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Chadwick's Discovery of the Neutron (1932)

Chemistry · Structure Of Atom · NEET

James Chadwick discovered the neutron in 1932. He fired alpha particles at a thin sheet of the metal beryllium. This knocked out a new particle that had no charge (neutral) and a mass a little more than a proton. He named it the neutron. Memory hook: "Cha-D-wick used Be-ryllium to find the neu-Tral one in 32."
Chadwick's Neutron Experiment (1932)Alphasourcealpha particlesBerylliumthin sheetneutral particlesNeutroncharge 0mass > proton
Alpha particles strike a thin beryllium sheet and knock out neutrons: neutral particles with mass slightly greater than a proton (Chadwick, 1932).

Your doubts, answered

Who discovered the neutron and in what year?

James Chadwick discovered the neutron in 1932. Remember: the electron came first (1897), then the proton was characterised (1919), and the neutron came last (1932). So neutron = the most recent of the three basic particles.

Which element did Chadwick bombard, and with what?

He bombarded a thin sheet of beryllium (Be) with alpha particles (helium nuclei). NEET loves this exact detail. Do not confuse it: the target is beryllium, and the bullets are alpha particles.

Why was the neutron discovered so late?

The neutron has no charge. Electric and magnetic fields cannot bend or push a neutral particle, so it is very hard to detect. The electron and proton are charged, so scientists spotted them earlier. The neutron stayed hidden until Chadwick's 1932 experiment.

Is the neutron heavier or lighter than a proton?

The neutron is slightly heavier than a proton. NCERT states the emitted neutral particle had a mass slightly greater than that of a proton. Neutron mass is about 1.675 x 10^-27 kg, just above the proton's 1.673 x 10^-27 kg.

What is the charge of a neutron?

Zero. The neutron is electrically neutral. That is exactly why it was so hard to find. Its symbol is often written as 0n1 (mass number 1, charge 0).

What comes out when alpha particles hit beryllium?

Neutral particles (neutrons) are emitted. You can write it as: beryllium + alpha particle -> carbon + neutron. For NEET you mainly need the fact that neutrons are released, not the full equation.

⚠️ The NEET trap
✗ The neutron was discovered by bombarding beryllium with cathode rays (electrons), and it is lighter than a proton.
✓ The neutron was found by bombarding beryllium with ALPHA particles, and it is slightly HEAVIER than a proton.
🧠 Bullets = alpha particles (not electrons). Target = beryllium. Result = neutron, mass just above the proton.

Real NEET questions

NEET 2020

The number of protons, neutrons and electrons in 175/71 Lu, respectively, are:

A · 71, 71 and 104
B · 175, 104 and 71
C · 71, 104 and 71 ✓
D · 104, 71 and 71
Solution: For 175/71 Lu: atomic number Z = 71 and mass number A = 175. Protons = Z = 71. Electrons (neutral atom) = Z = 71. Neutrons = A - Z = 175 - 71 = 104. So protons : neutrons : electrons = 71 : 104 : 71. Key idea from Chadwick's discovery: neutrons are the extra mass in the nucleus, found using A - Z.

Solved Structure Of Atom NEET PYQs

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Frequently asked

What is the exact NCERT line about the neutron?

NCERT says: neutrons were discovered by Chadwick (1932) by bombarding a thin sheet of beryllium with alpha particles, which emitted electrically neutral particles having a mass slightly greater than that of protons, named neutrons.

Why does NEET ask about the neutron discovery?

It is a direct one-mark fact question (who, year, target element, projectile). It also links to finding neutron number as A - Z, which appears in many numerical questions.

What is the symbol and mass number of a neutron?

A neutron is written 0n1: mass number 1 and charge 0. Its actual mass is about 1.675 x 10^-27 kg, or roughly 1 atomic mass unit.

Do neutrons stay outside or inside the atom?

Neutrons sit inside the nucleus, along with protons. Together protons and neutrons are called nucleons.

Related concepts:
Comparison of Proton, Electron and Neutron (Mass and Charge)Canal Rays (Anode Rays) and Discovery of ProtonNucleons: Protons and Neutrons in the NucleusFinding Protons, Neutrons and Electrons in IonsRutherford's Alpha-Particle Scattering Experiment
Practise Chemistry for NEET:
Chemistry MCQs (free, chapter-wise)Structure Of Atom PYQs