Chapter 2 - Structure of Atom

Structure of Atom explains what exists inside an atom and how scientists developed the modern picture of atomic structure. The chapter begins with experimental discoveries and moves through successive atomic models before introducing orbitals, quantum numbers and electronic configuration.

The best way to understand this chapter is as a sequence: each new model was needed because the earlier one could not explain an important observation.

Discovery of subatomic particles

Experiments with electrical discharge through gases showed that atoms are divisible. Students study cathode rays, anode rays and the discoveries of electrons, protons and neutrons.

Important ideas include:

• Charge-to-mass ratio of the electron

• Millikan’s oil-drop experiment

• Charge and approximate mass of each subatomic particle

• Location of particles inside an atom

Atomic number, mass number, isotopes and isobars

The atomic number is the number of protons in the nucleus and identifies the element. The mass number is the total number of protons and neutrons.

• Isotopes have the same atomic number but different mass numbers.

• Isobars have the same mass number but different atomic numbers.

Development of atomic models

Thomson proposed a positively charged sphere containing electrons. It explained electrical neutrality but not later scattering observations.

Rutherford’s alpha-particle scattering experiment showed that most of an atom is empty space and that its positive charge and mass are concentrated in a tiny nucleus. The model could not explain atomic stability or line spectra.

Electromagnetic radiation and quantum theory

Students learn wavelength, frequency, velocity, wave number and the electromagnetic spectrum. Planck proposed that energy is absorbed or emitted in discrete packets called quanta.

The photoelectric effect supports the particle nature of radiation. The line spectrum of hydrogen shows that atoms absorb and emit only specific energies.

Bohr’s model of hydrogen

Bohr proposed fixed permitted energy levels. Electrons do not radiate energy while remaining in a permitted orbit. Radiation is absorbed or emitted only when an electron moves between levels.

Students solve questions involving:

• Radius of a permitted orbit

• Energy of an electron

• Energy difference between levels

• Frequency and wavelength of radiation

• Hydrogen spectral series

From fixed orbits to orbitals

De Broglie proposed that moving particles possess wave character. Heisenberg’s uncertainty principle states that the exact position and exact momentum of a microscopic particle cannot be known simultaneously.

The quantum-mechanical model therefore describes the probability of finding an electron in an orbital, not a fixed path.

Quantum numbers

Four quantum numbers describe an electron:

• Principal quantum number: main energy level

• Azimuthal quantum number: subshell and shape

• Magnetic quantum number: orbital orientation

• Spin quantum number: electron spin

Students also study the shapes and orientations of s, p and d orbitals and the meaning of nodes.

Electronic configuration

Electrons are assigned to orbitals using:

• Aufbau principle

• Pauli exclusion principle

• Hund’s rule

Half-filled and completely filled subshells have extra stability, explaining special configurations such as chromium and copper.

What can students study on Teachoo?

• Atomic-model explanations and diagrams

• NCERT solutions

• Bohr-model numericals

• Hydrogen-spectrum questions

• de Broglie wavelength calculations

• Quantum-number validity questions

• Orbital diagrams

• Electronic-configuration practice

• MCQs, assertion-reason and case-based questions

Common mistakes

• Confusing an orbit with an orbital

• Treating shells, subshells and orbitals as identical

• Using incorrect combinations of quantum numbers

• Filling orbitals before applying Hund’s rule

• Forgetting the special configurations of chromium and copper

• Mixing absorption and emission transitions

Best way to study this chapter

Study the atomic models chronologically. For each model, learn its proposal, supporting observation and limitation. Then practise quantum numbers and electronic configurations by writing them, not by reading solved examples passively.

Frequently asked questions

What is the main idea of Structure of Atom?

The chapter explains the internal structure of atoms and the arrangement and behaviour of electrons.

What is the difference between an orbit and an orbital?

An orbit is a fixed path in Bohr’s model. An orbital is a three-dimensional region where the probability of finding an electron is high.

How many electrons can an orbital contain?

An orbital can contain a maximum of two electrons with opposite spins.

Why did Rutherford’s model fail?

It could not explain why revolving electrons did not lose energy and collapse into the nucleus, and it could not explain atomic line spectra.

What do quantum numbers describe?

They describe an electron’s energy level, subshell, orbital orientation and spin.

Why are chromium and copper exceptions?

An electron shifts to the d subshell because half-filled and completely filled d subshells have additional stability.

Learn Structure of Atom with Teachoo for atomic models, quantum numbers, electronic configuration, numerical practice and complete NCERT solutions.