Structure of Atom for NEET: Unit 2 Formulas, Rules and Traps
Structure of atom is the NCERT name for the chapter that explains what an atom is made of and where its electrons sit: a tiny nucleus of protons and neutrons, surrounded by electrons in orbitals that are described by four quantum numbers. For NEET it is Chemistry Unit 2, "Atomic Structure", and it is still in the syllabus. This guide shows what Unit 2 asks, works the key formulas with NCERT's own numbers and lists the traps that cost marks.
Is structure of atom in the NEET 2027 syllabus?
Yes. Atomic Structure is Unit 2 of the Chemistry section in the NMC syllabus for NEET (UG) 2026, dated 22 December 2025. It sits between Some Basic Concepts in Chemistry (Unit 1) and Chemical Bonding (Unit 3). No 2027 syllabus or bulletin had been published on 2 November 2026.
The NEET (UG) home page still carried only 2026-cycle notices when we checked on 2 November 2026. Some websites already print NEET 2027 dates and "new" syllabus lists, but none of that comes from NTA or NMC yet. Until the 2027 file appears, the 2026 syllabus is the one to prepare structure of atom from, and this post will be updated on the day it changes.
Worried that a topic was cut? Our NEET 2027 deleted syllabus guide covers the full list. Atomic Structure is not on it.
What does Unit 2 actually ask?
For structure of atom, Unit 2 asks for the hydrogen atom in two versions, Bohr's and the quantum mechanical one, plus the rules for filling electrons into orbitals. Thomson's and Rutherford's models and the discovery of the electron, proton and neutron are not named in the Chemistry Unit 2 text, although NCERT teaches them first.
Here is the syllabus line, split into the pieces you can tick off:
| Syllabus phrase (NMC, NEET UG 2026) | Where it lives in NCERT Unit 2 |
|---|---|
| Nature of electromagnetic radiation, photoelectric effect | 2.3.1 and 2.3.2, Table 2.2 work functions |
| Spectrum of the hydrogen atom | 2.3.3, Table 2.3 spectral series |
| Bohr model: postulates, energy and radius relations, limitations | 2.4, 2.4.1, 2.4.2 |
| Dual nature of matter, de Broglie's relationship | 2.5.1 |
| Heisenberg uncertainty principle | 2.5.2 |
| Quantum mechanical model and its important features | 2.6 |
| Orbitals as one-electron wave functions, ψ and ψ² against r for 1s and 2s | 2.6.2 |
| Quantum numbers n, l, ml and spin ms | 2.6.1 |
| Shapes of s, p and d orbitals | 2.6.2 |
| Aufbau principle, Pauli's exclusion principle, Hund's rule | 2.6.4 |
| Electronic configuration, extra stability of half-filled and filled orbitals | 2.6.5 and 2.6.6 |
So what about Rutherford? The alpha-scattering experiment and Rutherford's model do appear in the syllabus, but under Physics Unit 18, Atoms and Nuclei. The photoelectric effect and de Broglie also turn up in Physics Unit 17. That overlap is real, and our modern physics for NEET guide handles the Physics side. This structure of atom guide sticks to what Chemistry adds: quantum numbers, orbitals, nodes and configurations.
What are electrons, protons and neutrons, in numbers?
An atom is mostly empty space with a positive nucleus at the centre. Protons and neutrons sit in the nucleus and carry almost all the mass; electrons move around it and carry the negative charge. The electron's mass is about 1/1836 of a proton's. NCERT Table 2.1 gives the exact values used in every structure of atom numerical.
| Particle | Charge (C) | Relative charge | Mass (kg) | Mass (u) |
|---|---|---|---|---|
| Electron | -1.602176 × 10^-19 | -1 | 9.109382 × 10^-31 | 0.00054 |
| Proton | +1.602176 × 10^-19 | +1 | 1.6726216 × 10^-27 | 1.00727 |
| Neutron | 0 | 0 | 1.674927 × 10^-27 | 1.00867 |
Two dates from the same pages come up in assertion-reason questions: the proton was characterised in 1919, and Chadwick discovered the neutron in 1932 by bombarding beryllium with alpha particles. The neutron is slightly heavier than the proton, which is why "neutron and proton have equal mass" is false at the fourth decimal.
How does the Bohr model give energy and radius?
Bohr's model says the electron in hydrogen moves in fixed circular orbits with quantised angular momentum, mvr = nh/2π. Each orbit has a fixed energy, En = -2.18 × 10^-18 (1/n²) J, and a fixed radius, rn = 52.9 n² pm. For one-electron ions such as He⁺ and Li²⁺, multiply the energy by Z² and divide the radius by Z.
Those two lines cover most Bohr numericals in structure of atom. The negative sign is not a typo. NCERT sets the energy of a free electron at rest, infinitely far from the nucleus, to zero, so every bound state sits below zero. The ground state, n = 1, is the most negative and the most stable.
| n | Energy En (J) | Radius rn (pm) |
|---|---|---|
| 1 | -2.18 × 10^-18 | 52.9 |
| 2 | -0.545 × 10^-18 | 211.6 |
| 3 | -0.242 × 10^-18 | 476.1 |
| 4 | -0.136 × 10^-18 | 846.4 |
The first two energies and the 52.9 pm Bohr radius are printed in NCERT; the rest of the table is the same formula worked out by us. NCERT's Problem 2.11 shows the Z effect: for He⁺ in n = 1, the energy is -8.72 × 10^-18 J (four times hydrogen's) and the radius is 0.02645 nm (half of 0.0529 nm).
Bohr's model still fails in ways NEET likes to ask, and structure of atom questions often test exactly these limits. It can't explain the fine doublets in the hydrogen spectrum, the spectra of atoms with more than one electron, the splitting of lines in a magnetic field (Zeeman effect) or an electric field (Stark effect), or why atoms form molecules.
Which spectral series belongs to which region?
The hydrogen spectrum has five named series. Each one ends on a fixed lower orbit n1: Lyman ends on n = 1, Balmer on n = 2, Paschen on n = 3, Brackett on n = 4 and Pfund on n = 5. Only the Balmer series lies in the visible region; Lyman is ultraviolet and the other three are infrared, as NCERT Table 2.3 shows.
| Series | n1 (lower orbit) | n2 | Region |
|---|---|---|---|
| Lyman | 1 | 2, 3, ... | Ultraviolet |
| Balmer | 2 | 3, 4, ... | Visible |
| Paschen | 3 | 4, 5, ... | Infrared |
| Brackett | 4 | 5, 6, ... | Infrared |
| Pfund | 5 | 6, 7, ... | Infrared |
Work one line through. NCERT's Problem 2.10 takes the jump from n = 5 to n = 2: ΔE = 2.18 × 10^-18 J × (1/4 - 1/25) = 4.58 × 10^-19 J, given out as light. Divide by h and you get a frequency of 6.91 × 10^14 Hz. Divide the speed of light by that and the wavelength comes to about 434 nm, which is violet and right where a Balmer line should be (that last step is our arithmetic).
One more structure of atom number to know: NCERT gives the Rydberg constant for hydrogen as 109,677 cm^-1, which is 1.09677 × 10^7 m^-1. The NIST value of the Rydberg constant, 10,973,731.568 m^-1, is the general constant and is slightly larger; NCERT's 109,677 cm^-1 is the figure for hydrogen itself. Use NCERT's hydrogen value in NEET numericals.
Photoelectric effect, de Broglie and Heisenberg
The photoelectric effect shows that light behaves as particles: an electron leaves a metal only if one photon carries more energy than the metal's work function, whatever the brightness. De Broglie turned the idea around and said every moving particle has a wavelength, λ = h/mv. Both ideas pushed the structure of atom story from Bohr's orbits towards orbitals.
NCERT's potassium example is the cleanest way to remember it. Red light (about 4.3 to 4.6 × 10^14 Hz) of any brightness ejects nothing from potassium, while even weak yellow light (5.1 to 5.2 × 10^14 Hz) does, because potassium's threshold frequency is 5.0 × 10^14 Hz. Brighter light ejects more electrons, not faster ones. Table 2.2 lists work functions from 2.25 eV for potassium to 4.8 eV for copper.
For de Broglie, NCERT's Problem 2.13 takes an electron of mass 9.1 × 10^-31 kg with kinetic energy 3.0 × 10^-25 J. Its speed is 812 m s^-1 and its wavelength is 896.7 nm. Problem 2.12 puts a 0.1 kg ball at 10 m s^-1, and the wavelength drops to about 6.6 × 10^-34 m, far too small to detect. That contrast is the whole point: wave behaviour matters only for tiny masses. The CODATA value of the Planck constant is exactly 6.62607015 × 10^-34 J Hz^-1; NCERT rounds it to 6.626 × 10^-34 J s.
Heisenberg's uncertainty principle closes the section: you can't know the exact position and the exact momentum of an electron at the same time, Δx × Δp ≥ h/4π. Its consequence, as NCERT puts it, is that it rules out definite paths or trajectories for electrons. That is exactly why fixed Bohr orbits had to go.
What do the four quantum numbers tell you?
Four quantum numbers describe each electron in an atom. The principal number n sets the shell and most of the energy; the azimuthal number l sets the subshell and the shape; the magnetic number ml sets the orientation of the orbital; and the spin number ms, +1/2 or -1/2, sets the spin. No two electrons in an atom share all four values.
The rules chain together, and most structure of atom questions on quantum numbers only test this chain:
| Quantum number | Allowed values | What it fixes |
|---|---|---|
| n (principal) | 1, 2, 3, ... (shells K, L, M, N) | Shell, size, most of the energy |
| l (azimuthal) | 0 to n - 1 (s, p, d, f for 0, 1, 2, 3) | Subshell and shape |
| ml (magnetic) | -l to +l, so 2l + 1 values | Orientation, number of orbitals in the subshell |
| ms (spin) | +1/2 or -1/2 | Spin direction |
So n = 2 allows l = 0 and 1 (2s and 2p), and l = 2 needs n of at least 3, which is why there is no 2d orbital. NCERT's Exercise 2.24 asks the same thing for g orbitals: l = 4, so n must be at least 5.
How many nodes does an orbital have?
In structure of atom, a node is a region where the chance of finding the electron falls to zero. An orbital has n - l - 1 radial nodes and l angular nodes, so n - 1 nodes in total. A 2s orbital has one radial node, a 3p orbital has one radial and one angular node, and a 3d orbital has no radial node and two angular nodes.
| Orbital | n | l | Radial nodes (n - l - 1) | Angular nodes (l) | Total (n - 1) |
|---|---|---|---|---|---|
| 1s | 1 | 0 | 0 | 0 | 0 |
| 2s | 2 | 0 | 1 | 0 | 1 |
| 3s | 3 | 0 | 2 | 0 | 2 |
| 2p | 2 | 1 | 0 | 1 | 1 |
| 3p | 3 | 1 | 1 | 1 | 2 |
| 4p | 4 | 1 | 2 | 1 | 3 |
| 3d | 3 | 2 | 0 | 2 | 2 |
The structure of atom syllabus asks for the plots of ψ and ψ² against r for 1s and 2s. Remember one picture: for 1s, ψ² is highest at the nucleus and falls steadily; for 2s, it falls to zero once (the radial node) and then rises to a second, smaller maximum. Angular nodes are planes through the nucleus. For pz it is the xy plane, and dxy has two.
How do Aufbau, Pauli and Hund fill the orbitals?
Electrons fill orbitals in order of rising energy (the Aufbau principle), at most two per orbital with opposite spins (Pauli's exclusion principle), and they spread out with parallel spins across orbitals of equal energy before pairing (Hund's rule). The energy order follows the (n + l) rule: lower n + l fills first, and on a tie, lower n wins.
NCERT's working order is 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, 6s, 4f, 5d, 6p, 7s. Check the (n + l) rule on the first surprise: 4s has n + l = 4 and 3d has 5, so 4s fills first. That is why potassium's last electron goes into 4s, not 3d.
Follow these steps for any structure of atom configuration question:
- Count the electrons: Z for a neutral atom, then add or subtract for an ion.
- Fill in the (n + l) order above, two electrons per orbital.
- Spread electrons singly across p, d or f orbitals before pairing (Hund).
- Check for the half-filled or filled exception (next section).
- Count unpaired electrons from the box diagram, not from the written string.
Two more structure of atom lines from NCERT turn up as statements. In hydrogen, every subshell of a shell has the same energy, because its energy depends only on n. In multi-electron atoms, the energy depends on n and l, and the same subshell gets lower in energy as atomic number rises: E2s(H) > E2s(Li) > E2s(Na) > E2s(K).
Why are chromium and copper exceptions?
Chromium is 3d⁵ 4s¹, not 3d⁴ 4s², and copper is 3d¹⁰ 4s¹, not 3d⁹ 4s². One 4s electron moves into 3d because half-filled and completely filled subshells are extra stable. NCERT gives three reasons: a symmetrical distribution of electrons, relatively small shielding and the largest exchange energy.
Exchange energy is the stabilising effect of electrons with the same spin in orbitals of equal energy. The number of possible exchanges is highest in a half-filled or full subshell, so d⁵ and d¹⁰ win. NCERT lists p³, p⁶, d⁵, d¹⁰, f⁷ and f¹⁴ as the extra-stable set, and adds its own caution that exceptions exist beyond Cr and Cu.
Why does this part of structure of atom matter beyond Unit 2? Because the same configurations come back in chemical bonding and in the d block, where magnetic moment questions start from the number of unpaired electrons. Get Cr and Cu wrong here and you lose marks in two other units.
Which lines trap students in structure of atom?
Most mistakes in this chapter are not about hard physics. They come from a sign, a unit or a rule applied one step too far. These are the traps that show up again and again:
- Dropping the minus sign. Bohr energies are negative. Energy released in emission is a positive amount of energy given out, but ΔE (final minus initial) is negative.
- Using Z in the wrong place. Energy goes with Z², radius with 1/Z. He⁺ n = 1 has four times the energy magnitude and half the radius of hydrogen.
- Mixing up the Rydberg values. 2.18 × 10^-18 J is the energy constant; 109,677 cm^-1 is the wavenumber constant. They are not interchangeable.
- Calling Balmer ultraviolet. Only Balmer is visible; Lyman is ultraviolet.
- Allowing 2d or 1p. l can't equal or exceed n.
- Counting radial nodes as n - 1. That is the total. Radial is n - l - 1.
- Writing Cr as 3d⁴ 4s². Cr and Cu each move one 4s electron into 3d.
- Filling 3d before 4s. By the (n + l) rule 4s (4 + 0) comes before 3d (3 + 2), so K and Ca put their electrons in 4s.
Each wrong answer costs one mark under the plus four, minus one rule in the NEET (UG) 2026 Information Bulletin. If you want to see when guessing is worth it, our NEET negative marking guide runs the numbers.
A one-week revision plan for structure of atom
Structure of atom works best in short, mixed sessions, because the numericals and the rule questions use different muscles. Read the NCERT structure of atom chapter once, then practise from its end-of-chapter exercise, which has 67 questions.
- Day 1: Table 2.1 and the Bohr formulas. Build the energy and radius table above from memory for n = 1 to 4.
- Day 2: Spectral series. Do NCERT Exercises 2.13 to 2.18 and 2.55 on transitions.
- Day 3: Photoelectric effect and de Broglie. Do Exercises 2.9 to 2.12 and 2.20 to 2.21.
- Day 4: Quantum numbers. Do Exercises 2.24, 2.25, 2.29, 2.30 and 2.31.
- Day 5: Nodes and orbital shapes. Draw ψ² against r for 1s and 2s and fill in the node table without looking.
- Day 6: Configurations for Z = 1 to 30, with Cr and Cu, then Exercises 2.66 and 2.67.
- Day 7: A timed mixed set, then a list of every question you got wrong and why.
How many structure of atom questions will NEET 2027 have? Nobody can say honestly. NTA publishes the subject split, 45 Chemistry questions for 180 marks, not a chapter split. For a sensible revision order across the whole subject, futureMBBS's NEET 2027 chapter-wise weightage guide is a good next read, and its NEET UG 2027 preparation plan puts the units on a calendar. If you are unsure which NCERT edition to study from, its explainer on what the NCERT book is helps.
For the whole subject plan, start with our NEET 2027 Chemistry preparation strategy, and for practice by unit see the NEET Chemistry questions guide. Still catching up on Class 11? The Class 11 backlog plan puts this unit early for a reason: bonding, periodicity and the d block all lean on it.
Want to know whether structure of atom is where your Chemistry marks leak? NEETsheet's chapter-wise Chemistry sets and timed mocks show which trap you keep falling into, so your next revision goes there first. Start at neetsheet.in.
Sources
All links were checked on 2 November 2026.
- National Medical Commission (UGMEB), Syllabus for NEET (UG) 2026, public notice of 22 December 2025 (Chemistry Unit 2, Physics Units 17 and 18): https://cdnbbsr.s3waas.gov.in/s37bc1ec1d9c3426357e69acd5bf320061/uploads/2026/01/202601081066816297.pdf
- National Testing Agency, Information Bulletin NEET (UG) 2026 (pattern: Chemistry 45 questions and 180 of 720 marks; plus four and minus one marking): https://cdnbbsr.s3waas.gov.in/s37bc1ec1d9c3426357e69acd5bf320061/uploads/2026/02/202602231394640855.pdf
- National Testing Agency, NEET (UG) home page, checked on 2 November 2026 for any NEET (UG) 2027 notice: https://neet.nta.nic.in/
- NCERT, Chemistry Class 11 Part 1, Unit 2 Structure of Atom, reprint 2026-27 (Tables 2.1 to 2.4; equations 2.11 to 2.23; Problems 2.10 to 2.13; sections 2.1 to 2.6.6; Exercises 2.1 to 2.67): https://ncert.nic.in/textbook/pdf/kech102.pdf
- National Institute of Standards and Technology, CODATA value of the Planck constant, 6.62607015 × 10^-34 J Hz^-1 (exact): https://physics.nist.gov/cgi-bin/cuu/Value?h
- National Institute of Standards and Technology, CODATA value of the Rydberg constant, 10,973,731.568 m^-1: https://physics.nist.gov/cgi-bin/cuu/Value?ryd
Frequently Asked Questions
An atom has a tiny, dense nucleus of protons and neutrons at its centre, which holds almost all of its mass, and electrons around it in orbitals. The number of protons is the atomic number. In NCERT the electron carries a charge of -1.602176 × 10^-19 C and a mass of 9.109382 × 10^-31 kg.
Yes. Atomic Structure is Chemistry Unit 2 in the NMC syllabus for NEET (UG) 2026, the current file, and no 2027 syllabus had been published by 2 November 2026. It covers the Bohr model, the hydrogen spectrum, de Broglie, Heisenberg, quantum numbers, orbital shapes and electronic configuration. Check again when NTA releases the 2027 bulletin.
Structure of atom is a Class 11 topic. It is Unit 2 of NCERT Chemistry Class 11 Part 1 (reprint 2026-27). The Bohr model and the photoelectric effect come back in Class 12 Physics, in the chapters on atoms and on the dual nature of radiation and matter, so you meet them twice.
There are four quantum numbers: principal (n), azimuthal (l), magnetic (ml) and spin (ms). n can be 1, 2, 3 and so on; l runs from 0 to n - 1; ml runs from -l to +l; ms is +1/2 or -1/2. No two electrons in one atom can have the same set of all four.
Chromium is 3d5 4s1 and copper is 3d10 4s1 because half-filled and completely filled d subshells are extra stable. NCERT gives three reasons: a symmetrical distribution of electrons, relatively small shielding and the largest exchange energy, which is highest when a subshell is half full or full.
