Atomic Structure — Set 4
Chemistry · परमाणु संरचना · Questions 31–40 of 50
Which subatomic particle is not found in the nucleus of a typical atom?
Correct Answer: A. Electron
• **Electrons outside nucleus** = Electrons orbit in electron clouds at distances far from nucleus. • **Nucleons only in nucleus** — Protons and neutrons (nucleons) confined to tiny nucleus at atom center. • 💡 Wrong-option analysis: B: Protons are fundamental nucleon component; C: Neutrons are fundamental nucleon component; D: Nucleons by definition are nucleus constituents (protons + neutrons).
What determines the chemical properties of an element?
Correct Answer: C. Number of valence electrons
• **Valence electrons control bonding** = Outermost electrons responsible for forming chemical bonds. • **Group similarity** — Elements in same periodic group share valence electron count, so similar chemistry. • 💡 Wrong-option analysis: A: Neutron count creates isotopes not chemical change; B: Nucleon count affects mass not chemistry; D: Atomic mass correlates with chemistry but valence electrons are direct cause.
Which scientist developed the 'Plum Pudding' model of the atom?
Correct Answer: B. J.J. Thomson
• **J.J. Thomson (1897)** = Proposed positive 'pudding' with embedded negative electron 'plums' scattered throughout. • **First subatomic model** — Only theory to acknowledge electrons as atom parts; electron discovery was his achievement. • 💡 Wrong-option analysis: A: Rutherford disproved plum pudding with gold foil; C: Bohr came after Rutherford; D: Dalton proposed atoms as indivisible, no internal structure.
What is the maximum number of electrons that can be held in the first energy level (n=1)?
Correct Answer: A. 2
• **First shell = K shell** = Contains only 1s orbital; 2n^2 formula: 2(1)^2 = 2 electrons maximum. • **Hydrogen and Helium** — He with 2 electrons fills K shell completely, making it noble gas. • 💡 Wrong-option analysis: B: 18 is M shell capacity (n=3); C: 8 is L shell capacity (n=2); D: 32 exceeds any stable shell capacity.
Which of the following is true for neutral atoms of the same element?
Correct Answer: D. They have the same number of protons
• **Proton count defines element** = All atoms of element (C, O, H) have identical proton count. • **Isotopes differ in neutrons** — Same element can have different neutron counts but same proton number. • 💡 Wrong-option analysis: A: Different protons = different element entirely; B: Isotopes have different neutron counts; C: Atomic number (proton count) always same for same element.
The mass number of an atom is defined as the sum of?
Correct Answer: D. Protons and neutrons
• **Mass number (A)** = Sum of nucleons: protons (Z) + neutrons (N); denoted as superscript in notation. • **Electrons excluded** — Electrons negligible mass (~1/1836 proton mass); mass number only counts nucleus particles. • 💡 Wrong-option analysis: A: Total would include electrons incorrectly; B: Electrons negligible mass; C: Electrons wrongly included; electrons must be excluded from mass number.
Who proposed that electrons move in discrete orbits with fixed energy?
Correct Answer: A. Niels Bohr
• **Niels Bohr (1913)** = Proposed electrons occupy specific allowed orbits with quantized energy levels. • **Hydrogen spectral success** — Explained hydrogen emission spectrum using energy transitions between allowed orbits. • 💡 Wrong-option analysis: B: Schrödinger developed wave equation and orbitals concept; C: Planck provided photon quantization; D: Chadwick discovered neutron, not orbital theory.
What kind of charge does the nucleus of an atom carry?
Correct Answer: D. Positive
• **Nucleus = positive** = Contains only protons (positive charge) and neutrons (no charge); no electrons. • **Proton number determines charge** — Net nuclear charge = +Ze where Z = atomic number. • 💡 Wrong-option analysis: A: Nucleus never negative; electrons outside carry negative; B: Charge fixed by proton count; C: Neutrons neutral but protons are positive.
Which subatomic particle's mass is approximately equal to that of a proton?
Correct Answer: D. Neutron
• **Neutron mass ≈ Proton mass** = Both approximately 1 amu (1.007 amu proton, 1.009 amu neutron). • **Nucleon pair** — Proton and neutron are the only particles with similar mass, both ~1836× electron mass. • 💡 Wrong-option analysis: A: Positron same mass as electron, ~1/1836 proton; B: Electron is ~1/1836 proton mass; C: Neutrino nearly massless, extremely tiny.
An atom of an element has 11 protons and 12 neutrons. What is its mass number?
Correct Answer: A. 23
• **Mass number (A) = 23** = Sum of nucleons: 11 protons + 12 neutrons = 23. • **Sodium isotope** — This is Sodium-23 (^23Na); atomic number always 11 for Na, mass number identifies isotope. • 💡 Wrong-option analysis: B: 1 is wrong calculation; C: 12 is just neutron count; D: 11 is just proton count (atomic number).