Question

What are neutrons? What experimental evidence led us to assume the existence of neutrons?

Answer 1

Neutrons are the sub-atomic particles discovered by James Chadwick in 1932.

• They are represented as n.
• Neutrons have no charge on them.
• The mass of a neutron is almost similar to that of a proton i.e.; 1.674 × 10^-24 g. In fact, neutron is slightly heavier than that a proton.
• Neutrons are present inside the nucleus.

Experimental evidence that led us to assume the existence of neutrons:

• It was observed that the weight of an atom calculated from the addition of the weight of the total number of electrons and protons was lesser than the actual weight of the atom.
• This led to the proposal that there might be another particle that carries no charge but has a significant mass.
• Example: Helium (He) has 2 protons. As one proton weighs approximately 1 amu (atomic mass unit), therefore the weight of the Helium atom should have been 2 amu as it contains 2 protons. But the weight of the Helium atom was found to be 4 amu which led to conclude that there is another particle inside an atom that has significant mass but is electrically neutral.
• It was later found that Helium has 2 neutrons and neutrons almost weigh as much as protons.