Atomic Structure — Set 1
Chemistry · परमाणु संरचना · Questions 1–10 of 50
Which subatomic particle was discovered by J.J. Thomson using the cathode ray tube experiment?
Correct Answer: A. Electron
• **Electron** = The negatively charged particle discovered through cathode ray experiments. • **1897** — Thomson identified it as the first subatomic particle, proving atoms are divisible. • 💡 Wrong-option analysis: B: Proton was discovered later by Rutherford; C: Positron is the antimatter counterpart of electron; D: Neutron was identified even later by Chadwick.
What is the primary reason an atom is electrically neutral under normal conditions?
Correct Answer: D. Equal number of protons and electrons
• **Charge balance** = Protons are positive and electrons are negative; equal numbers cancel to zero net charge. • **1:1 ratio** — Every neutral atom must have exactly one electron per proton for electrical neutrality. • 💡 Wrong-option analysis: A: Neutrons are neutral and do not affect charge balance; B: Electrons definitely carry negative charge; C: Neutron count does not determine atomic charge.
Which component of the atom contains almost all of its mass but occupies very little space?
Correct Answer: B. Nucleus
• **Nucleus** = Protons and neutrons make up the nucleus; electrons contribute negligible mass. • **99.9%** — The nucleus is incredibly dense, containing nearly all atomic mass in a tiny central region. • 💡 Wrong-option analysis: A: Electron cloud is vast and diffuse, contributes little mass; C: Orbits are just pathways where electrons move; D: Valence shell refers only to outermost electrons.
What name is given to atoms of the same element that have different numbers of neutrons?
Correct Answer: C. Isotopes
• **Isotopes** = Same element (same atomic number) but different numbers of neutrons, so different mass numbers. • **Examples** — Carbon-12 and Carbon-14 are isotopes of carbon, both with 6 protons but different neutrons. • 💡 Wrong-option analysis: A: Isotones have same neutron count but different protons; B: Isobars have same mass number but different atomic numbers; D: Isomers are molecular structures not atomic variants.
Which quantum number determines the primary energy level or shell of an electron?
Correct Answer: C. Principal quantum number
• **Principal quantum number (n)** = Denoted as n = 1, 2, 3..., it defines which electron shell (K, L, M, N) an electron occupies. • **Size and energy** — Higher n values mean electrons are farther from nucleus with higher energy levels. • 💡 Wrong-option analysis: A: Azimuthal quantum number defines orbital shape (s, p, d, f); B: Magnetic quantum number specifies orbital orientation; D: Spin quantum number indicates electron spin direction.
What is the maximum number of electrons that can be accommodated in the 'M' shell?
Correct Answer: C. 18
• **M shell = n=3** = Using the formula 2n^2, we get 2 * (3)^2 = 2 * 9 = 18 electrons maximum. • **Third shell** — M shell is the third electron shell from nucleus, holding more electrons than L or K shell. • 💡 Wrong-option analysis: A: 8 electrons is capacity of L shell (n=2); B: 2 electrons is capacity of K shell (n=1); D: 32 electrons exceeds any stable shell capacity.
Who is credited with the discovery of the neutron in 1932?
Correct Answer: D. James Chadwick
• **James Chadwick** = Bombarded beryllium with alpha particles and observed the neutral particle escaping. • **1932** — Discovered the neutron, completing our understanding of nuclear structure with proton + neutron + electron. • 💡 Wrong-option analysis: A: Rutherford discovered proton and the nuclear model; B: Dalton proposed atomic theory but not neutron; C: Bohr contributed atomic theory but not neutron discovery.
Which of the following describes 'Isobars' in chemistry?
Correct Answer: D. Same mass number, different atomic number
• **Isobars** = Different elements with identical mass numbers (sum of protons + neutrons is equal). • **Example: Calcium-40 and Argon-40** — Both have mass number 40 but different atomic numbers (20 vs 18). • 💡 Wrong-option analysis: A: Same electrons would be same element; B: Same neutrons (isotones) is different classification; C: This describes isotopes, not isobars.
The 'Uncertainty Principle', which states it is impossible to know both position and momentum exactly, was proposed by?
Correct Answer: D. Werner Heisenberg
• **Werner Heisenberg** = Stated Δx * Δp >= h/4π where greater certainty in position means less certainty in momentum. • **1927** — Formulated as a fundamental law of quantum mechanics, not a limitation of measurement tools. • 💡 Wrong-option analysis: A: Einstein developed relativity theory; B: Schrödinger developed wave equation; C: Planck contributed quantization concept; none proposed uncertainty principle.
What is the charge of an Alpha (α) particle?
Correct Answer: A. +2
• **Alpha particle = Helium nucleus** = Contains 2 protons (each +1) and 2 neutrons (neutral). • **Net charge = +2** — Two protons give +2 charge; no electrons present to neutralize it. • 💡 Wrong-option analysis: B: +1 charge would be single proton only; C: -1 is electron charge, not alpha particle; D: 0 would require neutrons only.