1. In wave-mechanical model of atom, degenerate energy levels have:
a) Equal energy
b) Equal number of electrons
c) Same shape
d) Electrons with same spin
Answer: a [Reason:] Degenerate energy levels have equal energy. In an atom, the orbitals in same subshell are degenerate in absence of external magnetic field.
2. Paired electrons in an atom have equal values of all of the four quantum numbers.
Answer: b [Reason:] According to Pauli’s exclusion principle, no two electrons in an atom can have all four quantum numbers equal. Paired electrons have opposite spins.
3. Protons attract electrons. Then why do electrons not fall on the nucleus?
a) Neutrons repel the electrons
b) Electrons in ground state cannot radiate energy
c) At very small distances, protons repel electrons
d) Inner electrons repel those in outer orbitals
Answer: b [Reason:] In ground state, electrons form standing waves. Standing waves are those in which energy is not transferred. Hence, electrons are bound to revolve around the nucleus with constant potential energy.
4. Number of atomic orbitals in nth shell is:
c) 2n + 1
d) n − 1
Answer: a [Reason:] Every shell contains n2 orbitals and every orbital can accommodate a maximum of 2 electrons. Hence a maximum of 2n2 electrons can reside in nth shell.
5. Electronic configuration of Cu is [Ar] 3d10 4s1. Which rule is violated in this configuration?
a) Aufbau Principle
b) Hund’s Rule of Maximum Multiplicity
c) Pauli’s Exclusion Principle
d) None of the mentioned
Answer: a [Reason:] According to Aufbau principle, orbitals of lower energy are completely filled before electrons start occupying higher energy orbitals. But in copper, higher energy 3d is filled before lower energy 4s. Such violation of Aufbau principle appears in about 20 elements.
6. When an electron shifts to an inner shell, it:
a) Absorbs photon
b) Emits a photon
c) Emits a positron
d) Absorbs a positron
Answer: b [Reason:] Generally, lower potential energy means more stability. Excited electrons lose energy in the form of photons to shift to stable empty inner shells. Positron is anti-particle of electron.
7. Around 1911, Rutherford suggested planetary model of atomic structure. Which of the following was not a proposition of this model?
a) Electrons revolve in circular orbits around the nucleus
b) Nucleus consists of protons and neutrons
c) Mass of atom is concentrated in the nucleus
d) Most of the volume in atom is void
Answer: b [Reason:] Neutrons were not discovered by then. Rutherford only suggested that the nucleus has positive charge, but did not comment on its structure.
8. Which of the Bohr’s postulates was incorrect?
a) Energy of electrons is quantized
b) Momentum of electrons is quantized
c) Electrons release energy when shifting from excited state to ground state
d) Electrons revolve around nucleus in fixed circular orbits
Answer: d [Reason:] Heisenberg uncertainty principle implies that exact position and momentum of electrons cannot be known with certainty and hence, definite circular paths are not feasible. Certain regions around the nucleus (called orbitals) have high probability of existence of electrons.
9. 4f orbital has higher energy than 5p.
Answer: a [Reason:] Sum of principal and azimuthal quantum numbers for 4f is (4+3=) 7 and that for 5p is (5+1=) 6. As a general rule, higher the sum, higher is the energy.
10. Which of the following orbitals do not exist?
Answer: d [Reason:] For principal quantum number 3, only possible values of azimuthal quantum number are 0, 1, and 2. Azimuthal quantum number of an f-orbital is 3.
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