BPSC CCE Prelims
Atomic Structure Previous Year Questions (PYQs)

Detailed Explanation:

J.J. Thomson proposed the first atomic model in 1897, known as the plum pudding model, with electrons embedded in a positively charged sphere.

E. Goldstein discovered canal rays, Rutherford proposed the nuclear model in 1911, and Niels Bohr introduced the planetary model with fixed orbits in 1913.

Detailed Explanation:

Dalton's atomic theory (1808) explained three fundamental laws of chemical combination:

✅ Law of conservation of mass: Atoms are neither created nor destroyed in reactions, so total mass remains constant.

✅ Law of constant composition: A compound always contains the same elements in fixed proportions by mass.

✅ Law of multiple proportions: When two elements form multiple compounds, mass ratios are simple whole numbers.

❌ Law of radioactivity: Radioactivity (discovered by Becquerel, 1896) involves nuclear changes, not explained by Dalton's indivisible atom concept.

Detailed Explanation:

Alpha particles (α) are positively charged particles emitted during radioactive decay, consisting of 2 protons + 2 neutrons (essentially a helium nucleus).

Beta rays (β) are negatively charged electrons, while cathode rays are also streams of electrons, both carrying negative charge.

Therefore, Option 3 (Alpha ray) is the correct answer.

Detailed Explanation:

❌ Statement 1 – Incorrect: Diamond is an electrical insulator (no free electrons), while graphite is a good electrical conductor (has delocalized electrons in layers).

❌ Statement 2 – Incorrect: Diamond has a tetrahedral crystal structure (each carbon bonded to 4 others in 3D), while graphite has a hexagonal layered structure (each carbon bonded to 3 others in 2D layers).

❌ Statement 3 – Incorrect: Diamond is the hardest natural substance (Mohs scale 10), while graphite is soft and slippery (Mohs scale 1-2) due to weak van der Waals forces between layers.

Both are allotropes of carbon with vastly different physical properties due to different atomic arrangements.

Detailed Explanation:

The nucleus of an atom contains protons (positively charged) and neutrons (neutral), forming the dense central core discovered by Rutherford in 1911.

Electrons orbit outside the nucleus in electron clouds—they are not part of the nuclear structure.

Therefore, only protons and neutrons constitute the nucleus, making Option 2 the correct answer.

Detailed Explanation:

The mass number (A) of a nucleus is defined as the sum of protons and neutrons in the nucleus. It is a count (dimensionless integer), not the actual mass in kilograms or atomic mass units.

Option 2 incorrectly states "total mass" (which would have units); Option 3 is false—mass number is close to but not always more than atomic weight; Option 4 is absurd—mass number is always greater than atomic number (Z).

Therefore, Option 1 is the correct answer.

Detailed Explanation:

Isotopes are atoms of the same element with identical atomic numbers (same number of protons) but different mass numbers due to varying numbers of neutrons. For example, Carbon-12 and Carbon-14 both have 6 protons but differ in neutron count (6 vs 8), resulting in different mass numbers (12 vs 14).

The number of protons and electrons remains constant within isotopes of an element, as changing protons would create a different element entirely, and neutral atoms maintain equal protons and electrons.

Therefore, isotopes differ only in mass number, making Option 2 the correct answer.

Detailed Explanation:

Option 1 is Incorrect: Atoms are electrically neutral because protons (positive) are in the nucleus and electrons (negative) orbit outside it—electrons are NOT inside the nucleus.

Options 2, 3, and 4 are Correct: Electrons are negatively charged with negligible mass; atomic number equals proton count; isotopes have applications in nuclear energy, medicine (radiotherapy), and carbon-14 dating.

Therefore, more than one statement is incorrect/correct, making Option 5 the correct answer.