Atomic Structure

1. An element has an atomic number of 16. What is the principal quantum n

An element has an atomic number of 16. What is the principal quantum number (n) of its outermost electrons ?

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1
2
3
4
This question was previously asked in
UPSC CISF-AC-EXE – 2020
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An element with atomic number 16 is Sulfur (S). To find the principal quantum number (n) of its outermost electrons, we need to write its electron configuration. The atomic number represents the number of protons and, in a neutral atom, the number of electrons. So, Sulfur has 16 electrons. The electron configuration is filled according to the Aufbau principle, Hund’s rule, and the Pauli exclusion principle.
Filling order: 1s, 2s, 2p, 3s, 3p, 4s, 3d, …
Electrons:
1s² (2 electrons)
2s² (2 electrons)
2p⁶ (6 electrons) – total 2+2+6 = 10 electrons
3s² (2 electrons) – total 10+2 = 12 electrons
3p⁴ (4 electrons) – total 12+4 = 16 electrons
The electron configuration is 1s² 2s² 2p⁶ 3s² 3p⁴. The outermost electrons are those in the highest principal energy level, which is n=3 in this case (3s and 3p orbitals).
The principal quantum number (n) of the outermost electrons corresponds to the highest energy level occupied by electrons in the atom’s electron configuration. For Sulfur (atomic number 16), the configuration is 1s² 2s² 2p⁶ 3s² 3p⁴, so the outermost electrons are in the n=3 level.
The outer electron shell is often referred to as the valence shell. The principal quantum number ‘n’ defines the energy level and size of the electron shell. n=1 is the first shell, n=2 is the second shell, n=3 is the third shell, and so on.

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2. Which one among the following statements with respect to the atomic nu

Which one among the following statements with respect to the atomic number of an atom is correct ?

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The number of neutrons is same as atomic number
The sum of electrons and neutrons is same as atomic number
The number of protons is same as atomic number
The sum of protons and neutrons is same as atomic number
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UPSC CAPF – 2024
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The correct option is C. The atomic number of an atom is defined as the number of protons in the nucleus of that atom.
– Atomic number (Z) is a fundamental property of an element that uniquely identifies it.
– It represents the number of positive charges (protons) in the nucleus.
– In a neutral atom, the number of electrons is equal to the number of protons.
The mass number (A) of an atom is the sum of the number of protons and neutrons in the nucleus. Isotopes of an element have the same atomic number (same number of protons) but different mass numbers (different numbers of neutrons).

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3. The atomic radius of hydrogen atom is

The atomic radius of hydrogen atom is

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37 nanometer
37 picometer
17 picometer
57 picometer
This question was previously asked in
UPSC CAPF – 2022
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The atomic radius of a hydrogen atom is approximately 37 picometers (pm) when considered as a covalent radius, or about 52.9 pm for the Bohr radius. Given the options, 37 picometer is the closest and a commonly cited value for the covalent radius of hydrogen.
– The size of an atom is often described by its radius.
– Different definitions of atomic radius exist (e.g., covalent radius, van der Waals radius, Bohr radius) which yield different values.
– 1 picometer (pm) = 10⁻¹² meters.
– 1 nanometer (nm) = 10⁻⁹ meters = 1000 picometers.
– A radius of 37 nanometers would be extremely large, equivalent to 37,000 picometers, which is not the size of a hydrogen atom.
The Bohr radius (a₀) for the ground state of hydrogen is approximately 52.9 pm. The covalent radius of hydrogen, often determined from the H-H bond length in H₂ (about 74 pm), is taken as half of this length, i.e., 37 pm. The van der Waals radius of hydrogen is significantly larger, around 120 pm. The question does not specify which type of radius, but 37 pm is a valid and commonly used value for its covalent radius.

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4. Which one of the following properties decreases across the periodic ta

Which one of the following properties decreases across the periodic table from left to right and increases from top to bottom?

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Ionization energy
Electron affinity
Electronegativity
Atomic radius
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UPSC CAPF – 2021
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Atomic radius decreases across the periodic table from left to right and increases from top to bottom.
Across a period (left to right), the atomic radius decreases because the nuclear charge increases while electrons are added to the same energy shell, pulling the electron cloud closer to the nucleus. Down a group (top to bottom), the atomic radius increases because new electron shells are added, increasing the distance between the nucleus and the valence electrons, despite the increased nuclear charge.
Ionization energy, electron affinity (generally), and electronegativity all tend to increase across a period from left to right and decrease down a group from top to bottom, showing the opposite trend to atomic radius. This is because these properties are related to the attraction between the nucleus and electrons; a smaller radius generally means a stronger attraction.

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5. Which one of the following is the average atomic mass of chlorine atom

Which one of the following is the average atomic mass of chlorine atom ?

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35.9 u
35.5 u
35.0 u
37.0 u
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UPSC CAPF – 2020
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The average atomic mass of chlorine atom is approximately 35.5 u.
Chlorine occurs naturally as a mixture of two main isotopes: chlorine-35 (³⁵Cl) and chlorine-37 (³⁷Cl). Their approximate natural abundances are about 75% for ³⁵Cl (mass ≈ 34.97 u) and 25% for ³⁷Cl (mass ≈ 36.97 u). The average atomic mass is a weighted average of the masses of its isotopes based on their natural abundance.
Average atomic mass = (0.75 × 34.97 u) + (0.25 × 36.97 u) ≈ 26.23 u + 9.24 u ≈ 35.47 u, which is rounded to 35.5 u.

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6. The number of angular and radial nodes for 4d orbital is respectively

The number of angular and radial nodes for 4d orbital is respectively

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2 and 1
1 and 2
3 and 1
4 and 0
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UPSC CAPF – 2018
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The correct answer is A) 2 and 1.
For an atomic orbital described by the principal quantum number *n* and azimuthal (angular momentum) quantum number *l*:
– The number of angular nodes is equal to *l*.
– The total number of nodes is equal to *n* – 1.
– The number of radial nodes is the total number of nodes minus the number of angular nodes, i.e., (*n* – 1) – *l*.
For a 4d orbital:
– Principal quantum number *n* = 4.
– For a d orbital, the azimuthal quantum number *l* = 2 (s=0, p=1, d=2, f=3).
– Number of angular nodes = *l* = 2.
– Number of radial nodes = (*n* – 1) – *l* = (4 – 1) – 2 = 3 – 2 = 1.
The question asks for the number of angular and radial nodes *respectively*.
Angular nodes are surfaces where the probability of finding the electron is zero, and their shape depends on the value of *l* (e.g., for p orbitals, the angular node is a plane; for d orbitals, there are two angular nodes, often planes or conical surfaces). Radial nodes are spherical surfaces where the radial probability density (probability per unit volume) is zero. The number of radial nodes depends on both *n* and *l*.

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7. Match List I with List II and select the correct answer using the code

Match List I with List II and select the correct answer using the code given below the Lists :

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List I
(Isotope)		List II
(Application)
A. Carbon1. Treatment of goitre
B. Cobalt2. Calculation of age of the earth
C. Iodine3. Treatment of cancer
D. Uranium4. Fuel in nuclear reactors

Code :

2 3 1 4
2 1 3 4
4 1 3 2
4 3 1 2
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UPSC CAPF – 2016
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The correct answer is A) 2 3 1 4.
– B. Cobalt (specifically Cobalt-60) is widely used in radiotherapy for the treatment of cancer due to its emission of gamma rays. This matches with application 3.
– C. Iodine (specifically Iodine-131) is used in the diagnosis and treatment of thyroid conditions, including goitre, as the thyroid gland absorbs iodine. This matches with application 1.
– D. Uranium (specifically Uranium-235) is the primary fuel used in nuclear reactors for generating energy through nuclear fission. Uranium isotopes (like Uranium-238) are also used in radiometric dating (Uranium-Lead dating) to calculate the age of rocks and the Earth. Application 4 (Fuel in nuclear reactors) and Application 2 (Calculation of age of the earth) are both valid uses of Uranium isotopes.
– A. Carbon: Carbon-14 is used for radiocarbon dating, but this is for dating organic materials up to around 50,000 years old, not for the age of the Earth itself (which is billions of years, dated using isotopes like Uranium). Carbon (graphite) is used as a moderator in some nuclear reactors, but not as fuel. Neither application 2 nor 4 is a direct or primary application of Carbon isotopes compared to other options.
Let’s examine the pairings in option A: A-2 (Carbon for Age of Earth), B-3 (Cobalt for Cancer), C-1 (Iodine for Goitre), D-4 (Uranium for Fuel). B-3, C-1, and D-4 are correct pairings. A-2 is incorrect as Carbon-14 dating is for organic matter, not the age of the Earth.
Let’s examine the pairings in option D: A-4 (Carbon for Fuel), B-3 (Cobalt for Cancer), C-1 (Iodine for Goitre), D-2 (Uranium for Age of Earth). B-3, C-1, and D-2 are correct pairings. A-4 is incorrect as Carbon is a moderator, not fuel, in reactors.
Both option A and D contain correct pairings for B and C. Both contain a correct pairing for D (either 2 or 4, as both are valid uses). Both contain an incorrect pairing for A. Comparing the incorrect pairings for A, linking Carbon-14 dating to ‘Age of Earth’ might be considered slightly less incorrect in the context of general dating methods related to Earth’s history than linking Carbon (as moderator) to ‘Fuel’. Therefore, option A (A-2, B-3, C-1, D-4) is the most likely intended answer, despite the flaw in the A-2 pairing.

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8. The heaviest element known up till January 2015 has the atomic number

The heaviest element known up till January 2015 has the atomic number :

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117
118
119
120
This question was previously asked in
UPSC CAPF – 2015
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As of January 2015, the element with the highest atomic number that had been synthesized and reported was Element 118, Oganesson (Og). While its official naming and addition to the periodic table by IUPAC/IUPAP occurred later, its synthesis was first reported in 2002 and confirmed in 2006 by a Russian-American collaboration. Element 117 (Tennessine, Ts) synthesis was reported in 2010 and confirmed in 2014. Thus, both 117 and 118 were “known” (synthesized) before January 2015, with 118 having the highest atomic number.
The question asks for the element with the highest atomic number *known* up until a certain date, which refers to elements that have been synthesized and their existence tentatively accepted by the scientific community, even if not fully confirmed or officially named.
The heaviest elements are synthetic and highly unstable, existing for only fractions of a second. Elements with atomic numbers 113, 115, 117, and 118 were officially added to the periodic table by IUPAC in December 2015 and named in 2016. Elements 119 and 120 were theoretical or subjects of ongoing synthesis attempts by January 2015.

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9. For Aluminium (Al) (atomic number : 13, mass number : 27), which one a

For Aluminium (Al) (atomic number : 13, mass number : 27), which one among the following statements is NOT correct ?

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Number of electrons present in Al is 13
Number of protons present in Al is 13
Number of neutrons present in Al is 14
The valency of Al is 2
This question was previously asked in
UPSC NDA-2 – 2024
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For a neutral atom, the number of protons is equal to the atomic number (Z), and the number of electrons is also equal to the atomic number. Given the atomic number of Aluminium (Al) is 13, it has 13 protons and 13 electrons. The mass number (A) is the sum of protons and neutrons. Given the mass number is 27 and the number of protons is 13, the number of neutrons is 27 – 13 = 14. Aluminium is in Group 13 of the periodic table, with electronic configuration 1s² 2s² 2p⁶ 3s² 3p¹. It readily loses its 3 valence electrons to form a stable Al³⁺ ion. Therefore, the typical valency of Aluminium is 3, not 2.
Atomic number = Number of protons. For a neutral atom, Number of electrons = Number of protons. Mass number = Number of protons + Number of neutrons. Valency is related to the number of electrons an atom gains, loses, or shares to form chemical bonds.
Elements in Group 13 (like Aluminium) typically exhibit a valency of +3, forming ions with a +3 charge by losing their three valence electrons.

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10. Which one among the following has valency of 3 ?

Which one among the following has valency of 3 ?

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Carbon
Oxygen
Nitrogen
Sodium
This question was previously asked in
UPSC NDA-2 – 2024
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Nitrogen commonly has a valency of 3, particularly in compounds like ammonia (NH3).
– Valency is the measure of the combining capacity of an element. It is typically equal to the number of electrons an atom gains, loses, or shares to achieve a stable electron configuration (usually a full outermost electron shell).
– Carbon (Atomic number 6): Electron configuration is 2, 4. It needs 4 electrons to complete its outer shell, so its common valency is 4 (e.g., CH4, CO2).
– Oxygen (Atomic number 8): Electron configuration is 2, 6. It needs 2 electrons, so its common valency is 2 (e.g., H2O, O2).
– Nitrogen (Atomic number 7): Electron configuration is 2, 5. It needs 3 electrons to complete its outer shell. Thus, its common valency is 3 (e.g., NH3, NCl3). Nitrogen can also exhibit other valencies (e.g., +5 in nitrates), but 3 is a very common valency.
– Sodium (Atomic number 11): Electron configuration is 2, 8, 1. It tends to lose 1 electron to achieve a stable configuration. Its valency is 1 (e.g., NaCl, Na2O).
The concept of valency is simplified and primarily applies to covalent bonding and simple ionic compounds. More rigorous descriptions of bonding use oxidation states and formal charges, especially for elements in higher periods or in complex compounds. However, for basic elements like N, O, C, and Na, their typical valencies are characteristic.

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