Question 1

Radioactivity is best defined as?

Accepted Answer

Spontaneous emission of radiation from an unstable nucleus. Option D is correct because an unstable nucleus emits radiation on its own. This process is a nuclear change, not a chemical change. The rate of decay is not controlled by temperature or pressure in basic cases.

Question 2

The atomic number of an element represents the number of?

Accepted Answer

Protons. Option B is correct because atomic number equals the number of protons in the nucleus. It uniquely identifies the element in chemistry and physics. In a neutral atom, electrons equal protons.

Question 3

Isotopes are atoms of the same element having?

Accepted Answer

Same atomic number but different mass numbers. Option C is correct because isotopes have the same atomic number but different mass numbers. They have the same number of protons but different numbers of neutrons. Many isotopes are stable while some are radioactive.

Question 4

Mass defect in a nucleus mainly explains?

Accepted Answer

Nuclear binding energy. Option D is correct because mass defect is converted into binding energy. A nucleus has slightly less mass than its separated nucleons. The relation E = mc^2 connects mass loss to energy.

Question 5

The SI unit of activity of a radioactive sample is?

Accepted Answer

Becquerel (Bq). Option C is correct because becquerel means one decay per second. Activity measures how fast nuclei are disintegrating. Curie is an older non-SI unit of activity.

Nuclear & Radioactivity — Set 1