The neutron and the mass of the atom

Almost all of an atom's mass is in its nucleus. Electrons are so light that we can ignore their mass.

A puzzle. Hydrogen has 1 proton; helium has 2. So helium should weigh about twice as much as hydrogen. But it weighs about four times as much. Something else in the nucleus must add mass without adding charge.

In 1932, James Chadwick, Rutherford's student, found it: the neutron. It has about the same mass as a proton but no charge. He won the Nobel Prize in Physics in 1935.

The three particles:

Light atoms often have equal numbers of protons and neutrons: carbon 6 and 6, oxygen 8 and 8. Heavier atoms have more neutrons than protons: iron 26 protons and 30 neutrons; uranium 92 and 146.

"Why doesn't the nucleus fly apart?" Protons are all positive, so they push each other away. Neutrons sit between them, increasing the distance between protons, and add to the nuclear force that holds the nucleus together.

Because neutrons have no charge, they can enter other nuclei easily. That discovery started the atomic age: nuclear power (and also weapons).

In India, BARC (Bhabha Atomic Research Centre) in Mumbai uses reactors such as Dhruva to study materials with neutrons. Homi Jehangir Bhabha, the father of India's nuclear programme, set up TIFR and BARC for peaceful uses of atomic energy: electricity, farming and medicine.