This set of Physical Chemistry Multiple Choice Questions & Answers (MCQs) focuses on “Periodic Table and Aufbau Principle”.
1. What is the Aufbau Principle?
a) Electrons fill orbitals with the highest energy first
b) Electrons fill orbitals with the lowest energy first
c) Filling of orbitals is a random process which does not depend on energy levels
d) Electrons reside in whichever orbital matches their spin
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Explanation: Electrons fill orbitals in the order of increasing energy levels. This is energetically favorable as electrons are ejected from their outermost subshells or orbitals upon ionization. This process cannot be arbitrary as atomic energy levels are quantized and electron spin has nothing to do with the Aufbau Principle.
2. What does the letter “l” denote with respect to quantum number?
a) L is not a quantum number
b) Magnetic quantum number
c) Spin quantum number
d) Azimuthal quantum number
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Explanation: “l” (L uncapitalized) is the azimuthal quantum number, which is the orbital angular momentum and describes the shape of an orbital in which an electron resides. Magnetic quantum number is denoted by “ml“, spin by “s”, and principal quantum number by “n”. All these numbers together can explain the position of an electron in an atom.
3. How many electrons does each subshell with azimuthal quantum number “l” contain?
a) 2l
b) 2l + 1
c) 1
d) 2(2l + 1)
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Explanation: The s shell can hold 2 electrons, p shell can hold 6, d shell can hold 10, and so on. Using this formulation and extrapolating trends based on orbital shape, it can be shown that 2(2L + 1) electrons can be held in an orbital with azimuthal quantum number l. This can be proven rigorously using mathematical induction.
4. What does the atomic orbital radii mean in quantum mechanics?
a) Radial probability density of finding an electron in the outermost shell
b) Distance between center of nuclei to outermost shell
c) Surface area of an electron
d) Volume of an atom
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Explanation: Since electrons are both waves and particles and an atom is not a sphere, atomic radii has to account for probability distribution of finding the outermost electron. This means that distance is no longer a singular number. In classical mechanics, this can be interpreted as the distance between the center of nuclei to the outermost shell.
5. What is the periodic trend of atomic radius within a row of the periodic table?
a) Atomic radius is independent of row position
b) Atomic radius stays the same
c) Atomic radius decreases with increasing atomic number
d) Atomic radius increases with increasing atomic number
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Explanation: Atomic radius across a row decreases with increasing atomic number. This is because the number of electrons in the outermost orbital increase, which pulls the atom closer together (increasing effective nuclear charge), hence decreasing atomic number. This is caused by electrostatic forces. Thus, bromine has a smaller atomic radius than potassium.
6. What is the periodic trend of atomic radius down a column of the periodic table?
a) Atomic radius
b) Atomic radius stays the same
c) Atomic radius decreases with increasing atomic number
d) Atomic radius increases with increasing atomic number
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Explanation: Atomic radius down a column increases with increasing atomic number. This is because an additional subshell increases, which increases the principal quantum number, and hence the atomic number. Thus, sodium has a smaller atomic radius than potassium.
7. What is electron affinity?
a) Energy released in the process of adding an electron
b) Tendency of an atom to attract a shared pair of electrons towards itself
c) Energy absorbed in the process of ejecting an electron
d) An electron’s tendency to attract itself to an atom and not let go
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Explanation: Electron Affinity of an atom is the energy released in the process of adding an electron to an atom. This normally occurs when an atom becomes an anion (negative charge). Energy absorbed in the process of ejecting an electron is ionization energy.
8. What is the electronic configuration of lithium?
a) 1s2
b) 1s2 2s
c) [Ne]3s2 3p6
d) [Ne]3s2 3p5
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Explanation: Lithium has 3 electrons, so filling these using Aufbau’s principle gives 1s2 2s. This unique electronic configuration is useful in real life applications as the lithium ion is very stable and mobile, making it useful in lithium-ion battery applications.
9. What is the electronic configuration of helium?
a) 1s2
b) 1s2 2s
c) [Ne]3s2 3p6
d) [Ne]3s2 3p5
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Explanation: Helium is a stable inert element and has only two electrons that fill up the innermost 1s orbital. Absence of additional electrons prevents further filling of outer shells.
10. What is the electronic configuration of argon?
a) 1s2
b) 1s2 2s
c) [Ne]3s2 3p6
d) [Ne]3s2 3p5
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Explanation: Being a noble gas, argon has filled subshells. This manifests as filled 3s and 3d shells over the normal neon electronic configuration. Hence, its electronic configuration is [Ne]3s2 3p6.
11. What is the electronic configuration of chlorine?
a) 1s2
b) 1s2 2s
c) [Ne]3s2 3p6
d) [Ne]3s2 3p5
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Explanation: Chlorine has one less electron than being the noble gas argon. It has a total of 17 electrons, and hence the electronic configuration of [Ne]3s2 3p5.
12. What is the electronic configuration of vanadium?
a) [Ne]3s2 3p2
b) [Ar]4s2
c) [Ar]4s2 3d5
d) [Ar]4s2 3d3
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Explanation: Vanadium is a rare transition metal and can be isolated artificially from its metal oxides. With a total of 23 electrons, it has an electronic configuration of [Ar]4s2 3d3.
13. What is the electronic configuration of silicon?
a) [Ne]3s2 3p2
b) [Ar]4s2
c) [Ar]4s2 3d5
d) [Ar]4s2 3d3
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Explanation: Silicon is a semi-metal used to make solar cells and other electronics applications. It is the most common element found in the Earth’s crust. With a total of 14 electrons, it fills subshells according to Aufbau’s principle with an electronic configuration of [Ne]3s2 3p2.
14. What is the electronic configuration of manganese?
a) [Ne]3s2 3p2
b) [Ar]4s2
c) [Ar]4s2 3d5
d) [Ar]4s2 3d3
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Explanation: Manganese has a total of 25 electrons and the 4s subshell fills before 3d as it is lower in energy (Aufbau principle). Keeping this in mind, manganese has an electron configuration of [Ar]4s2 3d5.
15. What is the electronic configuration of calcium?
a) [Ne]3s2 3p2
b) [Ar]4s2
c) [Ar]4s2 3d5
d) [Ar]4s2 3d3
View Answer
Explanation: Calcium is one of the most common elements found in minerals, vitamins, and oxide rocks. It is an alkali earth metal and reacts strongly with water. It has 20 electrons and filling these into shells using Aufbau principle gives an electronic configuration of [Ar]4s2.
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