What went wrong when the king rode under the arch?

It was too low and knocked off his crown

How does the chain of blame unfold in the poem?

Each accused person blames another, ending with the king's own changes

Who discovered the neutron and how?

Chadwick (1932) discovered the neutron by bombarding beryllium with \(\alpha\)-particles, releasing a neutral particle slightly heavier than the proton.

State the conclusions of Rutherford's scattering experiment.

Most of the atom is empty space; the positive charge and nearly all the mass lie in a tiny central nucleus; electrons revolve around it.

Define atomic number and mass number.

Atomic number \(Z\) is the number of protons (and electrons in a neutral atom); mass number \(A\) is the sum of protons and neutrons.

State Planck's quantum theory.

Atoms emit or absorb energy only in discrete quanta, each of energy \(E=h\nu\), where \(h=6.626\times10^{-34}\,\text{J s}\).

State the postulates of Bohr's model.

Electrons revolve in fixed stationary orbits of definite energy; energy is emitted or absorbed only on transition between orbits (\(\Delta E=h\nu\)); angular momentum is quantised as \(mvr=nh/2\pi\).

Name the five spectral series of hydrogen.

Lyman (UV), Balmer (visible), Paschen, Brackett and Pfund (infrared).

State the de Broglie relation.

Every moving particle has an associated wavelength \(\lambda=h/mv\).

Why is \(O_2\) configuration relevant to Hund's rule?

By Hund's rule of maximum multiplicity, degenerate orbitals are singly filled before pairing, leaving two unpaired electrons that make dioxygen paramagnetic.

What are the four quantum numbers?

Principal \(n\), azimuthal \(l\), magnetic \(m_l\) and spin \(m_s\), which together specify the state of an electron.

State the Pauli exclusion principle.

No two electrons in an atom can have the same set of all four quantum numbers, so an orbital holds at most two electrons of opposite spin.

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Chapter 2: Structure of Atom — Class 11 Chemistry

Chemistry · 9 chapters

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Chapter 2: Structure of Atom

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The neutron was discovered by:

Summary

This unit traces the experimental discovery of the atom's internal structure and the models built from it. Cathode-ray experiments revealed the electron, whose charge-to-mass ratio J. J. Thomson measured (\(1.758\times10^{11}\,\text{C kg}^{-1}\)) and whose charge Millikan's oil-drop experiment fixed (\(-1.602\times10^{-19}\,\text{C}\)). Canal rays revealed the proton, and Chadwick discovered the neutron in 1932. Thomson's plum-pudding model gave way to Rutherford's nuclear model after the gold-foil \(\alpha\)-scattering experiment showed a tiny, dense, positively charged nucleus. The atomic number \(Z\) equals the proton count, the mass number \(A\) equals protons plus neutrons, and isotopes and isobars are defined. Rutherford's model could not explain atomic stability or line spectra. Planck's quantum theory (\(E=h\nu\)) and Einstein's explanation of the photoelectric effect established the dual wave-particle nature of radiation. Bohr quantised electron orbits and energy, successfully explaining the hydrogen line spectrum (Lyman, Balmer, Paschen, Brackett and Pfund series), but failed for multi-electron atoms. de Broglie's matter-wave relation and Heisenberg's uncertainty principle led to the quantum-mechanical model based on Schrödinger's equation, in which orbitals are described by four quantum numbers and electrons are filled following the Aufbau principle, Pauli exclusion principle and Hund's rule.

Discovery of electron, proton and neutronThomson and Rutherford atomic modelsAtomic number, mass number, isotopes and isobarsPlanck's quantum theory and photoelectric effectBohr's model and hydrogen spectrumde Broglie relation and uncertainty principleQuantum-mechanical model and quantum numbersShapes of orbitals and electronic configuration

Key terms

Isotopes

Atoms of the same element with the same atomic number but different mass numbers, differing in number of neutrons.

Quantum

The smallest discrete packet of energy that can be emitted or absorbed as electromagnetic radiation, \(E=h\nu\).

Photoelectric effect

The ejection of electrons from a metal surface when radiation of frequency above a threshold frequency strikes it.

Atomic orbital

The wave function \(\psi\) of an electron in an atom; \(|\psi|^{2}\) gives the probability of finding the electron at a point.

Uncertainty principle

Heisenberg's rule that the position and momentum of an electron cannot both be determined exactly at the same instant.

Aufbau principle

Orbitals are filled in order of increasing energy, the lower \((n+l)\) orbital filling first.

Important questions

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Isotopes

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Atoms of the same element with the same atomic number but different mass numbers, differing in number of neutrons.

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Practice quiz · Structure of Atom

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Structure of Atom

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