Structure of an Atom - Class 11 NCERT

What is the fundamental building block of matter according to early philosophers?

Atom.

What does the Greek word 'atom' mean?

Uncutable or non-divisible.

Who proposed the first atomic theory based on scientific observations?

John Dalton in 1808.

What are the three subatomic particles identified in atomic structure?

Electrons, protons, and neutrons.

State one major problem for scientists after the discovery of subatomic particles.

To explain the stability of the atom.

Define the term 'electron'.

Negatively charged subatomic particle discovered in cathode ray tubes.

What did J.J. Thomson contribute to atomic theory in 1898?

Proposed the plum pudding model of the atom.

What experimental evidence supported the discovery of the electron?

Electrical discharge through gases in Cathode ray tubes.

What is the charge-to-mass ratio of the electron as determined by J.J. Thomson?

1.758820 × 10^11 C kg^–1.

Who conducted the oil drop experiment to measure the charge of the electron?

R.A. Millikan.

What is the charge of an electron?

–1.6022 × 10^–19 C.

What is a proton's charge and relation to an electron's charge?

Positive charge equal in magnitude to that of an electron but opposite in sign.

What was the key conclusion of Rutherford's α-particle scattering experiment?

Most of an atom is empty space, with a dense positively charged nucleus.

What does the quantum mechanical model of the atom describe?

It describes electron distributions in terms of probabilities rather than fixed positions.

Describe the nature of electromagnetic radiation according to Planck's theory.

Electromagnetic radiation emits energy in discrete quanta, known as photons.

What is the photoelectric effect?

Emission of electrons from a material when exposed to light of certain frequencies.

What relationship exists between frequency and energy of light according to Planck's equation?

E = hν, where E is energy, h is Planck's constant, and ν is frequency.

What is de Broglie's hypothesis regarding particles?

Matter, like light, exhibits both wave and particle properties.

State Heisenberg's uncertainty principle.

It is impossible to know both the exact position and momentum of an electron simultaneously.

What is an atomic orbital?

A mathematical function (wave function) representing the probability of finding an electron in an atom.

What are quantum numbers?

Numbers that describe the properties of atomic orbitals and the electrons in those orbitals.

What is the Aufbau principle?

Electrons occupy the lowest energy orbitals available before filling higher energy orbitals.

What does the Pauli Exclusion Principle state?

No two electrons in an atom can have the same set of four quantum numbers.

State Hund's rule of maximum multiplicity.

Electrons will occupy degenerate orbitals singly before pairing up.

What does the principal quantum number (n) indicate?

The size and energy level of an orbital.

What is the meaning of azimuthal quantum number (l)?

It indicates the shape of the orbital.

What is the significance of the magnetic quantum number (ml)?

It specifies the orientation of the orbital in space.

What is the purpose of electrons occupying orbitals according to quantum mechanics?

To minimize energy through a defined distribution.

How did Bohr improve previous atomic models?

By introducing quantized orbits for electrons.

What do the terms isobars and isotopes refer to?

Isobars are atoms with the same mass number but different atomic numbers; isotopes have the same atomic number but different mass numbers.

How is the electronic configuration of an atom determined?

Based on the order of filling of orbitals following Aufbau's principles, Pauli's exclusion principle, and Hund's rule.

Define what an emission spectrum indicates.

Buffer a spectrum of light emitted by an excited substance.

What is meant by the term 'ground state'?

The lowest energy state of an electron in an atom.

State the formula for calculating the energy of an electron in a hydrogen atom.

E_n = -2.18 × 10⁻¹⁸ J / n².

How does the shape of s, p, and d orbitals differ?

s orbitals are spherical, p orbitals have lobes, and d orbitals are more complex with different shapes.

What are the defining characteristics of the Schrödinger equation?

It incorporates wave-particle duality and determines discrete energy levels of electrons.

What constitutes the visual representation of atomic orbitals?

Boundary surface diagrams depicting regions with high probabilities of finding electrons.

What does |ψ|² represent in quantum mechanics?

The probability density of finding an electron in a given region.

Why does the electron configuration of transition metals show exceptions?

Due to increased stability associated with half-filled and fully filled subshells.

What is a core electron?

Electrons in fully filled inner shells that do not participate in bonding.

What are unpaired electrons?

Electrons that occupy degenerate orbitals singly before pairing.

List the types of electromagnetic radiation from lowest to highest frequency.

Radio waves, microwaves, infrared, visible light, ultraviolet, X-rays, gamma rays.

What is the role of the effective nuclear charge (Zeff)?

The net positive charge experienced by valence electrons after accounting for shielding.