Atomic Structure

5 min

Master fundamental subatomic particles, isotopes, and valency for competitive exams.

Golden Rule

Atoms consist of a dense nucleus containing protons and neutrons, surrounded by revolving electrons. The atomic number equals the number of protons, while the mass number is the sum of protons and neutrons.

Why This Matters

Direct 1-2 questions appear in SSC, Railway, and Defence General Science sections.
Forms the absolute foundation for Chemical Bonding, Periodic Table, and Radioactivity.
Frequently tests discovery names and basic isotope/isobar numerical differences.

Memory Hooks

E-T-P, P-R-C, N-J-C: Electron-Thomson-Plum, Proton-Rutherford-Canal, Neutron-Chadwick-Cricket.
IsoBARs have same mass number (BAR = heavy mass), IsoTOPs have same atomic number (TOP = atomic position).
Valency rule: If valence electrons are > 4, valency = 8 minus valence electrons.

Subatomic Particles Summary

Particle	Discoverer	Charge / Mass
Electron	J.J. Thomson	-1 / Negligible
Proton	Rutherford	+1 / 1 amu
Neutron	James Chadwick	0 / 1 amu

Core Concepts Visualised

Isotopes

(Same Z, Diff A)

Same Z, Diff A

Carbon-12 and Carbon-14 are isotopes.

Isobars

(Diff Z, Same A)

Diff Z, Same A

Argon-40 and Calcium-40 are isobars.

Valency

(Combining capacity)

Combining capacity

Sodium has a valency of +1.

Nucleons

(Protons + Neutrons)

Protons + Neutrons

Nucleons reside inside the atom core.

Exam Traps & Pitfalls

Proton discovered by RutherfordGoldstein discovered proton

Goldstein named/discovered canal rays (protons), but Rutherford named the proton particle. Exams credit Goldstein for discovery and Rutherford for the name.

Neutron has positive chargeNeutron is neutral (zero charge)

Never confuse neutron with proton. Neutrons contribute mass but have zero electrical charge.

Isotopes have same physical propertiesIsotopes differ in physical properties

Due to different mass numbers, physical properties like density and boiling point differ, though chemical properties remain identical.

Quick Exam Test

Q1.Who discovered the neutron in the year 1932?

Q2.Which of the following pairs represents isobars?

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