Chemistry Atomic Structure Syllabus for NEET 2026

Many NEET Chemistry questions become easier when you clearly understand how atoms are organised and how electrons behave inside them. While studying Atomic Structure, you may find it confusing to connect concepts like atomic models, electromagnetic radiation, quantum theory, hydrogen spectrum, and electronic transitions because all these topics are interlinked. 

This chapter explains the discovery of sub-atomic particles, Bohr’s model, the photoelectric effect, isotopes, spectral series, and quantum concepts that form the foundation of modern Chemistry. Regular practice of formulas, spectral calculations, and model-based concepts helps you improve accuracy in numerical and theory-based NEET questions.

Sub-Atomic Particles and Their Discovery

The discovery of sub-atomic particles proved that atoms are made of smaller charged particles and are not indivisible, as proposed in Dalton’s atomic theory. These discoveries helped scientists understand the internal structure of atoms and laid the foundation for modern Atomic Structure.

Cathode Rays (Discovery of the Electron)

Cathode rays were discovered by Michael Faraday and later studied in detail by J.J. Thomson using discharge tubes at low pressure and high voltage. These experiments proved the existence of negatively charged particles called electrons.

• They travel in straight lines from the cathode to the anode.

• They consist of negatively charged particles called electrons.

Charge-to-Mass Ratio (e/me):
e/me = 1.758820 × 10¹¹ C kg⁻¹

Charge on Electron:
−1.602 × 10⁻¹⁹ C

Mass of Electron (me):
9.109 × 10⁻³¹ kg

Anode Rays / Canal Rays (Discovery of Proton)

Anode rays were discovered by Goldstein using a perforated cathode discharge tube. These rays helped in the discovery of positively charged particles present inside atoms.

• They consist of positively charged gaseous ions.

• Their e/m ratio depends on the nature of the gas present in the tube.

Mass of Proton:
1.6726 × 10⁻²⁷ kg

Discovery of the Neutron

The neutron was discovered by James Chadwick in 1932 by bombarding Beryllium with high-energy alpha particles. Neutrons are electrically neutral particles present inside the nucleus.

Mass of Neutron:
1.6749 × 10⁻²⁷ kg

NEET Order Checklist (e/m ratio):
Neutron (0) < Alpha Particle < Proton < Electron

Atomic Number, Mass Number, and Isotopes

Atomic composition is represented using the notation:

ᴬZX

These quantities help identify atoms and distinguish between different forms of the same element.

Atomic Number (Z)

The atomic number represents the number of protons present in the nucleus. In a neutral atom, it is also equal to the number of electrons.

Z = Number of protons = Number of electrons (in a neutral atom)

Mass Number (A)

Mass number represents the total number of protons and neutrons present in the nucleus.

A = Z + n

n = A − Z

Isotopes

Isotopes are atoms of the same element having the same atomic number but different mass numbers because of different numbers of neutrons.

Examples:
Protium = ¹₁H
Deuterium = ²₁D
Tritium = ³₁T

Isobars

Isobars are atoms having the same mass number but different atomic numbers.

Example:
¹⁴₆C and ¹⁴₇N

Isoelectronic Species

Isoelectronic species contain the same number of electrons, even though they may belong to different elements.

Example:
O⁻ and N²⁻ both contain 9 electrons.

Early Atomic Models

Scientists proposed different atomic models to explain the arrangement of particles inside an atom. These models gradually improved as new experimental observations were discovered.

Thomson Model of the Atom (Plum Pudding Model)

J.J. Thomson proposed that the atom is a positively charged sphere with electrons embedded inside it like seeds in a watermelon.

• Explained the electrical neutrality of atoms.

• Failed to explain atomic stability and line spectra.

Rutherford’s Nuclear Model of the Atom

Rutherford proposed this model based on the alpha-particle scattering experiment performed using a thin gold foil.

Observations:

• Most alpha particles passed undeflected.

• Some deflected through small angles.

• Very few bounced back.

Conclusions:

• Most of the space inside an atom is empty.

• Positive charge and mass are concentrated in the nucleus.

Radius of atom ≈ 10⁻¹⁰ m
Radius of nucleus ≈ 10⁻¹⁵ m

Wave Nature of Electromagnetic Radiation

Light behaves as electromagnetic radiation and shows wave properties such as wavelength and frequency. These properties help explain the behavior of radiation in atomic transitions.

Wavelength (λ)

Distance between two successive crests or troughs.

Frequency (ν)

Number of waves passing through a point per second.

Relationship Between Wavelength and Frequency

c = νλ

ν = c/λ

Where:
c = 3 × 10⁸ m s⁻¹

Wavenumber (ν̅)

ν̅ = 1/λ

Electromagnetic Spectrum

Radio Waves < Microwaves < Infrared < Visible < Ultraviolet < X-rays < Gamma Rays

Particle Nature of Light and Quantum Theory

Certain phenomena, such as blackbody radiation and the photoelectric effect could not be explained using wave theory alone. This led to the development of quantum theory.

Planck’s Quantum Theory

Max Planck proposed that energy is emitted or absorbed in small packets called quanta.

Energy of radiation:

E = hν = hc/λ

Where:
h = 6.626 × 10⁻³⁴ J s

Photoelectric Effect

When light of sufficient frequency falls on a metal surface, electrons are emitted from the surface.

• The minimum frequency required is called the threshold frequency.

Work Function

W₀ = hν₀ = hc/λ₀

Einstein’s Photoelectric Equation

hν = hν₀ + ½mv²

Bohr’s Model for the Hydrogen Atom

Bohr explained the stability of the hydrogen atom by introducing fixed energy levels and quantized electron orbits. This model successfully explained the hydrogen line spectrum.

Quantization of Angular Momentum

mvr = nh/2π

Where:
n = 1, 2, 3, ...

Energy Transition

ΔE = E₂ − E₁ = hν = hc/λ

Radius of Orbit

rn = 52.9 × n²/Z pm

rn = 0.529 × n²/Z Å

Energy of Orbit

En = −2.18 × 10⁻¹⁸ (Z²/n²) J atom⁻¹

En = −13.6 (Z²/n²) eV atom⁻¹

Atomic Hydrogen Spectrum

When excited electrons return to lower energy levels, hydrogen emits radiation of specific wavelengths, producing line spectra. These spectral lines are grouped into different series.

Rydberg Equation

1/λ = RH Z² [ (1/n₁²) − (1/n₂²) ]

Where:
RH = 1.097 × 10⁷ m⁻¹

Spectral Series

Total Number of Spectral Lines

Total Lines = n(n − 1)/2

Atomic Structure: Study Resources By PW

Physics Wallah provides complete study resources for Atomic Structure preparation, including revision notes, PYQs, MCQs, formula sheets, mind maps, sample papers, and practice questions to help you revise concepts and improve problem-solving accuracy for NEET 2026.