20. Who argued in the early nineteenth century that each element consists of its own kind of atoms, differing from other elements in mass?
[SSC CGL Tier-I — 24 Jul 2023, Shift 1]
(A) Antoine Lavoisier
(B) John Dalton
(C) Amedeo Avogadro
(D) Robert Boyle
Answer: (B) John Dalton
Explanation: Dalton formulated an influential atomic theory in the early 1800s, proposing distinct atoms for different elements and explaining fixed composition in compounds. Modern science recognizes isotopes, so atoms of one element need not all have identical masses.
19. Which of the following elements has three valence electrons?
[RRB Group D — 7 Oct 2022, Shift 2]
(A) Al
(B) Ca
(C) N
(D) Mg
Answer: (A) Al
Explanation: Aluminium has atomic number 13 and the electronic configuration 2, 8, 3. Its outermost shell therefore contains three electrons. Calcium and magnesium each have two valence electrons, while nitrogen has five.
18. What mainly decides the valency of an atom?
[RRB Group D — 27 Sept 2022, Shift 1]
(A) The number of electrons in its outermost shell
(B) The number of electrons in its L shell
(C) The number of electrons in its innermost shell
(D) The number of electrons in its K shell
Answer: (A) The number of electrons in its outermost shell
Explanation: Valency reflects the combining capacity of an atom and is closely related to its outermost, or valence, electron configuration. Atoms may lose, gain or share electrons to approach stable configurations. Inner-shell electron totals do not generally decide ordinary valency.
17. Arrange B, Ne, K and O in increasing order of atomic number.
[RRB Group D — 27 Sept 2022, Shift 2]
(A) Ne, K, B, O
(B) K, B, O, Ne
(C) B, K, O, Ne
(D) B, O, Ne, K
Answer: (D) B, O, Ne, K
Explanation: The respective atomic numbers are boron 5, oxygen 8, neon 10 and potassium 19. Atomic number denotes the number of protons in an atomic nucleus. Ordering these proton counts from smallest to largest gives B, O, Ne, K.
16. According to Henry Moseley, which property is more fundamental to an element than its atomic mass?
[RRB Group D — 22 Sept 2022, Shift 2]
(A) Atomic number
(B) Atomic density
(C) Atomic radius
(D) Atomic size
Answer: (A) Atomic number
Explanation: Moseley’s X-ray experiments linked elemental identity and ordering to atomic number, later understood as nuclear proton count. Unlike average atomic mass, atomic number uniquely specifies an element. This principle also explains why isotopes share a position in the periodic table.
15. How does the number of valence electrons generally change while moving down a group of the periodic table?
[RRB Group D — 30 Aug 2022, Shift 2]
(A) It decreases.
(B) It increases.
(C) It first increases and then decreases.
(D) It remains constant.
Answer: (D) It remains constant.
Explanation: Elements in the same main-group column have comparable outer electronic configurations and hence typically the same number of valence electrons. Descending the group adds occupied shells, increasing atomic size, but the characteristic outer-electron count remains similar.
14. Which element has the greatest number of valence electrons among F, Al, H and N?
[RRB Group D — 22 Aug 2022, Shift 2]
(A) F
(B) Al
(C) H
(D) N
Answer: (A) F
Explanation: Fluorine has seven valence electrons; nitrogen has five, aluminium three and hydrogen one. The outermost electron count largely governs chemical bonding and reactivity. Fluorine typically gains or shares one electron to complete its valence octet.
13. The present atomic weight scale is based on which of the following?
[WBCS Mains 2021, Paper IV — held May 2022]
(A) Carbon-12
(B) Hydrogen-1
(C) Carbon-13
(D) Oxygen-16
Answer: (A) Carbon-12
Explanation: The unified atomic mass unit is defined as exactly one-twelfth the mass of a neutral carbon-12 atom in its ground state. Relative atomic masses are measured using this reference. Natural elemental atomic weights may be fractional because isotopes occur in mixtures.
12. The term “isotope” refers to which of the following?
[RRB NTPC CBT-I — 3 Apr 2021, Shift 1]
(A) Elements having the same bond structure
(B) Atoms with the same atomic number but different mass numbers
(C) Atoms with different atomic numbers but the same mass number
(D) Atoms with identical physical and chemical properties
Answer: (B) Atoms with the same atomic number but different mass numbers
Explanation: Isotopes share their nuclear proton number, which defines the element, but differ in neutron count and thus mass number. For example, chlorine-35 and chlorine-37 both have seventeen protons, yet possess eighteen and twenty neutrons respectively.
11. What are the atomic number and mass number, respectively, of tritium?
[RRB NTPC CBT-I — 13 Mar 2021, Shift 1]
(A) 1 and 3
(B) 3 and 1
(C) 1 and 2
(D) 1 and 1
Answer: (A) 1 and 3
Explanation: Tritium is hydrogen-3, containing one proton and two neutrons. Proton count gives atomic number 1; total nucleon count gives mass number 3. It is a radioactive hydrogen isotope, whereas protium has no neutron and deuterium has one.
