This set of Physical Chemistry Multiple Choice Questions & Answers (MCQs) focuses on “Periodic Table and Aufbau Principle – Set 2”.
1. What is the electronic configuration of Chromium (atomic number = 24)?
a) [Ne]3s23p2
b) [Ar]4s2
c) [Ar]4s3d5
d) [Ar]4s3d6
View Answer
Explanation: The first 18 electrons fill up principal orbitals 1, 2, and 3 with number of electrons being 2, 8, and 8 respectively. The remaining 6 electrons distribute themselves amongst 4s and 3d such that they all are half filled. This half-filled shell configuration of Chromium is more stable than the rules determined by the Aufbau Principle, hence is preferred over filling up the 4s subshell with two electrons
2. What is the electronic configuration of Copper (atomic number = 29)?
a) [Ne]3s23p6
b) [Ar]4s23d
c) [Ar]4s23d9
d) [Ar]4s3d10
View Answer
Explanation: Principal orbitals 1, 2, and 3 are filled by the first 18 electrons for the argon noble gas configuration. The remaining 11 electrons distribute themselves in the most stable manner; in this case, it is an exception to Aufbau’s principle. This full shell of 3d and half shell of 4s is more stable than the full shell of 4s and 9 electrons in 3d
3. Why does the filing of electrons in Copper’s orbital not follow the Aufbau Principle?
a) Copper is different from all other elements in its physical and chemical properties
b) With its atomic number of 29, filling the orbitals with half and full shell combinations offer higher stability than predicted by Aufbau’s principle
c) Half shell is more stable than a full shell
d) Copper does not follow Aufbau’s principle
View Answer
Explanation: The 4s3d10 combination for the outer two shells is more stable than having 2 electrons in 4s and 9 electrons in 3d. This is what gives copper a relatively inert nature, and it does not react vigorously with most compounds.
4. Why does the filing of electrons in Chromium’s orbital not follow the Aufbau Principle?
a) The atomic number of 24 allows the 3d and 4s subshell to be half filled, allowing greater stability
b) The atomic number of 24 allows the 3d and 4s subshell to be filled, allowing greater stability
c) Chromium does not follow Aufbau’s principle due to unique physical and thermal properties
d) The electrons in Chromium’s atom behave differently than other electrons
View Answer
Explanation: After filling the first three shells with 18 electrons, Chromium requires an additional six electrons to be distributed within the 3d and 4s subshells. Filling half of them is more stable than completely filling 3d and leaving 4s with 4 electrons.
5. What element has the highest first ionization energy?
a) Hydrogen
b) Chlorine
c) Fluorine
d) Helium
View Answer
Explanation: Helium has the highest ionization energy as the two opposite spin electrons are paired together and close to the nucleus. Hence, removing one electron requires more energy than any other element.
6. What is the electronic configuration of Vanadium (atomic number = 23)?
a) [Ne]3s23p3
b) [Ar]4s2
c) [Ar]4s23d3
d) [Ar]4s3d4
View Answer
Explanation: With an atomic number of 23, the 3d and 4s subshells need to be filled with 5 electrons. Using Aufbau’s principle, 4s is filled followed by 3d which gives the desired electron configuration.
7. What are Actinide elements?
a) Elements with atomic number between 89-103 that are highly electropositive and radioactive
b) Elements with atomic number between 57-71 with at least one electron in the valence 5d shell
c) Elements with atomic number between 89-103 that are stable and used as catalysts
d) Elements that are not part of the Periodic table
View Answer
Explanation: Actinide elements are part of the third group on the periodic table and is referred to as “Rare Earth Metals”, along with Lanthanoids. All of them are instable and radioactive and not all of them are found in nature. They form stable complexes with several ligands.
8. What are Lanthanoid elements?
a) Elements with atomic number between 89-103 that are unstable and used as detergents
b) The group of compounds that were discovered by Albert Einstein
c) Elements that are solely coined as “Rare Earth Metals”
d) Elements with atomic number between 57-71 with at least one electron in the valence 5d shell
View Answer
Explanation: Lanthanoids, also known as the 5d block elements, are reactive silver colored metals. These are known as rare earth metals along with Actinides as they are not found readily in the Earth’s crust. Their compounds are used in catalysts in industrial reactions and manufacturing synthetic products and petroleum.
9. What is the electronic configuration of Krypton (atomic number = 36)?
a) [Ne]3s23p2
b) [Ar]4s2
c) [Ar]4s23d64p10
d) [Ar]4s23d104p6
View Answer
Explanation: Krypton has all filled subshells and principal shells by completely occupying 4s, 3d, and 4p subshells. This are filled by Aufbau’s principle and fill all the orbitals. It is hence a noble gas with this complete and stable electronic configuration.
10. What are alkali metals?
a) Radioactive compounds synthesized in the laboratory that are not found in nature
b) Group 1A, the left most group on the periodic table
c) Group 2A, the second most left group on the periodic table
d) Noble inert gases that do not react readily with other elements
View Answer
Explanation: The six elements lithium, sodium, potassium, rubidium, cesium, and francium are known as alkali metals. They lie in the “s block” region of the periodic table. Overall, they are reactive and not found readily in nature. All of them have one electron in their outermost shell
11. What are alkali earth metals?
a) Shiny, silvery metals part of group 2A of the periodic table
b) Radioactive elements found on the bottom row of the periodic table
c) Deform under stress to form unique crystal lattice compounds
d) Transition metals that occupy most of the center of the periodic table
View Answer
Explanation: Alkali earth metals are six elements with similar properties in group 2A of the periodic table whose compounds are found readily in the Earth’s crust. They react vigorously with water and their cations have a charge of +2. All have a filled s subshell in their outermost orbital.
12. What are noble gases?
a) Special gases formed under stimulated laboratory conditions
b) Transition elements found in the center of the periodic table
c) Radioactive actinide gases that react vigorously with any other element
d) Stable, inert gases that do not react with most compounds under regular conditions
View Answer
Explanation: Noble gases are colorless, odorless gases that form group 18 of the periodic table. They have filled subshells, hence have very low chemical activity under standard conditions. They have a complete octet, making it unlikely for them to lose electrons and form charged ions.
13. What is the electronic configuration of Scandium (atomic number = 21)?
a) [Ne]3s23p
b) [Ar]4s23d
c) [Ar]4s43d
d) [Ar]4s23d6
View Answer
Explanation: Since Scandium has an atomic number of 21, only 3 electrons fill the 4s and 3d subshells. Applying Aufbau’s principle, this completely occupies 4s and one electron goes to 3d. The first 18 electrons fill the first three shells with the Argon noble gas configuration.
14. Which element has the smallest orbital radius?
a) Hydrogen
b) Helium
c) Beryllium
d) Lithium
View Answer
Explanation: Although Helium has three electrons while hydrogen has two, they have the same principal quantum number (n = 2). Helium has a noble gas configuration with the outermost shell having a filled duplet. This draws the two outermost electrons closer to the nucleus, hence decreasing orbital atomic radius.
15. What is the electronic configuration of Gallium (atomic number = 31)?
a) [Ne]4s23d54p6
b) [Ar]
c) [Ar]4s23d54p6
d) [Ar]4s23d104p
View Answer
Explanation: The first 18 electrons fill themselves according to Argon’s noble gas configuration. The remaining 13 are occupied using Aufbau’s principle. This completely fills the 4s and 3d subshells, leaving one electron in the 4p subshell.
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