21. The species that has the same number of electrons as 35 17 Cl is

The species that has the same number of electrons as 3517Cl is

<sup>32</sup><sub>16</sub>S
<sup>34</sup><sub>16</sub>S<sup>+</sup>
<sup>40</sup><sub>18</sub>Ar<sup>+</sup>
<sup>35</sup><sub>16</sub>S<sup>2-</sup>
This question was previously asked in
UPSC NDA-2 – 2017
The number of electrons in a neutral atom is equal to its atomic number (the subscript). For an ion, the number of electrons is adjusted based on the charge.
The species is 3517Cl. The atomic number is 17, so a neutral Chlorine atom has 17 protons and 17 electrons. We need to find the species with 17 electrons.
A) 3216S: Atomic number 16. Neutral Sulfur has 16 electrons.
B) 3416S+: Atomic number 16. Neutral Sulfur has 16 electrons. S⁺ means it lost 1 electron (16 – 1 = 15 electrons).
C) 4018Ar+: Atomic number 18. Neutral Argon has 18 electrons. Ar⁺ means it lost 1 electron (18 – 1 = 17 electrons).
D) 3516S2-: Atomic number 16. Neutral Sulfur has 16 electrons. S²⁻ means it gained 2 electrons (16 + 2 = 18 electrons).
Therefore, 4018Ar+ has 17 electrons, the same number as neutral 3517Cl. These species are isoelectronic.
The superscript number (mass number) is the sum of protons and neutrons and is irrelevant to the number of electrons in this calculation, unless isotopes are being considered, which is not the case here for electron count. The subscript number is the atomic number, which is crucial as it determines the number of protons and thus the number of electrons in a neutral atom.

22. Which one of the following statements is correct ?

Which one of the following statements is correct ?

Covalent bonds are directional
Ionic bonds are directional
Both covalent and ionic bonds are directional
Both covalent and ionic bonds are non-directional
This question was previously asked in
UPSC NDA-2 – 2016
Covalent bonds are directional.
Covalent bonds involve the sharing of electrons between specific atoms, leading to electron density concentrated in defined regions (orbitals) oriented in space. This directional nature of covalent bonds determines the geometry and shape of molecules, including bond angles and lengths.
Ionic bonds are formed by the electrostatic attraction between oppositely charged ions. This attraction is isotropic and extends in all directions equally around the ion. Thus, ionic bonds are non-directional, leading to the formation of extended crystal lattices rather than discrete molecules with fixed shapes determined by bond angles.

23. There are six electrons, six protons and six neutrons in an atom of an

There are six electrons, six protons and six neutrons in an atom of an element. What is the atomic number of the element ?

6
12
18
24
This question was previously asked in
UPSC NDA-2 – 2016
The atomic number of an element is defined by the number of protons found in the nucleus of an atom of that element. The question states that the atom has six protons. Therefore, the atomic number of the element is 6.
– Atomic Number (Z) = Number of Protons.
– In a neutral atom, Number of Protons = Number of Electrons.
– The mass number is the total number of protons and neutrons in the nucleus (Mass Number = Protons + Neutrons).
– In this case, Protons = 6, Electrons = 6, Neutrons = 6.
– Atomic Number = 6.
– Mass Number = 6 + 6 = 12. This describes an atom of Carbon-12.
The number of neutrons can vary for atoms of the same element, leading to different isotopes. However, the number of protons is unique to each element and determines its identity.

24. The relative atomic mass of boron (which exists in two isotopic forms

The relative atomic mass of boron (which exists in two isotopic forms 10B and 11B) is 10·81. What will be the abundance of 10B and 11B, respectively (consider a sample of 100 atoms) ?

19% and 81%
81% and 19%
38% and 62%
62% and 38%
This question was previously asked in
UPSC NDA-1 – 2024
Let the abundance of 10B be x and the abundance of 11B be y. The total abundance is 100%, so x + y = 1 (or 100 if using percentages). The average atomic mass is calculated as the weighted average of the isotopic masses: (mass of 10B * x) + (mass of 11B * y) = Average Atomic Mass. Using approximate integer masses (10 for 10B and 11 for 11B), we have: 10x + 11y = 10.81. Since x + y = 1, we can write y = 1 – x. Substituting this into the equation: 10x + 11(1 – x) = 10.81 => 10x + 11 – 11x = 10.81 => 11 – x = 10.81 => x = 11 – 10.81 = 0.19. So, the abundance of 10B is 0.19 or 19%. Then, y = 1 – x = 1 – 0.19 = 0.81 or 81%. The abundance of 10B and 11B, respectively, is 19% and 81%.
The average atomic mass of an element is the weighted average of the masses of its isotopes, where the weights are their natural abundances. This principle is used to determine the relative abundance of isotopes if the average atomic mass and isotopic masses are known.
Isotopes are atoms of the same element that have the same number of protons but different numbers of neutrons, resulting in different atomic masses. Boron’s two stable isotopes are 10B (with 5 protons and 5 neutrons) and 11B (with 5 protons and 6 neutrons). The natural abundance of isotopes is relatively constant for a given element across the Earth.

25. For an element with atomic number 35, which one of the following will

For an element with atomic number 35, which one of the following will be the correct number of electrons in its valence shell based on Bohr’s model of an atom?

1
3
5
7
This question was previously asked in
UPSC NDA-1 – 2022
The correct answer is 7. An element with atomic number 35 is Bromine (Br), which is a halogen in Group 17 of the periodic table and typically has 7 valence electrons.
– Bohr’s model describes electrons orbiting the nucleus in specific energy levels or shells (K, L, M, N, etc.).
– The maximum number of electrons a shell can hold is generally given by 2n², where n is the shell number (n=1 for K, n=2 for L, etc.).
– For atomic number 35 (Bromine), we fill the shells sequentially:
– K shell (n=1): 2 electrons (capacity 2)
– L shell (n=2): 8 electrons (capacity 8)
– M shell (n=3): 18 electrons (capacity 18)
– N shell (n=4): The remaining electrons: 35 – (2 + 8 + 18) = 35 – 28 = 7 electrons.
– The outermost shell is the valence shell, which is the N shell in this case.
– The number of electrons in the valence shell is 7.
While the simple Bohr model doesn’t fully explain the filling order of electrons in multi-electron atoms due to subshells, the concept of valence electrons in the outermost shell is fundamental. Elements in the same group of the periodic table generally have the same number of valence electrons (e.g., Halogens in Group 17 all have 7 valence electrons), which determines their chemical properties.

26. Which one of the following is the correct order of the valencies of el

Which one of the following is the correct order of the valencies of elements Ne, Si, N and Mg ?

[amp_mcq option1=”Ne < Mg < N < Si" option2="Si < N < Mg < Ne" option3="Ne < N < Si < Mg" option4="Mg < Ne < N < Si" correct="option1"]

This question was previously asked in
UPSC NDA-1 – 2022
The correct order of the valencies of elements Ne, Si, N and Mg is Ne < Mg < N < Si.
Valency is the combining capacity of an element, typically the number of electrons an atom needs to gain, lose, or share to form a chemical bond and achieve a stable electron configuration.
Neon (Ne) is a noble gas with a full outer electron shell, so its valency is generally 0. Magnesium (Mg) is in Group 2 and typically loses 2 electrons, so its valency is 2. Nitrogen (N) is in Group 15 and typically gains 3 electrons or forms 3 covalent bonds, so its valency is 3. Silicon (Si) is in Group 14 and typically forms 4 covalent bonds, so its valency is 4. Ordering these valencies (0, 2, 3, 4) gives Ne < Mg < N < Si.

27. The mass number of argon is 40. Which one of the following statements

The mass number of argon is 40. Which one of the following statements is correct?

The number of protons in argon is 22.
The number of neutrons in argon is 18.
The number of electrons in argon is 18.
The sum of numbers of protons and electrons in argon is 40.
This question was previously asked in
UPSC NDA-1 – 2022
The number of electrons in a neutral atom of argon is 18.
The mass number of an element is the sum of the number of protons and neutrons in the nucleus. The atomic number, which defines the element, is equal to the number of protons. In a neutral atom, the number of protons is equal to the number of electrons. Argon (Ar) has an atomic number of 18.
Given mass number = 40. Atomic number of Argon = 18. Number of protons = 18. For a neutral atom, number of electrons = number of protons = 18. Number of neutrons = Mass number – Number of protons = 40 – 18 = 22.

28. Which of the following sets of elements has the same valency ?

Which of the following sets of elements has the same valency ?

Na, Mg, Ca
Na, Mg, Al
Mg, Ca, K
Mg, Ca, Ba
This question was previously asked in
UPSC NDA-1 – 2021
The set of elements Mg, Ca, and Ba has the same valency. All three elements belong to Group 2 (Alkaline Earth Metals) of the periodic table.
Elements in the same group of the periodic table typically have the same number of valence electrons and thus exhibit similar chemical properties, including the same common valency. Magnesium (Mg), Calcium (Ca), and Barium (Ba) are all in Group 2, and they tend to lose their two valence electrons to form ions with a +2 charge. Their valency is 2.
In option A, Na (Group 1, valency 1), Mg (Group 2, valency 2), Ca (Group 2, valency 2). In option B, Na (valency 1), Mg (valency 2), Al (Group 13, valency 3). In option C, Mg (valency 2), Ca (valency 2), K (Group 1, valency 1). Only option D shows elements exclusively from the same group (Group 2) with the same characteristic valency.

29. In which of the following pairs are the ions isoelectronic ?

In which of the following pairs are the ions isoelectronic ?

Mg$^{2+}$, Ar
Na$^{+}$, O$^{2-}$
Al$^{3+}$, Cl$^{-}$
K$^{+}$, Ne
This question was previously asked in
UPSC NDA-1 – 2019
The pair Na⁺ and O²⁻ are isoelectronic.
Isoelectronic species are atoms or ions that have the same number of electrons. Let’s check the electron count for each species:
– Mg²⁺: Magnesium (Mg) has atomic number 12, so it has 12 electrons. Mg²⁺ has lost 2 electrons, so it has 10 electrons.
– Ar: Argon (Ar) has atomic number 18, so it has 18 electrons.
– Na⁺: Sodium (Na) has atomic number 11, so it has 11 electrons. Na⁺ has lost 1 electron, so it has 10 electrons.
– O²⁻: Oxygen (O) has atomic number 8, so it has 8 electrons. O²⁻ has gained 2 electrons, so it has 10 electrons.
– Al³⁺: Aluminum (Al) has atomic number 13, so it has 13 electrons. Al³⁺ has lost 3 electrons, so it has 10 electrons.
– Cl⁻: Chlorine (Cl) has atomic number 17, so it has 17 electrons. Cl⁻ has gained 1 electron, so it has 18 electrons.
– K⁺: Potassium (K) has atomic number 19, so it has 19 electrons. K⁺ has lost 1 electron, so it has 18 electrons.
– Ne: Neon (Ne) has atomic number 10, so it has 10 electrons.

Comparing the pairs:
A) Mg²⁺ (10e⁻), Ar (18e⁻) – Not isoelectronic.
B) Na⁺ (10e⁻), O²⁻ (10e⁻) – Isoelectronic.
C) Al³⁺ (10e⁻), Cl⁻ (18e⁻) – Not isoelectronic.
D) K⁺ (18e⁻), Ne (10e⁻) – Not isoelectronic.

Many ions of elements close to noble gases are isoelectronic with the nearest noble gas. For example, Na⁺, Mg²⁺, Al³⁺, O²⁻, F⁻ are all isoelectronic with Neon (10 electrons). K⁺, Ca²⁺, S²⁻, Cl⁻ are all isoelectronic with Argon (18 electrons). Isoelectronic species often have similar chemical properties related to their electron configuration.

30. The valency of an element depends upon the

The valency of an element depends upon the

total number of protons in an atom
mass number of an atom
total number of neutrons in an atom
total number of electrons in the outer most shell of an atom
This question was previously asked in
UPSC NDA-1 – 2017
The valency of an element depends upon the total number of electrons in the outermost shell of an atom. These electrons are called valence electrons.
Valency is the combining capacity of an element, which is determined by how readily an atom gains, loses, or shares electrons to achieve a stable electron configuration, typically like that of a noble gas (usually a full outer shell). The number of electrons in the outermost shell dictates how an atom will participate in chemical bonding. Elements with a few valence electrons tend to lose them (forming positive ions), elements with nearly full outer shells tend to gain electrons (forming negative ions), and elements with partially filled outer shells often share electrons (forming covalent bonds).
The total number of protons determines the atomic number and identifies the element. The mass number is the sum of protons and neutrons and relates to the atom’s mass and isotopes. The number of neutrons affects isotopes but generally not the chemical properties or valency of the element. The arrangement of electrons in the outer shell is the primary factor governing chemical reactivity and valency.