad format
• Achievers Section questions in the olympiad format
• Reading worked solutions to find the step that went wrong

How to use it

1. 1There are 18 questions in this worksheet. All questions are compulsory.
2. 2Each question has four options (A), (B), (C) and (D). Exactly one option is correct.
3. 3Questions 1–15 carry 1 mark each. Questions 16–18 (Achievers Section) carry 3 marks each.
4. 4There is no negative marking. Study every diagram and table carefully.

Sample questions from this worksheet

A preview of what's inside Atoms Molecules and Structure of the Atom (Class 9) — the PDF has the full set, and every question comes with a worked solution like these.

1. 1. The law of conservation of mass states that in a chemical reaction, mass is _______.

   • ACreated
   • BDestroyed
   • CNeither created nor destroyedcorrect
   • DConverted into energy
   Solution: In an ordinary chemical reaction the atoms are only rearranged, so the total mass of the products equals the total mass of the reactants — mass is neither created nor destroyed. This is why chemical equations must be balanced.

2. 2. The chemical formula of calcium chloride is _______.

   • ACaCl₂correct
   • BCaCl
   • CCa₂Cl
   • DCa₂Cl₃
   Solution: Calcium has valency 2 and chlorine valency 1, so two chloride ions are needed to balance one calcium ion: CaCl₂. Crossing over the valencies is the quickest way to write such formulae.

3. 3. Assertion (A): The atomic mass of chlorine is 35.5 u, although no single chlorine atom has that mass.Reason (R): Naturally occurring chlorine is a mixture of two isotopes, and the value quoted is their weighted average.

   • ABoth A and R are true and R is the correct explanation of Acorrect
   • BBoth A and R are true but R is not the correct explanation of A
   • CA is true but R is false
   • DA is false but R is true
   Solution: Chlorine occurs as Cl-35 and Cl-37 in roughly a 3 : 1 ratio, giving an average of (3 × 35 + 1 × 37) ÷ 4 = 35.5 u. Every individual atom has mass 35 u or 37 u, never 35.5 u. Both statements are true and the isotope mixture is exactly why the average is fractional.