Is Atoms part of the EAPCET Physics syllabus?

Yes. Atoms is taught in Intermediate second year Physics, and the EAPCET Engineering syllabus is based on the Intermediate first- and second-year syllabus. Check the official notification each year for any changes.

How many Physics questions are there in EAPCET?

40 of the 160 questions in the Engineering paper are Physics, spread across 30 chapters.

What mistakes do students make in Atoms?

Taking the longest wavelength from the largest jump — longest wavelength means smallest energy difference, so it comes from the nearest level. Forgetting Z² for hydrogen-like ions such as He⁺ or Li²⁺ when finding energies or wavelengths. Mixing up the signs of energies — kinetic energy is +13.6 Z²/n² eV, potential energy is twice the total energy, and the total energy is negative. Inverting the 1/λ formula incorrectly — first compute 1/λ from R(1/n₁² − 1/n₂²), then take its reciprocal.

EAPCET Physics · Intermediate second year

Atoms

Atoms covers Rutherford’s model, the Bohr model of the hydrogen atom and the hydrogen spectrum.

Subject: Physics
Taught in: Intermediate second year
Physics in the paper: 40 of 160 questions

Key formulas and results

$r_{n} \propto n^{2}/Z$ , with $r_{1} = 0.529\ \text{Å}$ for hydrogen
$E_{n} = -\frac{13.6\,Z^{2}}{n^{2}}\ \text{eV}$
Angular momentum $mvr = \frac{nh}{2\pi}$
$\frac{1}{\lambda} = RZ^{2}\left(\frac{1}{n_{1}^{2}} - \frac{1}{n_{2}^{2}}\right)$
Lyman series ( $n_{1} = 1$ ) is ultraviolet; Balmer ( $n_{1} = 2$ ) visible; Paschen ( $n_{1} = 3$ ) infrared

What this chapter covers

Rutherford’s scattering experiment
Bohr’s postulates
Energy levels
Hydrogen spectrum

How it is usually asked

Energy levels and transitions
Wavelength of spectral lines
Bohr radius and speed

Understanding Atoms

Rutherford’s alpha-scattering experiment showed that an atom’s positive charge and almost all its mass sit in a tiny nucleus, with electrons outside. Classical physics, however, predicts that orbiting electrons should radiate energy and spiral in. Bohr solved this for hydrogen by postulating stationary orbits in which the angular momentum is a whole multiple of h/2π and no radiation is emitted.

From these postulates, the orbit radius grows as n²/Z, the speed falls as Z/n, and the energy is Eₙ = −13.6 Z²/n² eV. Energy is negative because the electron is bound. When an electron jumps from a higher level to a lower one, it emits a photon whose energy equals the difference between the levels, giving the hydrogen spectral series.

EAPCET usually asks for energies, wavelengths and wavelength ratios of transitions, the number of possible spectral lines, and how radius, speed or energy scale with n and Z. The ionisation energy of hydrogen in its ground state is 13.6 eV. The Lyman series lies in the ultraviolet, the Balmer series mainly in the visible and the Paschen series in the infrared, a frequent recall question.

Worked example

EAPCET-style question

The ratio of the longest wavelength in the Lyman series to the longest wavelength in the Balmer series of hydrogen is

(1) $\frac{5}{27}$
(2) $\frac{27}{5}$
(3) $\frac{4}{9}$
(4) $\frac{3}{4}$

Solution

The longest wavelength in a series comes from the smallest energy gap, the transition from the next level up.
Lyman ( $2 \to 1$ ): $\frac{1}{\lambda_{L}} = R\left(1 - \frac{1}{4}\right) = \frac{3R}{4}$ .
Balmer ( $3 \to 2$ ): $\frac{1}{\lambda_{B}} = R\left(\frac{1}{4} - \frac{1}{9}\right) = \frac{5R}{36}$ .
$\frac{\lambda_{L}}{\lambda_{B}} = \dfrac{5R/36}{3R/4} = \frac{20}{108} = \frac{5}{27}$ .

Answer: $\frac{5}{27}$

Common mistakes to avoid

Taking the longest wavelength from the largest jump — longest wavelength means smallest energy difference, so it comes from the nearest level.
Forgetting Z² for hydrogen-like ions such as He⁺ or Li²⁺ when finding energies or wavelengths.
Mixing up the signs of energies — kinetic energy is +13.6 Z²/n² eV, potential energy is twice the total energy, and the total energy is negative.
Inverting the 1/λ formula incorrectly — first compute 1/λ from R(1/n₁² − 1/n₂²), then take its reciprocal.

How to prepare it

Transitions from level n can produce n(n − 1)/2 different spectral lines.

Once the formulas feel familiar, test them the way the exam does: practise Atoms questions chapter-wise, then attempt a full previous year paper to see whether it holds up under a 3-hour timer.

Related Physics chapters

See every chapter in the EAPCET Physics syllabus.

Practise Atoms chapter-wise

Chapter-wise quizzes show exactly which topics in this chapter you still get wrong — before a full paper does.

Frequently asked questions

Is Atoms part of the EAPCET Physics syllabus?: Yes. Atoms is taught in Intermediate second year Physics, and the EAPCET Engineering syllabus is based on the Intermediate first- and second-year syllabus. Check the official notification each year for any changes.
How should I prepare Atoms for EAPCET?: Transitions from level n can produce n(n − 1)/2 different spectral lines.
How many Physics questions are there in EAPCET?: 40 of the 160 questions in the Engineering paper are Physics, spread across 30 chapters.
What mistakes do students make in Atoms?: Taking the longest wavelength from the largest jump — longest wavelength means smallest energy difference, so it comes from the nearest level. Forgetting Z² for hydrogen-like ions such as He⁺ or Li²⁺ when finding energies or wavelengths. Mixing up the signs of energies — kinetic energy is +13.6 Z²/n² eV, potential energy is twice the total energy, and the total energy is negative. Inverting the 1/λ formula incorrectly — first compute 1/λ from R(1/n₁² − 1/n₂²), then take its reciprocal.