Chapter 12 - Atoms

The idea that matter is made of atoms is ancient, but the internal structure of atoms became clear only through experiments.

Atoms Class 12 follows the development from Rutherford’s nuclear model to Bohr’s quantised model of hydrogen. Students learn how atomic energy levels explain the hydrogen spectrum.

Rutherford’s alpha-particle scattering experiment

Rutherford’s experiment directed alpha particles towards a thin metal foil.

The main observations were:

• Most particles passed through with little or no deflection

• Some were deflected through small angles

• A very small number were deflected through large angles or returned

These results showed that an atom is mostly empty space and that most of its mass and positive charge are concentrated in a tiny central nucleus.

Rutherford’s atomic model

In Rutherford’s model:

• A small, dense, positively charged nucleus lies at the centre

• Electrons move around the nucleus

• Most atomic volume is empty space

The model explained scattering results but had serious limitations.

Limitations of Rutherford’s model

According to classical electromagnetic theory, an accelerating charged particle should radiate energy.

An electron moving in a circular orbit is accelerating and should lose energy, spiral inward and collapse into the nucleus.

The model also could not explain discrete atomic line spectra.

Bohr’s model

Bohr introduced quantisation to explain hydrogen.

His main postulates include:

• Electrons occupy certain permitted stationary orbits

• They do not radiate energy while remaining in a stationary orbit

• Angular momentum is quantised

• Radiation is emitted or absorbed during transitions between allowed states

Radius of permitted orbits

Students derive the radius of the nth permitted orbit.

The radius increases with the square of the orbit number for hydrogen-like systems under the model.

Higher orbits are therefore farther from the nucleus.

Velocity of the electron

The electron’s orbital speed changes with orbit number.

Students derive the prescribed expression using electrostatic attraction as the centripetal force and Bohr’s quantisation condition.

Energy of the electron

The total energy of an electron in a bound hydrogen orbit is negative when zero energy is chosen for an electron infinitely far from the nucleus.

A more negative energy represents a more tightly bound state.

The lowest energy state is the ground state. Higher allowed states are excited states.

Absorption and emission

An electron absorbs energy to move to a higher level.

It emits a photon while moving to a lower level.

The photon energy equals the difference between the initial and final atomic energies.

Students should not use the energy of either orbit alone as the photon energy.

Hydrogen line spectrum

Hydrogen produces discrete spectral lines rather than a continuous spectrum.

Each line corresponds to an electron transition between specific energy levels.

The spectrum supports the idea of quantised atomic energies.

Common student difficulties

Students often:

• Assume most alpha particles were strongly deflected

• Ignore the instability of Rutherford’s orbiting electron

• Treat Bohr orbits as arbitrary

• Confuse orbit radius with nuclear size

• Forget the negative sign of bound-state energy

• Use the wrong initial and final levels

• Calculate photon energy without taking the level difference

• Confuse absorption with emission

Learning with Teachoo

Teachoo provides:

• Scattering-experiment diagrams

• Rutherford and Bohr model comparisons

• Radius, velocity and energy derivations

• Energy-level diagrams

• Hydrogen-spectrum questions

• NCERT solutions

• MCQs and application-based problems

Frequently Asked Questions

What did Rutherford’s experiment prove?

It showed that atoms are mostly empty space and contain a small, dense, positively charged nucleus.

Why was Rutherford’s model unstable?

A classically orbiting electron should radiate energy and collapse into the nucleus.

What did Bohr add to the atomic model?

Bohr proposed discrete stationary orbits and quantised energy levels.

Why is electron energy negative in a bound atom?

Zero energy is chosen at infinite separation. Energy must be supplied to remove the bound electron completely.

What produces an atomic spectral line?

A photon is emitted or absorbed when an electron moves between two allowed energy levels.

Is Bohr’s model valid for every atom?

It works best for hydrogen and hydrogen-like one-electron systems and has limitations for more complex atoms.