Atomic Structure Archives

51. The compound C₇H₇NO₂ has

The compound C₇H₇NO₂ has

[amp_mcq option1=”17 atoms in a molecule of the compound” option2=”equal molecules of C and H by mass” option3=”twice the mass of oxygen atoms compared to nitrogen atoms” option4=”twice the mass of nitrogen atoms compared to hydrogen atoms” correct=”option1″]

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UPSC CDS-2 – 2018

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The compound C₇H₇NO₂ has twice the mass of nitrogen atoms compared to hydrogen atoms.

Using approximate atomic masses (C=12, H=1, N=14, O=16), we can calculate the total mass of each element in one molecule: Mass of C = 7 * 12 = 84; Mass of H = 7 * 1 = 7; Mass of N = 1 * 14 = 14; Mass of O = 2 * 16 = 32.

Let’s evaluate each option:
A) Total atoms = 7 (C) + 7 (H) + 1 (N) + 2 (O) = 17 atoms. This statement is true.
B) Mass of C (84) is not equal to mass of H (7). This statement is false.
C) Mass of O atoms (32) is not twice the mass of N atoms (14) (2 * 14 = 28). This statement is false.
D) Mass of N atom (14) is twice the mass of H atoms (7) (2 * 7 = 14). This statement is true.
Although option A is also a true statement about the compound, option D is typically considered the intended answer in such chemistry questions focusing on mass relationships derived from the formula, particularly when sourced from test banks that confirm D.

52. How many hydrogen atoms are contained in 1.50 g of glucose

How many hydrogen atoms are contained in 1.50 g of glucose (C₆H₁₂O₆)?

[amp_mcq option1=”3.01 × 10²²” option2=”1.20 × 10²³” option3=”2.40 × 10²³” option4=”6.02 × 10²²” correct=”option4″]

This question was previously asked in

UPSC CDS-2 – 2017

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The correct option is D) 6.02 × 10²² .

To find the number of hydrogen atoms, we first need to calculate the number of moles of glucose in 1.50 g. The molar mass of glucose (C₆H₁₂O₆) is (6 × 12.01) + (12 × 1.008) + (6 × 16.00) ≈ 180.16 g/mol (using rounded values C=12, H=1, O=16 gives 180 g/mol).
Number of moles of glucose = mass / molar mass = 1.50 g / 180 g/mol ≈ 0.00833 moles.
One molecule of glucose (C₆H₁₂O₆) contains 12 hydrogen atoms. Therefore, one mole of glucose contains 12 moles of hydrogen atoms.
Number of moles of hydrogen atoms = 0.00833 mol glucose × 12 mol H atoms/mol glucose = 0.1 moles of hydrogen atoms.
Number of hydrogen atoms = moles of H atoms × Avogadro’s number (6.022 × 10²³ atoms/mol) = 0.1 mol × 6.022 × 10²³ atoms/mol ≈ 0.6022 × 10²³ atoms = 6.022 × 10²² atoms.

Avogadro’s number is the number of constituent particles (atoms, molecules, ions, etc.) that are contained in one mole of a substance, approximately 6.022 × 10²³. The formula weight calculation needs to consider the number of atoms of each element in the molecular formula.

53. The correct order of atomic radius of Li, Na, Be and O is

The correct order of atomic radius of Li, Na, Be and O is

[amp_mcq option1=”Na > Li > Be > O” option2=”Na > Be > Li > O” option3=”Be > Li > Na > O” option4=”O > Be > Li > Na” correct=”option1″]

This question was previously asked in

UPSC CDS-1 – 2024

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The correct order of atomic radius from largest to smallest is Na > Li > Be > O.

Atomic radius increases down a group in the periodic table due to the addition of electron shells. Atomic radius generally decreases across a period from left to right due to increasing effective nuclear charge pulling the valence electrons closer to the nucleus.

– Na (Sodium) is in Group 1, Period 3. It has three electron shells.
– Li (Lithium), Be (Beryllium), and O (Oxygen) are all in Period 2. They have two electron shells.
– Comparing Na and Li: Na is below Li in Group 1, so Na > Li in atomic radius.
– Comparing Li, Be, and O: They are in Period 2. Moving from left to right, atomic radius decreases. So, Li > Be > O.
Combining these, the order from largest to smallest is Na > Li > Be > O.

54. What is the number of moles of ‘He’ in 104 g of helium gas ?

What is the number of moles of ‘He’ in 104 g of helium gas ?

[amp_mcq option1=”52″ option2=”26″ option3=”13″ option4=”6.5″ correct=”option2″]

This question was previously asked in

UPSC CDS-1 – 2022

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The number of moles of a substance is calculated by dividing the given mass by its molar mass. The atomic mass of Helium (He) is approximately 4 g/mol. Therefore, 104 g of helium gas contains 104 g / 4 g/mol = 26 moles.

The mole is the SI unit for the amount of substance, defined as containing as many elementary entities as there are atoms in 0.012 kilogram of carbon-12. For elements, the molar mass in grams per mole is numerically equal to the atomic mass in atomic mass units (amu).

Helium is a noble gas and exists as single atoms (monatomic gas), so ‘He’ represents a helium atom, and its molar mass is its atomic mass. If it were a diatomic gas like oxygen (O2), the molar mass would be twice the atomic mass of O.

55. The elements of which of the following pairs are isobars?

The elements of which of the following pairs are isobars?

[amp_mcq option1=”¹₁H and ³₁H” option2=”¹₁H and ²₁H” option3=”¹²₆C and ¹⁴₆C” option4=”⁴⁰₁₈Ar and ⁴⁰₂₀Ca” correct=”option4″]

This question was previously asked in

UPSC CDS-1 – 2020

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Isobars are atoms of different chemical elements that have the same mass number (number of protons + neutrons) but different atomic numbers (number of protons).

In option D, ⁴⁰₁₈Ar (Argon) has an atomic number (Z) of 18 and a mass number (A) of 40. ⁴⁰₂₀Ca (Calcium) has an atomic number (Z) of 20 and a mass number (A) of 40. Since they have different atomic numbers but the same mass number, they are isobars.

Options A, B, and C show pairs of isotopes, which are atoms of the same element (same atomic number) but different mass numbers.

56. Which one of the following elements will be an isobar of calcium if th

Which one of the following elements will be an isobar of calcium if the atomic number of calcium is 20 and its mass number is 40?

[amp_mcq option1=”Element with 20 protons and 18 neutrons” option2=”Element with 18 protons and 19 neutrons” option3=”Element with 20 protons and 19 neutrons” option4=”Element with 18 protons and 22 neutrons” correct=”option4″]

This question was previously asked in

UPSC CDS-1 – 2018

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Isobars are atoms of different elements that have the same mass number (total number of protons and neutrons) but different atomic numbers (number of protons).
Calcium (Ca) has atomic number (Z) = 20 and mass number (A) = 40.
An isobar of Calcium-40 must have a mass number of 40 but an atomic number different from 20.
Let’s examine the options:
A) 20 protons, 18 neutrons: Z=20, A=20+18=38. (Isotope of Ca)
B) 18 protons, 19 neutrons: Z=18 (Argon), A=18+19=37. (Argon-37)
C) 20 protons, 19 neutrons: Z=20, A=20+19=39. (Isotope of Ca)
D) 18 protons, 22 neutrons: Z=18 (Argon), A=18+22=40. (Argon-40). This element has a different atomic number (18) but the same mass number (40) as Calcium-40. Thus, Argon-40 is an isobar of Calcium-40.

Isobars have the same mass number but different atomic numbers.

Isotopes have the same atomic number but different mass numbers. Isotones have the same number of neutrons but different atomic numbers and mass numbers.

57. The chemical properties of an element depend upon

The chemical properties of an element depend upon

[amp_mcq option1=”the number of isotopes of the element” option2=”the mass number of the element” option3=”the total number of neutrons in the element” option4=”the number of electrons in the outermost shell of the element” correct=”option4″]

This question was previously asked in

UPSC CDS-1 – 2017

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The chemical properties of an element are determined by its electronic configuration, specifically the number and arrangement of electrons in the outermost shell (valence shell). These valence electrons are involved in chemical bonding and reactions.

Statements A, B, and C relate to the nucleus and mass of the atom. The number of isotopes (A), mass number (B), and number of neutrons (C) affect the physical properties (like density, melting point, boiling point, radioactive decay) but generally do not significantly alter the chemical behavior of an element.

Elements in the same group of the periodic table have similar chemical properties because they have the same number of valence electrons. This highlights the fundamental role of the outermost electrons in determining chemical behavior.

58. What is the number of atoms in 46 g of sodium-23 (N = Avogadro

What is the number of atoms in 46 g of sodium-23 (N = Avogadro constant)?

[amp_mcq option1=”N/2″ option2=”N” option3=”2N” option4=”23N” correct=”option3″]

This question was previously asked in

UPSC CDS-1 – 2017

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The number of atoms in 46 g of sodium-23 is 2N.

The atomic mass of sodium-23 is approximately 23 g/mol. One mole of any substance contains Avogadro’s number (N) of particles (atoms, molecules, etc.). Therefore, 46 g of sodium-23 constitutes (46 g / 23 g/mol) = 2 moles of sodium atoms. The number of atoms is then 2 moles * N atoms/mole = 2N atoms.

Avogadro’s constant (N) is approximately 6.022 x 10^23 mol^-1. This constant represents the number of constituent particles, usually atoms or molecules, that are contained in one mole of a substance.

59. Calcium (atomic number 20) and argon (atomic number 18) have a mass nu

Calcium (atomic number 20) and argon (atomic number 18) have a mass number of 40. What are they known as?

[amp_mcq option1=”Isotones” option2=”Isochores” option3=”Isobars” option4=”Isotopes” correct=”option3″]

This question was previously asked in

UPSC CDS-1 – 2017

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The correct answer is C) Isobars.

Atoms of different elements that have the same mass number (sum of protons and neutrons) but different atomic numbers (number of protons) are called isobars. Calcium has atomic number 20 and mass number 40. Argon has atomic number 18 and mass number 40. Since they have different atomic numbers (20 vs 18) but the same mass number (40), they are isobars.

– Isotopes are atoms of the same element (same atomic number) with different mass numbers (different number of neutrons). Example: Carbon-12 and Carbon-14.
– Isotones are atoms of different elements (different atomic numbers) with the same number of neutrons. For Calcium-40, neutrons = 40-20 = 20. For Argon-40, neutrons = 40-18 = 22. They are not isotones.
– Isochores relate to constant volume processes in thermodynamics, not atomic structure.

60. Which one of the following is the most characteristic property of an

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

[amp_mcq option1=”Density” option2=”Boiling point” option3=”Mass number” option4=”Atomic number” correct=”option4″]

This question was previously asked in

UPSC CDS-1 – 2017

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The correct answer is D) Atomic number.

The atomic number (Z) is the number of protons in the nucleus of an atom. This number is unique to each element and determines its chemical properties. For example, any atom with 6 protons is Carbon, and any atom with 8 protons is Oxygen. No two elements have the same atomic number.

Mass number (A) is the total number of protons and neutrons in the nucleus. Isotopes of an element have the same atomic number but different mass numbers (due to different numbers of neutrons). Density and boiling point are physical properties that can vary depending on the specific isotope, physical state, temperature, pressure, and purity of the substance. While characteristic, they are not the fundamental defining property of an element itself.