Atomic Structure

11. What is the maximum number of electrons that can occupy the first shel

What is the maximum number of electrons that can occupy the first shell of an atom ?

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2
4
8
16
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The maximum number of electrons that can occupy the first electron shell of an atom is 2.
– Electron shells (also called energy levels) surround the nucleus of an atom. They are denoted by the principal quantum number, n (n=1, 2, 3, …), where n=1 is the shell closest to the nucleus.
– The maximum number of electrons that can occupy a given shell is determined by the formula 2n², where n is the shell number.
– For the first shell (n=1), the maximum number of electrons is 2 * (1)² = 2 * 1 = 2.
– For the second shell (n=2), the maximum number of electrons is 2 * (2)² = 2 * 4 = 8.
– For the third shell (n=3), the maximum number of electrons is 2 * (3)² = 2 * 9 = 18, and so on.
The first shell (n=1) contains only one subshell, the ‘s’ subshell, which can hold a maximum of 2 electrons. The electron configuration follows the Aufbau principle, Hund’s rule, and the Pauli exclusion principle, filling lower energy levels before higher ones.

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12. As proposed by Dalton, which of the following symbol represents phosph

As proposed by Dalton, which of the following symbol represents phosphorus ?

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Circle with P inside
Circle with L inside
Circle with vertical and horizontal lines crossing in the center
Circle with three lines radiating from the center to the circumference
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As proposed by Dalton in his atomic theory, each element had a unique symbol. Phosphorus was represented by a circle with three lines radiating outwards from the center to the circumference.
Dalton’s symbols were pictorial representations of elements and compounds, unlike the modern alphabetical symbols.
Other examples of Dalton’s symbols include a simple circle for Oxygen, a circle with a dot in the center for Hydrogen, and a circle with vertical and horizontal lines crossing for Sulphur (often shown as + inside circle or vertical/horizontal lines). Option A represents the modern symbol for Phosphorus inside a circle, which was not Dalton’s representation.

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13. Which one of the following ions is not iso-electronic with F⁻ ?

Which one of the following ions is not iso-electronic with F⁻ ?

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O²⁻
Na⁺
Ne
N⁻
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The correct option is D. Iso-electronic species are atoms or ions that have the same number of electrons. We need to find which species among the options does not have the same number of electrons as F⁻.
A neutral Fluorine atom (F) has 9 electrons (atomic number = 9). The F⁻ ion has gained one electron, so it has $9 + 1 = 10$ electrons.
Let’s find the number of electrons for each option:
A) O²⁻: Neutral Oxygen (O) has 8 electrons (atomic number = 8). O²⁻ has gained two electrons, so $8 + 2 = 10$ electrons.
B) Na⁺: Neutral Sodium (Na) has 11 electrons (atomic number = 11). Na⁺ has lost one electron, so $11 – 1 = 10$ electrons.
C) Ne: Neutral Neon (Ne) has 10 electrons (atomic number = 10).
D) N⁻: Neutral Nitrogen (N) has 7 electrons (atomic number = 7). N⁻ has gained one electron, so $7 + 1 = 8$ electrons.
Thus, N⁻ has 8 electrons, while F⁻ has 10 electrons. N⁻ is not isoelectronic with F⁻.
Isoelectronic species often have similar chemical and physical properties due to having the same electron configuration. The species O²⁻, F⁻, Ne, Na⁺, Mg²⁺, Al³⁺ are all isoelectronic with 10 electrons, having the electron configuration of Neon.

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14. Chlorine occurs in nature in two isotopic forms of masses 35 u and 37

Chlorine occurs in nature in two isotopic forms of masses 35 u and 37 u in the ratio of 3 : 1 respectively. What is the average atomic mass of the Chlorine atom ?

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36·1 u
35·5 u
36·5 u
35·1 u
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Chlorine exists as two isotopes: $^{35}$Cl with a mass of 35 u and $^{37}$Cl with a mass of 37 u. They are present in nature in a ratio of 3:1. This means that out of every 4 chlorine atoms, approximately 3 have a mass of 35 u and 1 has a mass of 37 u. The average atomic mass is calculated as a weighted average of the masses of the isotopes based on their relative abundance. The relative abundance of $^{35}$Cl is 3/4 or 0.75, and the relative abundance of $^{37}$Cl is 1/4 or 0.25.
Average atomic mass = ($\text{Mass of isotope 1} \times \text{Fractional abundance of isotope 1}$) + ($\text{Mass of isotope 2} \times \text{Fractional abundance of isotope 2}$)
Average atomic mass = $(35 \text{ u} \times 3/4) + (37 \text{ u} \times 1/4)$
Average atomic mass = $(105/4) \text{ u} + (37/4) \text{ u}$
Average atomic mass = $(105 + 37)/4 \text{ u}$
Average atomic mass = $142/4 \text{ u}$
Average atomic mass = $35.5 \text{ u}$.
Atomic mass unit (u) is a standard unit used for indicating atomic and molecular masses. Average atomic mass is the value listed for an element on the periodic table. It reflects the natural isotopic composition of the element.

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15. What is the maximum number of electrons in the M-Shell ?

What is the maximum number of electrons in the M-Shell ?

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6
8
18
32
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Electron shells are denoted by letters K, L, M, N, … or by principal quantum numbers n = 1, 2, 3, 4, … The K-shell corresponds to n=1, the L-shell to n=2, the M-shell to n=3, and so on. The maximum number of electrons that can occupy a shell with principal quantum number ‘n’ is given by the formula $2n^2$. For the M-shell, n=3, so the maximum number of electrons is $2 \times (3)^2 = 2 \times 9 = 18$.
The formula for the maximum number of electrons in a shell with principal quantum number ‘n’ is $2n^2$.
K-shell (n=1): $2 \times 1^2 = 2$ electrons.
L-shell (n=2): $2 \times 2^2 = 8$ electrons.
M-shell (n=3): $2 \times 3^2 = 18$ electrons.
N-shell (n=4): $2 \times 4^2 = 32$ electrons.
While the formula $2n^2$ gives the maximum capacity of a shell, the actual filling of electrons follows the Aufbau principle, Hund’s rule, and the Pauli exclusion principle, which dictates the filling order based on subshells (s, p, d, f) and energy levels. The outermost shell cannot accommodate more than 8 electrons (octet rule, with some exceptions), and the second last shell cannot accommodate more than 18 electrons.

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16. Atomic mass of an element is equal to the sum of number of

Atomic mass of an element is equal to the sum of number of

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electrons and protons only
protons and neutrons only
electrons and neutrons only
electrons, protons and neutrons
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The correct answer is (B) protons and neutrons only. The atomic mass of an element (more accurately, the mass number of a specific isotope or the relative atomic mass) is primarily determined by the mass of the particles in the nucleus. Protons and neutrons are located in the nucleus and account for almost all of an atom’s mass. The mass of electrons is negligible in comparison.
The nucleus of an atom contains protons and neutrons, which contribute significantly to its mass. Electrons have very little mass.
The Mass Number (A) of an isotope is the sum of the number of protons (Z) and neutrons (N) in its nucleus (A = Z + N). The atomic mass listed on the periodic table for an element is the weighted average of the masses of its naturally occurring isotopes, taking into account their relative abundances.

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17. Which one of the following is the most fundamental characteristic of a

Which one of the following is the most fundamental characteristic of an element ?

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Melting point
Atomic number
Colour
Atomic weight
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The correct answer is (B) Atomic number. The atomic number (Z) of an element is defined as the number of protons in the nucleus of an atom of that element. This number is unique to each element and determines its fundamental chemical identity.
The atomic number is the defining characteristic of an element, determining its position in the periodic table and its chemical properties.
Melting point and color are physical properties that can vary depending on conditions or even within different forms (allotropes) of the same element. Atomic weight (more precisely, relative atomic mass) is the weighted average mass of isotopes of an element and is determined by the number of protons and neutrons, but the identity is fixed by the number of protons alone.

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18. The number of maximum electrons in N Shell is

The number of maximum electrons in N Shell is

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2
8
18
32
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According to the rules governing electron shells, the maximum number of electrons in the N shell (n=4) is 32.
– Electron shells are designated by principal quantum numbers (n), starting from n=1 for the innermost shell (K shell).
– The formula for the maximum number of electrons that can occupy a shell with principal quantum number ‘n’ is 2n².
– For the N shell, n=4.
– Applying the formula 2n² for n=4: Maximum electrons = 2 * (4)² = 2 * 16 = 32.
– The distribution of electrons in shells follows specific rules, filling the lower energy shells first, but the maximum capacity is given by 2n².

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19. A sample of oxygen contains two isotopes of oxygen with masses 16 u an

A sample of oxygen contains two isotopes of oxygen with masses 16 u and 18 u respectively. The proportion of these isotopes in the sample is 3 : 1. What will be the average atomic mass of oxygen in this sample?

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17.5 u
17 u
16 u
16.5 u
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The average atomic mass of oxygen in this sample is 16.5 u.
– Average atomic mass is calculated as the weighted average of the masses of its isotopes based on their relative abundance.
– The sample contains isotopes with masses 16 u and 18 u in a proportion of 3:1.
– This means out of 4 parts, 3 parts have mass 16 u and 1 part has mass 18 u.
– Relative abundance of 16u isotope = 3 / (3+1) = 3/4 = 0.75
– Relative abundance of 18u isotope = 1 / (3+1) = 1/4 = 0.25
– Average atomic mass = (Mass₁ × Abundance₁) + (Mass₂ × Abundance₂)
– Average atomic mass = (16 u × 0.75) + (18 u × 0.25)
– Average atomic mass = 12 u + 4.5 u = 16.5 u.
Naturally occurring oxygen is predominantly isotope ¹⁶O (around 99.76%), with smaller amounts of ¹⁷O (around 0.038%) and ¹⁸O (around 0.205%). The standard average atomic mass of oxygen listed on the periodic table is approximately 15.999 u, reflecting the natural isotopic abundance. This question presents a hypothetical sample with a specific, non-natural isotopic ratio.

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20. The atomic number of an element is 8. How many electrons will it gain

The atomic number of an element is 8. How many electrons will it gain to form a compound with sodium?

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One
Two
Three
Four
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The element with atomic number 8 (Oxygen) will gain two electrons to form a compound with sodium.
– The element with atomic number 8 is Oxygen (O). Its electronic configuration is 2, 6.
– To achieve a stable octet configuration in its outermost shell, Oxygen needs to gain 8 – 6 = 2 electrons, forming the O²⁻ ion.
– Sodium (Na, atomic number 11) has electronic configuration 2, 8, 1 and tends to lose 1 electron to form the Na⁺ ion.
– In the formation of sodium oxide (Na₂O), two sodium atoms each lose one electron, and one oxygen atom gains two electrons.
Elements form chemical bonds (ionic or covalent) to achieve a stable electron configuration, usually a full valence shell (like noble gases). Oxygen is a non-metal in Group 16 and typically forms -2 ions by gaining two electrons. Sodium is an alkali metal in Group 1 and typically forms +1 ions by losing one electron.

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