10. The atomic number of an element is the number of which particles in the nucleus of each atom?
[RRB NTPC CBT-I — 4 Mar 2021, Shift 1]
(A) Protons
(B) Mesons
(C) Neutrons
(D) Electrons
Answer: (A) Protons
Explanation: Atomic number, denoted Z, counts the protons in the nucleus. For an electrically neutral atom this also equals the electron count, but an ion can have a different number of electrons. The proton count remains the defining feature of its element.
9. An isotope of which element is used in the treatment of cancer?
[UPSC NDA — 6 Sept 2020, General Ability Test]
(A) Uranium
(B) Cobalt
(C) Sodium
(D) Iodine
Answer: (B) Cobalt
Explanation: Cobalt-60 emits penetrating gamma radiation and has been used for external-beam cancer radiotherapy. Radioisotopes of iodine are commonly used for certain thyroid disorders instead. Medical use depends upon the kind, energy and location of the radiation emitted.
8. Who explained the phenomenon of the photoelectric effect?
[UPSC NDA — 21 Apr 2019, General Ability Test]
(A) Max Planck
(B) Albert Einstein
(C) Niels Bohr
(D) Ernest Rutherford
Answer: (B) Albert Einstein
Explanation: Einstein explained photoelectric emission by treating light as packets of energy, now called photons. An electron leaves a material only if the photon provides enough energy to overcome the work function. This established crucial evidence for light’s quantum behaviour.
7. What is the radius of an atom used to measure?
[RRB ALP CBT-I — 29 Aug 2018, Shift 1]
(A) Atomic number
(B) Atomic mass
(C) Atomic size
(D) Atomicity
Answer: (C) Atomic size
Explanation: Atomic radius serves as a measure of atomic size, although exact definitions vary for bonded and isolated atoms. Across a period it generally decreases as effective nuclear attraction rises; down a group it usually increases with additional electron shells.
6. The chemical properties of an element chiefly depend on which of the following?
[UPSC CDS General Knowledge — 5 Feb 2017]
(A) Its number of isotopes
(B) Its mass number
(C) Its total number of neutrons
(D) Its number of electrons in the outermost shell
Answer: (D) Its number of electrons in the outermost shell
Explanation: The outer-shell, or valence, electrons participate in ordinary chemical bonding and therefore strongly govern reactivity and compound formation. Isotope abundance and neutron counts can alter mass-dependent behaviour, but they do not usually change an element’s basic valence configuration.
5. Calcium (atomic number 20) and argon (atomic number 18) both have mass number 40. What are they called?
[UPSC CDS General Knowledge — 5 Feb 2017]
(A) Isotones
(B) Isochors
(C) Isobars
(D) Isotopes
Answer: (C) Isobars
Explanation: Isobars are atoms of different elements having identical mass numbers but different proton numbers. Calcium-40 contains twenty protons and twenty neutrons, while argon-40 contains eighteen protons and twenty-two neutrons. Their common nucleon total is forty.
4. Which one of the following statements is correct?
[UPSC NDA & NA (II) — 2015]
(A) Rutherford’s alpha-particle scattering experiment discovered the electron.
(B) J. J. Thomson suggested that the nucleus contains protons.
(C) The atomic number equals the number of protons in the atomic nucleus.
(D) The mass number equals the number of electrons in the shells.
Answer: (C) The atomic number equals the number of protons in the atomic nucleus.
Explanation: Atomic number counts nuclear protons and determines the chemical element. Rutherford’s scattering experiment revealed the nucleus, whereas Thomson discovered the electron and proposed a diffuse positive background. Mass number counts protons plus neutrons, not electrons.
3. Which transition of an electron in a hydrogen atom emits radiation of the shortest wavelength?
[UPSC NDA & NA (I) — 2012]
(A) n = 2 to n = 1
(B) n = 3 to n = 2
(C) n = 4 to n = 3
(D) n = 5 to n = 4
Answer: (A) n = 2 to n = 1
Explanation: Shorter wavelength means greater photon energy, because E = hc/λ. The n = 2 to n = 1 transition produces the largest energy drop among those listed for hydrogen. Therefore it emits the shortest-wavelength radiation.
2. Regarding an atom of a chemical element, what does the magnetic quantum number refer to?
[UPSC Civil Services Prelims — 2003]
(A) Orientation
(B) Shape
(C) Size
(D) Spin
Answer: (A) Orientation
Explanation: The magnetic quantum number, mₗ, distinguishes orientations of orbitals within a particular subshell. Its permitted values run from −l to +l. The azimuthal quantum number l controls orbital shape, while the spin quantum number describes electron spin.
1. Which of the following carries a negative charge?
[45th BPSC Preliminary Examination — 2002]
(A) X-rays
(B) Alpha particles
(C) Beta particles
(D) Gamma rays
Answer: (C) Beta particles
Explanation: In the conventional beta-minus form of nuclear radiation, a beta particle is a high-speed negatively charged electron. Alpha particles are positively charged helium nuclei; X-rays and gamma rays are electrically neutral photons. Beta-plus particles are positrons, so the qualification matters.
