Atomic Structure — Set 5
Chemistry · परमाणु संरचना · Questions 41–50 of 50
Which principle states that electrons fill lower-energy orbitals first before moving to higher ones?
Correct Answer: D. Aufbau Principle
• **Aufbau Principle** = 'Building up' principle; electrons fill in order of increasing orbital energy (1s, 2s, 2p, 3s...). • **Lowest energy first** — Atoms naturally seek lowest energy state; filled orbitals more stable than unfilled. • 💡 Wrong-option analysis: A: Hund's Rule about parallel spins in degenerate orbitals; B: Pauli's about no two electrons same quantum numbers; C: Le Chatelier about equilibrium shifts.
What is the atomic number of Carbon?
Correct Answer: C. 6
• **Carbon atomic number = 6** = Has 6 protons in nucleus; defines carbon as element. • **Common isotope C-12** — Most abundant is Carbon-12 with 6 neutrons; gives mass number 12. • 💡 Wrong-option analysis: A: 4 is Beryllium's atomic number; B: 14 is mass number of C-14 isotope; D: 12 is mass number not atomic number.
Which particle is identical to a Helium nucleus?
Correct Answer: A. Alpha particle
• **Alpha particle = ^4He nucleus** = 2 protons + 2 neutrons; emitted in alpha decay. • **Helium-4 nucleus** — When alpha particles slow down in matter, they capture electrons and become neutral helium atoms. • 💡 Wrong-option analysis: B: Gamma ray is electromagnetic radiation; C: Beta particle is electron or positron; D: Neutron is single particle, not alpha particle.
Who is considered the 'Father of Nuclear Physics'?
Correct Answer: A. Ernest Rutherford
• **Ernest Rutherford** = Discovered alpha/beta radiation, nuclear model, transmutation of elements. • **Gold foil experiment** — Proved existence of nucleus through alpha particle scattering studies (1909). • 💡 Wrong-option analysis: B: Marie Curie discovered radioactivity and isotopes but came before Rutherford; C: Einstein developed relativity; D: Newton worked on gravity/motion, ancient era.
What is the total number of electrons in a Magnesium ion ($Mg^{2+}$) if its atomic number is 12?
Correct Answer: A. 10
• **Mg^2+ means 2 electrons lost** = Neutral Mg has 12 electrons; losing 2 gives 12 - 2 = 10 electrons. • **Cation formation** — +2 charge indicates 2 more protons than electrons (12 protons, 10 electrons). • 💡 Wrong-option analysis: B: 24 doubles atomic number incorrectly; C: 14 adds instead of subtracts electrons; D: 12 is neutral atom not cation.
Which of the following elements has zero neutrons in its nucleus?
Correct Answer: D. Hydrogen (Protium)
• **Protium (^1H) nucleus** = Single proton only; no neutrons; lightest stable nucleus. • **Pure proton nucleus** — Unique among all elements in having no neutrons; basis of hydrogen atom. • 💡 Wrong-option analysis: A: Carbon minimum 6 neutrons; B: Helium minimum 2 neutrons; C: Oxygen minimum 8 neutrons; all others have neutrons.
What is the charge of a neutron?
Correct Answer: D. 0
• **Neutron charge = 0** = Electrically neutral; no electric field to deflect it in radiation. • **Composite particle** — Made of quarks (1 up, 2 down) that cancel to net zero charge overall. • 💡 Wrong-option analysis: A: -1 is electron charge; B: +1 is proton charge; C: +2 is alpha particle charge.
The name 'atom' is derived from a Greek word meaning?
Correct Answer: B. Indivisible
• **Greek 'atomos'** = 'a' (not) + 'tomos' (cut); means cannot be cut or divided. • **Historical naming** — Ancient Greeks thought atoms were indivisible building blocks, though later found divisible. • 💡 Wrong-option analysis: A: Tiny refers to size not word meaning; C: Invisible refers to visibility not etymology; D: Spherical refers to shape not origin of word.
How many electrons are present in the outermost shell of Noble Gases (except Helium)?
Correct Answer: A. 8
• **Octet rule** = Noble gases (Ne, Ar, Kr, Xe) have 8 valence electrons in outermost shell. • **Chemical inertness** — Complete valence shell (octet) makes noble gases extremely stable and unreactive. • 💡 Wrong-option analysis: B: 4 is half-filled p orbital; C: 6 is p orbitals with 2 vacancies; D: 2 is Helium only (special case, exception).
Which subatomic particle was the last to be discovered among the following?
Correct Answer: A. Neutron
• **Neutron discovered 1932** = Chadwick discovered it last among these particles; hardest to detect (no charge). • **Discovery timeline** — Electron (1897), Proton (1919), Photon (1905), Neutron (1932) in order. • 💡 Wrong-option analysis: B: Proton discovered 1919 by Rutherford; C: Photon concept emerged 1905 from Planck/Einstein; D: Electron discovered 1897 by Thomson.