Master the fundamentals of Chemistry and the structure of the atom with our meticulously curated collection of topic-wise questions for the UPSC Prelims. Dive deep into the basics of Chemistry, understanding essential concepts without the fluff. From atomic structure to chemical bonding, our questions cover everything you need to know to excel in your exam. Explore the intricacies of atomic models, electron configurations, and periodic trends with clarity and precision. Whether you’re revising key principles or aiming to strengthen your foundation, our resource provides comprehensive coverage for effective preparation. Boost your confidence and enhance your understanding of Chemistry with our UPSC Prelims Topic Wise Questions – Chemistry – Basics of Chemistry and Structure of Atom. Ace your exam with a solid grasp of fundamental concepts essential for success.
Q1. The alpha particle carries two positive charges. Its mass is very nearly equal to that of (1996)
(a) two protons
(b) an atom of helium
(c) sum of masses of two positrons and two neutrons
(d) two positrons as each positron carries a single positive charge
Ans. (b)
The alpha particle (α) bears two positive charges and has a mass very close to that of a helium atom. As we know, each alpha particle consists of two protons and two neutrons. The total number of nucleons in an alpha particle equals the sum of protons and neutrons, which is 2 + 2 = 4 atomic mass units (amu). This total number of nucleons is equivalent to the mass number of an element. Hence, the mass number of an alpha particle is 4 amu, which is identical to the molecular weight of a helium atom.
Q2. In an atom, the order of filling up of the orbitals is governed by (2001)
(a) Aufbau principle
(b) Heisenberg’s uncertainty principle
(c) Hund’s rule
(d) Pauli’s exclusion principle
Ans. (a)
In an atom, the sequence of filling up orbitals follows the Aufbau principle. According to this principle, electrons are added to an atom one by one, beginning with the lower energy orbitals and proceeding to higher energy ones until all the electrons are placed in the appropriate orbitals.
Q3. Which one of the following is the correct sequence in increasing order, of molecular weights of the hydrocarbons? (2001)
(a) Methane, ethane, propane and butane
(b) Propane, butane, ethane and methane
(c) Butane, ethane, propane and methane
(d) Butane, propane, ethane and methane
Ans. (a)
The molecular weight of hydrocarbons can be determined by composing the formula of each hydrocarbon. For example:
- Methane: CH4 = 12 + 4 = 16
- Ethane: C2H6 = 12 × 2 + 1 × 6 = 30
- Propane: C3H8 = 12 × 3 + 1 × 8 = 44
- Butane: C4H10 = 12 × 4 + 1 × 10 = 58
The correct sequence in increasing order of molecular weight of the hydrocarbons is CH4, C2H6, C3H8, C4H10. In other words, as the number of carbon atoms increases, so does the molecular weight of the hydrocarbon.
Q4. Match List I with List II and select the correct answer using the codes given below the list. (2001)
| List I (Characteristics) | List II (Particles) |
| A. Zero Mass | 1. Positron |
| B. Fractional charge | 2. Neutrino |
| C. Fractional spin | 3. Quark |
| D. Integral spin | 4. Photon |
Codes
A B C D
(a) 2 3 1 4
(b) 3 2 4 1
(c) 2 3 4 1
(d) 3 2 1 4
Ans. (a)
The correct matching is A-2, B-3, C-1, D-4.
- Positron is similar to an electron but carries a positive charge (+1) instead of a negative charge (-1). Both have fractional spins denoted as +1/2 or -1/2. Positron is the antiparticle of an electron.
- Neutrino is considered a massless particle by physicists. It is believed to have properties opposite to those of a neutron and is also referred to as the antiparticle of a neutron.
- Quarks are fundamental particles that constitute particles such as protons and neutrons. They possess fractional charges like +1/3 or +2/3.
- Photon, like quarks, is a fundamental particle (classified as a boson) with an integer spin of +1 or -1.
Q5. Regarding the atom of a chemical element, magnetic quantum number refers to (2003)
(a) orientation
(b) shape
(c) size
(d) spin
Ans. (a)
Regarding the atom of a chemical element, the magnetic quantum number refers to orientation. The magnetic quantum number (m) specifies the orientation of an atomic orbital in space relative to an external magnetic field.
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