NEET (UG)

Practice Test 1 — Structure of Atom

12 questions • 18 minutes • auto-graded with full solutions

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Section A — MCQ (Single Correct)

Question 1

The charge on an electron is:

Solution: Electron charge $= -1.6\times10^{-19}\ \text{C}$.

Question 2

The number of neutrons in an atom with $A = 40$, $Z = 20$ is:

Solution: Neutrons $= A - Z = 20$.

Question 3

Rutherford concluded that the atom has a tiny dense:

Solution: Most mass and positive charge sit in the nucleus.

Question 4

The energy of the ground state of hydrogen is:

Solution: $E_1 = -13.6\ \text{eV}$.

Question 5

The number of orbitals in the $d$ subshell is:

Solution: $d$: $l = 2$, so $2l+1 = 5$ orbitals.

Question 6

The maximum number of electrons in the $n = 3$ shell is:

Solution: $2n^2 = 2\times9 = 18$.

Question 7

By the Aufbau principle, which fills first?

Solution: $4s$ fills before $3d$.

Question 8

The electronic configuration of oxygen ($Z = 8$) is:

Solution: $\text{O}: 1s^2 2s^2 2p^4$.

Section B — Assertion & Reason

Question 9

A: Rutherford's atomic model could not explain the stability of the atom.
R: A revolving electron should continuously radiate energy and spiral into the nucleus.

Solution: Radiating, in-spiralling electrons would make the atom unstable — R explains A.

Question 10

A: Chromium has the configuration $[\text{Ar}]3d^5 4s^1$.
R: Half-filled subshells have extra stability due to symmetry and exchange energy.

Solution: The stability of $3d^5$ causes the anomalous configuration — R explains A.

Question 11

A: Bohr's model successfully explains the spectra of all multi-electron atoms.
R: The model quantises the angular momentum of the electron.

Solution: Bohr's model works only for one-electron species, so A is false; it does quantise angular momentum, so R is true.

Question 12

A: Degenerate orbitals are singly occupied before pairing of electrons begins.
R: Single occupation with parallel spins minimises electron-electron repulsion (Hund's rule).

Solution: Minimising repulsion is exactly the reason for Hund's rule — R explains A.