Question 1

The maximum number of electrons that can be accommodated in the M shell (n = 3) is:

Accepted Answer

The maximum number of electrons in any shell is given by 2n², so for M shell (n=3): 2(3)² = 18.

Question 2

An atom has 15 protons and 16 neutrons. What is its mass number?

Accepted Answer

Mass number = Number of protons + Number of neutrons = 15 + 16 = 31.

Question 3

Which of the following correctly represents the electron configuration of Iron (Fe, atomic number 26)?

Accepted Answer

Iron (Z = 26) has configuration [Ar] 3d⁶ 4s², filling d-orbitals before completing the 4s orbital.

Question 4

The number of protons, neutrons, and electrons in ₃₇Cl⁻¹ are respectively:

Accepted Answer

Chlorine-37 has 17 protons and 20 neutrons (37 - 17); the -1 charge means one extra electron, so 18 electrons total.

Question 5

According to Bohr's model, the energy of an electron in the nth orbital of a hydrogen atom is proportional to:

Accepted Answer

Bohr's energy formula is E = -13.6 eV/n², showing energy is proportional to -1/n².

Question 6

The maximum number of electrons that can be accommodated in the M shell (n=3) is:

Accepted Answer

Maximum electrons in any shell = 2n², so for M shell (n=3): 2(3)² = 18 electrons.

Question 7

Which of the following correctly represents the electron configuration of Chromium (Cr, Z=24)?

Accepted Answer

Chromium exhibits anomalous configuration with one electron in 4s orbital for extra stability of half-filled d subshell: [Ar] 3d⁵ 4s¹.

Question 8

An atom has 35 electrons and 44 neutrons. The mass number of this atom is:

Accepted Answer

Mass number = Number of protons + Number of neutrons = 35 + 44 = 79.

Question 9

According to Heisenberg's Uncertainty Principle, if the uncertainty in position (Δx) of an electron is zero, then the uncertainty in momentum (Δp) would be:

Accepted Answer

Heisenberg's principle states Δx·Δp ≥ h/4π; if Δx = 0, then Δp must be infinite to satisfy the inequality.

Question 10

The number of orbitals present in the d subshell is:

Accepted Answer

For d subshell, l=2, which gives ml values from -2 to +2, resulting in 5 orbitals.

Question 11

An element has the valence electron configuration 3s² 3p⁵. The number of unpaired electrons in its ground state is:

Accepted Answer

Configuration 3s² 3p⁵ shows 3s² is paired and 3p⁵ has 4 paired electrons and 1 unpaired electron in the p orbitals.

Question 12

According to Bohr's model, the radius of the first Bohr orbit (a₀) is approximately:

Accepted Answer

The Bohr radius a₀ = 0.53 Å or 52.9 pm, representing the most probable distance of electron from nucleus in hydrogen atom.

Question 13

Two atoms are isotopes of the same element. Which of the following statements is true about them?

Accepted Answer

Isotopes are atoms of the same element (same atomic number/protons) with different numbers of neutrons, resulting in different mass numbers.

Question 14

An atom of element X has 18 electrons and 20 neutrons. What is the mass number of this atom?

Accepted Answer

Mass number = number of protons + number of neutrons = 18 + 20 = 38 (electrons equal protons in a neutral atom).

Question 15

According to Bohr's model, if an electron jumps from the 3rd orbit to the 1st orbit of a hydrogen atom, how many spectral lines can be emitted?

Accepted Answer

The electron can transition: 3→2, 2→1, and 3→1, producing 3 distinct spectral lines.

Question 16

The de Broglie wavelength of an electron is 0.727 nm. If Planck's constant h = 6.63 × 10⁻³⁴ J·s and electron mass m = 9.1 × 10⁻³¹ kg, what is the velocity of the electron (approximately)?

Accepted Answer

Using λ = h/(mv), velocity v = h/(m×λ) = (6.63 × 10⁻³⁴)/[(9.1 × 10⁻³¹) × (0.727 × 10⁻⁹)] ≈ 1.0 × 10⁶ m/s.

AFCAT Atomic Structure

Atomic Structure — Practice Questions