A sample of a uranium compound is found to be losing mass gradually. Explain what is happening to the sample.

Uranium have an unstable nuclei, hence it is a radioactive element, and radioactive element can undergo radioactive decay like the alpha decay, beta decay.

Radioactive decay is a spontaneous breakdown of the atomic nucleus which, results in the release of energy along with radioactive particles, like alpha, beta, gamma etc.

Hence,

The mass is lost from a sample of uranium.