Atomic Foundations of Matter MCQs

Q1. Why is the atomic mass of chlorine about 35.5, not a whole number?

• A.it is the average mass of its isotopes in their natural proportions
• B.chlorine has half a proton
• C.chlorine atoms are unstable
• D.electrons add 0.5 to the mass

Answer: A. it is the average mass of its isotopes in their natural proportions

Q2. Which property would be the SAME for two isotopes of the same element?

• A.the way they react chemically
• B.their mass
• C.their number of neutrons
• D.their density

Answer: A. the way they react chemically

NCERT fact (chemistry, chapter 'Journey Inside the Atom'): Isotopes have the same atomic number Z but different mass number A, since A = protons + neutrons; comparing the given pair => only the neutron count differs while the chemistry is identical; hence the chosen option = the way they react chemically. Why option A fits - Correct. Distractor analysis: option B) 'their mass' is incorrect: Isotopes differ in mass; option C) 'their number of neutrons' doesn't hold - Isotopes differ in neutron number. Final answer - A) the way they react chemically.

Q3. Which statement correctly contrasts isotopes and isobars?

• A.isotopes: same Z, different A; isobars: same A, different Z
• B.isotopes: same A, different Z; isobars: same Z, different A
• C.both have the same Z and the same A
• D.both are always radioactive

Answer: A. isotopes: same Z, different A; isobars: same A, different Z