Chapter 2: Structure of Atom Short Questions | chemistry Class 11 CBSE

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Chapter 2: Structure of Atom Short Questions | chemistry Class 11 CBSE | ByteNotes.in

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Question 1

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What are cathode rays and what do they consist of?

Cathode rays are streams of particles produced in a cathode ray discharge tube when high voltage is applied across electrodes at very low pressure.
These rays travel from the cathode (negative electrode) to the anode (positive electrode) in straight lines in the absence of electric or magnetic fields.
They consist of negatively charged particles called electrons, whose properties are independent of the electrode material and the gas in the tube.

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Question 2

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Define atomic number and mass number. Write the relationship between them.

Atomic number (Z) is the number of protons present in the nucleus of an atom; it also equals the number of electrons in a neutral atom.
Mass number (A) is the total number of nucleons (protons + neutrons) present in the nucleus of an atom.
The relationship is: A = Z + n (number of neutrons), so the number of neutrons = A − Z.

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Question 3

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Distinguish between isotopes and isobars with one example each.

Isotopes are atoms of the same element with the same atomic number but different mass numbers; they differ in the number of neutrons. Example: ¹H, ²H (deuterium), ³H (tritium).
Isobars are atoms of different elements with the same mass number but different atomic numbers. Example: ¹⁴₆C and ¹⁴₇N.
Isotopes of an element show identical chemical behaviour because chemical properties are governed by the number of electrons (determined by atomic number, not neutron number).

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Question 4

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State two important conclusions Rutherford drew from his alpha-particle scattering experiment.

Most of the atom is empty space, since the vast majority of alpha particles passed through the gold foil undeflected.
The atom has a tiny, dense, positively charged nucleus at its centre, since a very small fraction of alpha particles were deflected by large angles (up to 180°) due to enormous repulsive force.
The nucleus is extremely small compared to the atom: the atomic radius is ~10^-10 m while the nuclear radius is ~10^-15 m — a ratio comparable to a cricket ball inside a 5 km sphere.

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Question 5

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State Planck's quantum theory and write the expression for the energy of a quantum.

Planck's quantum theory states that atoms and molecules can emit or absorb energy only in discrete quantities called quanta, not continuously.
The smallest indivisible quantity of energy that can be emitted or absorbed in the form of electromagnetic radiation is called a quantum (or photon).
The energy E of a quantum is directly proportional to its frequency: E = hν, where h = 6.626 × 10^-34 J·s is Planck's constant and ν is the frequency of radiation.

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Question 6

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What is the photoelectric effect? State the role of threshold frequency.

The photoelectric effect is the ejection of electrons from the surface of certain metals when they are irradiated with light of sufficiently high frequency.
Threshold frequency (ν₀) is the minimum frequency of incident radiation below which no photoelectric effect is observed, regardless of the intensity of light.
At frequency ν > ν₀, the kinetic energy of ejected electrons equals h(ν − ν₀); the minimum energy required to eject an electron (hν₀) is called the work function W₀.

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Question 7

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What is dual behaviour of electromagnetic radiation? Give one experimental evidence for each behaviour.

Electromagnetic radiation exhibits both wave-like and particle-like properties, which is called its dual behaviour.
Wave-like behaviour is evidenced by phenomena such as diffraction and interference, which can only be explained if light travels as a wave.
Particle-like behaviour is evidenced by the photoelectric effect and black body radiation, which can only be explained if light consists of discrete packets of energy (photons).

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Question 8

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State the de Broglie relation and explain its significance for sub-atomic particles.

de Broglie proposed that every moving particle has an associated wavelength given by λ = h/mv = h/p, where m is the mass, v is the velocity, and p is the momentum.
For macroscopic objects, the associated wavelength is negligibly small (due to large mass) and cannot be detected experimentally.
For sub-atomic particles like electrons (very small mass), the wavelength is significant and detectable — confirmed by electron diffraction experiments — making wave behaviour relevant for atoms and molecules.

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Question 9

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State Heisenberg's uncertainty principle and write its mathematical expression.

Heisenberg's uncertainty principle states that it is impossible to determine simultaneously the exact position and exact momentum (or velocity) of an electron.
Mathematically: Δx × Δpₓ ≥ h/4π, or equivalently Δx × mΔvₓ ≥ h/4π, where Δx is the uncertainty in position and Δpₓ is the uncertainty in momentum.
The principle is significant only for microscopic objects like electrons; for macroscopic objects the uncertainties are negligibly small and have no practical consequence.

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Question 10

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State the four postulates of Bohr's model of the hydrogen atom.

Electrons move in circular paths of fixed radius and energy called stationary states or orbits, arranged concentrically around the nucleus.
Energy of an electron in a stationary orbit does not change with time; energy is absorbed when moving to a higher orbit and emitted when moving to a lower orbit.
The frequency of radiation emitted or absorbed during a transition is given by ν = ΔE/h, known as Bohr's frequency rule.
The angular momentum of the electron in a stationary state is quantised: mevr = nh/2π, where n = 1, 2, 3… is the principal quantum number.

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Question 11

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Give two drawbacks of Rutherford's nuclear model of atom.

Rutherford's model could not explain atomic stability: according to classical electromagnetic theory, an accelerating electron should continuously emit radiation, lose energy, and spiral into the nucleus within ~10^-8 s.
The model provided no information about the distribution of electrons around the nucleus or the energies of these electrons.
If electrons were stationary to avoid radiation, the electrostatic attraction would pull them into the nucleus, creating a miniature Thomson model — neither option explained a stable, real atom.

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Question 12

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Define the principal quantum number and azimuthal quantum number. What information does each provide?

The principal quantum number (n) is a positive integer (1, 2, 3…) that determines the size of the orbital and, to a large extent, the energy of the electron in the orbital.
The azimuthal (or orbital angular momentum) quantum number (l) can take values from 0 to (n−1) and defines the three-dimensional shape of the orbital and identifies the subshell (s, p, d, f).
For a given n, there are n subshells; the number of orbitals in a shell is n², and the maximum number of electrons in a shell with principal quantum number n is 2n².

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Question 13

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What is the magnetic quantum number? For l = 2, list all possible values of ml.

The magnetic orbital quantum number (ml) gives information about the spatial orientation of the orbital with respect to a standard set of coordinate axes.
For any subshell with azimuthal quantum number l, the possible values of ml are: −l, −(l−1), ..., 0, ..., (l−1), l, giving (2l+1) values in total.
For l = 2 (d subshell), the possible values of ml are −2, −1, 0, +1, and +2, corresponding to the five d orbitals.

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Chapter 2: Structure of Atom | Short Questions

What are cathode rays and what do they consist of?

Define atomic number and mass number. Write the relationship between them.

Distinguish between isotopes and isobars with one example each.

State two important conclusions Rutherford drew from his alpha-particle scattering experiment.

State Planck's quantum theory and write the expression for the energy of a quantum.

What is the photoelectric effect? State the role of threshold frequency.

What is dual behaviour of electromagnetic radiation? Give one experimental evidence for each behaviour.

State the de Broglie relation and explain its significance for sub-atomic particles.

State Heisenberg's uncertainty principle and write its mathematical expression.

State the four postulates of Bohr's model of the hydrogen atom.

Give two drawbacks of Rutherford's nuclear model of atom.

Define the principal quantum number and azimuthal quantum number. What information does each provide?

What is the magnetic quantum number? For l = 2, list all possible values of ml.

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