the sub-atomic particles protons and neutrons were discovered early in which century?

Protons and neutrons were both discovered in the early 20th century.

Quick answer

• The proton as a nuclear particle is credited to Ernest Rutherford (around 1917), in the first decades of the 1900s.

• The neutron was discovered by James Chadwick in 1932, also in the first half of the 1900s.

So, the sub-atomic particles protons and neutrons were discovered early in the 20th century.