JEE Mains Chemistry · Teaching notes
Structure of Atom — JEE Mains Chemistry
Structure of Atom has 130 past-year questions from 2021 to 2026, and 46 of them ask for a numerical answer. The numbers come from a handful of relations: a photon's energy, the Bohr orbit, the Rydberg equation, the de Broglie wavelength and the uncertainty principle. The rest of the chapter is counting and ordering, of quantum numbers, nodes and orbital energies, and it rewards knowing the rules exactly.
Every subtopic, worked example, formula and trap in one printable document — answers shown, ready to share.
Subtopic notes
Photons, Planck's Quantum and the Photoelectric Effect
19 PYQsLight comes in packets of energy hν. This page turns wavelength, frequency and wavenumber into photon energy, per photon or per mole, and uses it to eject electrons from a metal.
Bohr Model: Radius, Energy and Velocity
25 PYQsIn a one-electron atom or ion, Bohr's orbits have fixed radii, energies and speeds, and each scales with the orbit number n and the nuclear charge Z.
Hydrogen Spectrum and Spectral Series
18 PYQsWhen the electron of a hydrogen-like species drops between orbits it emits one line; the Rydberg equation gives each line's wavenumber, and the lines group into series by the orbit they land on.
de Broglie Waves and the Uncertainty Principle
15 PYQsA moving particle has a wavelength h/mv, a Bohr orbit holds a whole number of these waves, and the position and momentum of a particle can never both be known exactly.
Quantum Numbers and Electron Counting
16 PYQsFour quantum numbers, n, l, mₗ and mₛ, label every electron in an atom. Their allowed values decide how many orbitals and electrons a shell or subshell can hold.
Orbitals: Nodes, Shapes and Probability Plots
15 PYQsAn orbital is a region where the electron is likely to be found. Its nodes, its shape and its probability plots all follow from n and l.
Orbital Energies and Electronic Configuration
22 PYQsIn a many-electron atom orbital energy follows the (n + l) rule; in a one-electron atom it depends on n alone. Filling orbitals in that order gives the configuration, with Cr and Cu as the exceptions.
Formula & revision sheet
13 formulas · 3 reference tables · 32 gotchas across all subtopics — the exam-eve cheat-sheet
Formula & revision sheet
13 formulas · 3 reference tables · 32 gotchas across all subtopics — the exam-eve cheat-sheet
Formulas (2)
Watch out for (4)
- Wavenumber in cm⁻¹ needs c in cm s⁻¹→ Photon energy from wavelength, frequency or wavenumber
- Energy ratio is the inverse wavelength ratio→ Photon energy from wavelength, frequency or wavenumber
- Intensity changes the current, not the energy→ Photoelectric effect and work function
- Put eV and J on the same footing→ Photoelectric effect and work function
Formulas (2)
Reference tables (1)
Thomson, Rutherford, Bohr and the quantum model4 rows
| Model | Picture of the atom | What it explained | Where it failed |
|---|---|---|---|
| Thomson | Uniform sphere of positive charge with electrons embedded | The atom is neutral overall | Predicts only small deflections, so cannot explain large-angle scatteringQ If Thomson were right, α-particles would pass through gold foil with only small deflections. |
| Rutherford | Tiny dense positive nucleus with electrons around it | Large-angle scattering; a few α-particles bounce back | An orbiting electron should radiate and spiral in; no line spectrum |
| Bohr | Electrons in fixed circular orbits, mvr = nh/2π | Stability and line spectrum of H, He⁺, Li²⁺ | Many-electron atoms (even Li⁺), Zeeman and Stark splitting; a definite path breaks the uncertainty principleQ Li⁺ has two electrons, so Bohr's theory does not apply to it. Li²⁺ has one. |
| Quantum mechanical | Electron as a wave; orbitals are regions of probability ψ² | All atoms, including many-electron ones | Keeps stationary states and ΔE = hν, but drops the definite orbit |
Watch out for (5)
- The radius goes as n², not n→ Orbit radius and its scaling
- First excited state is n = 2→ Orbit radius and its scaling
- Every orbit energy is negative→ Orbit energy, kinetic and potential energy, and speed
- KE is minus E, PE is twice E→ Orbit energy, kinetic and potential energy, and speed
- Li⁺ is not a one-electron ion→ Thomson, Rutherford, Bohr and the quantum model
Reference tables (1)
Spectral series, line counts and other spectra5 rows
| Series | Lands on (n₁) | First line | Series limit | Region |
|---|---|---|---|---|
| Lyman | 1 | 2 → 1, about 122 nm | ∞ → 1, about 91 nm | UltravioletQ |
| Balmer | 2 | 3 → 2, about 656 nm | ∞ → 2, about 365 nm | Visible |
| Paschen | 3 | 4 → 3, about 1875 nm | ∞ → 3, about 821 nm | InfraredQ With R rounded to 10⁵ cm⁻¹, the Paschen limit ∞ → 3 is 9/R = 900 nm, the infrared used for heat therapy. |
| Brackett | 4 | 5 → 4, about 4052 nm | ∞ → 4, about 1459 nm | Infrared |
| Pfund | 5 | 6 → 5, about 7460 nm | ∞ → 5, about 2280 nm | Infrared |
Watch out for (4)
- Count lines from the landing level→ Rydberg equation for hydrogen-like species
- Longest wavelength is the first line→ Rydberg equation for hydrogen-like species
- One electron is not many atoms→ Spectral series, line counts and other spectra
- Moseley used atomic number, not mass→ Spectral series, line counts and other spectra
Formulas (3)
Watch out for (5)
- Equal wavelength means equal momentum→ de Broglie wavelength
- Work in kg and J→ de Broglie wavelength
- The wavelength grows as n, not n²→ de Broglie waves in a Bohr orbit
- Mass in kg→ Heisenberg's uncertainty principle
- Δx = Δp is not Δx = Δv→ Heisenberg's uncertainty principle
Watch out for (4)
- l stops at n − 1→ Allowed values and counting orbitals and electrons
- Angular momentum uses l, not n→ Allowed values and counting orbitals and electrons
- Ions lose the highest-n electrons first→ Quantum numbers of a particular electron
- "By Aufbau" ignores the exceptions→ Quantum numbers of a particular electron
Formulas (1)
Reference tables (1)
Probability plots, boundary surfaces and shapes6 rows
| Orbital | ψ² at the nucleus | Radial nodes | Peaks in 4πr²ψ² | Shape |
|---|---|---|---|---|
| 1s | Maximum | 0 | One, at a₀ for H | SphereQ The density ψ² peaks at the nucleus; only the radial probability peaks at a₀. |
| 2s | Maximum | 1, at 2a₀ for H | Two, the outer one larger | SphereQ |
| 2p | Zero | 0 | One, at 4a₀ for H | Two lobes, one nodal plane |
| 3s | Maximum | 2 | Three | Sphere |
| 3p | Zero | 1 | Two | Two lobes, one nodal plane |
| 3d | Zero | 0 | One, at 9a₀ for H | Four lobes (d z² has two lobes and a ring), two nodal surfaces |
Watch out for (4)
- Radial nodes are n − l − 1→ Radial and angular nodes
- A nodal plane is an angular node→ Radial and angular nodes
- Density is not radial probability→ Probability plots, boundary surfaces and shapes
- Signs on lobes are not charges→ Probability plots, boundary surfaces and shapes
Formulas (3)
Watch out for (6)
- A tie goes to the lower n→ The (n + l) rule in many-electron atoms
- m does not enter→ The (n + l) rule in many-electron atoms
- Hydrogen does not follow Aufbau→ One-electron atoms and the effect of Z
- Higher Z lowers the orbital→ One-electron atoms and the effect of Z
- Exchange energy needs degenerate orbitals→ Configurations, exceptions and electron counts
- Half-filled is not filled→ Configurations, exceptions and electron counts
PYQ weightage by concept
17 concepts · 130 PYQs — where the marks actually sit, so you know what to drill first
PYQ weightage by concept
17 concepts · 130 PYQs — where the marks actually sit, so you know what to drill first
| Concept | PYQs | Share |
|---|---|---|
| Photon energy from wavelength, frequency or wavenumber | 11 | 8% |
| Photoelectric effect and work function | 8 | 6% |
| Concept | PYQs | Share |
|---|---|---|
| Orbit energy, kinetic and potential energy, and speed | 13 | 10% |
| Orbit radius and its scaling | 7 | 5% |
| Thomson, Rutherford, Bohr and the quantum model | 5 | 4% |
| Concept | PYQs | Share |
|---|---|---|
| Rydberg equation for hydrogen-like species | 10 | 8% |
| Spectral series, line counts and other spectra | 8 | 6% |
| Concept | PYQs | Share |
|---|---|---|
| de Broglie wavelength | 6 | 5% |
| Heisenberg's uncertainty principle | 5 | 4% |
| de Broglie waves in a Bohr orbit | 4 | 3% |
| Concept | PYQs | Share |
|---|---|---|
| Allowed values and counting orbitals and electrons | 11 | 8% |
| Quantum numbers of a particular electron | 5 | 4% |
| Concept | PYQs | Share |
|---|---|---|
| Probability plots, boundary surfaces and shapes | 8 | 6% |
| Radial and angular nodes | 7 | 5% |
| Concept | PYQs | Share |
|---|---|---|
| Configurations, exceptions and electron counts | 8 | 6% |
| The (n + l) rule in many-electron atoms | 7 | 5% |
| One-electron atoms and the effect of Z | 7 | 5% |
Test yourself on Structure of Atom
20 past JEE Mains questions from this chapter, timed at 48 minutes and marked the way the exam marks it. You see your score and every answer the moment you finish. Free to start.