Question

During early research in nuclear energy, why were scientists surprised that the products from splitting a uranium nucleus had less mass than the reactants?

They expected that the large release of energy would cause an increase in mass.
Up to that time, only chemical changes were known to form less mass than they started with.
All known changes always started and ended with the same amount of mass.
Other known nuclear changes resulted in the same mass of products as reactants.

Answer 1

Correct choice: "All known changes always started and ended with the same amount of mass."

Explanation: before nuclear reactions were studied, conservation of mass had been observed in all ordinary (chemical) processes, so scientists expected mass to be conserved. The discovery of fission showed a small "mass defect" — the products weigh slightly less because binding energy is released. Einstein's E = mc² explains how the lost mass is converted to the large amount of energy released.