Atomic Structure — Set 2
Chemistry · परमाणु संरचना · Questions 11–20 of 50
Which subatomic particle is the lightest in weight?
Correct Answer: B. Electron
• **Electron mass** = Approximately 1/1836th the mass of a proton (0.000549 amu vs 1.007 amu). • **Negligible contribution** — Electrons make almost no contribution to atomic mass, why mass number counts only nucleus. • 💡 Wrong-option analysis: A: Proton is ~1836 times heavier than electron; C: Alpha particle (4 nucleons) is thousands of times heavier; D: Neutron is only slightly lighter than proton.
The atomic number of an element is always equal to the number of?
Correct Answer: D. Protons
• **Atomic number (Z)** = Defined as total number of protons in nucleus; uniquely identifies each element. • **Determining identity** — Change proton count and you change the element entirely; neutron changes create isotopes instead. • 💡 Wrong-option analysis: A: Nucleons include both protons and neutrons; B: Valence electrons vary in isotopes and ions; C: Neutron count varies even within same element (isotopes).
According to Bohr's model, what happens when an electron jumps from a lower energy level to a higher one?
Correct Answer: B. Energy is absorbed
• **Excitation process** = Moving to higher shell requires energy input; electron in excited state away from nucleus. • **Energy difference absorbed** — Specific energy photons (light) absorbed; amount equals difference between orbital energy levels. • 💡 Wrong-option analysis: A: Energy level change always involves energy change; C: Energy emitted happens when electron falls to lower level; D: Atoms do not become neutrons.
What are 'Isotones' defined as in atomic physics?
Correct Answer: D. Atoms with the same number of neutrons
• **Isotones** = Different elements but identical neutron count; example Carbon-14 and Oxygen-16 both have 8 neutrons. • **Different proton and atomic numbers** — Isotones are NOT same element; they are different elements grouped by neutron count. • 💡 Wrong-option analysis: A: Different atomic numbers mean different chemical properties; B: Same mass number defines isobars not isotones; C: Same protons defines same element (isotopes).
Which scientist proposed the first modern atomic theory based on scientific evidence in 1803?
Correct Answer: C. John Dalton
• **John Dalton** = Proposed atoms are indivisible and unbreakable; each element has atoms of unique mass. • **1803-1808** — Formulated from law of conservation of mass, law of constant composition using experimental evidence. • 💡 Wrong-option analysis: A: Avogadro proposed number hypothesis later; B: Lavoisier studied gas laws not atomic structure; D: Democritus (ancient) proposed philosophy not scientific theory.
The dual nature of matter (wave-particle duality) was proposed by which physicist?
Correct Answer: A. Louis de Broglie
• **Louis de Broglie** = Proposed wavelength λ = h/p where all moving matter has wave properties. • **1924 hypothesis** — Electrons and particles act as both particles and waves depending on how measured. • 💡 Wrong-option analysis: B: Born developed probability interpretation of wave function; C: Dirac unified quantum mechanics and relativity; D: Fermi developed statistical methods and beta decay theory.
Which region of the atom is responsible for chemical bonding and reactivity?
Correct Answer: C. Valence electrons
• **Valence electrons** = Electrons in outermost shell involved in forming bonds with other atoms. • **Chemical behavior determined** — Elements in same group have same number of valence electrons, so similar chemical properties. • 💡 Wrong-option analysis: A: Nucleus is too far inside, does not bond directly; B: Neutrons are neutral, do not participate in bonding; D: Inner electrons are shielded, not available for bonding.
In the notation $_6C^{14}$, what does the number 14 represent?
Correct Answer: B. Mass number
• **Mass number (A) = 14** = Sum of protons and neutrons in nucleus (6 + 8 = 14). • **Subscript 6 = atomic number** — Carbon always has 6 protons; the superscript 14 differentiates carbon isotopes. • 💡 Wrong-option analysis: A: Electrons in neutral atom equals protons not mass number; C: Protons is subscript (6) not superscript; D: Atomic number is shown as subscript, not superscript.
Which rule states that no two electrons in an atom can have the same set of four quantum numbers?
Correct Answer: A. Pauli Exclusion Principle
• **Pauli Exclusion Principle** = Maximum two electrons per orbital must have opposite spins (paired electrons). • **Four quantum numbers** — n, l, m_l, and m_s must differ for any two electrons in same atom. • 💡 Wrong-option analysis: B: Aufbau deals with order of orbital filling; C: Octet Rule about electron shell stability; D: Hund's Rule about parallel spins in degenerate orbitals.
The 'Gold Foil Experiment' led to the discovery of which atomic feature?
Correct Answer: D. Nucleus
• **Nuclear model** = Rutherford observed alpha particles bouncing back; proved dense positive center exists. • **1909 experiment** — Most particles passed through; some deflected 180° indicating concentrated positive charge. • 💡 Wrong-option analysis: A: Neutron discovered later by Chadwick; B: Electron known before Rutherford; C: Orbitals part of later quantum model, not gold foil discovery.