Question

which best describes nuclear fission?
a nucleus spontaneously splits and absorbs energy.
two nuclei spontaneously combine and absorb energy.
a nucleus collides with a neutron and splits, releasing energy.
nuclei combine to form a heavier nucleus, releasing energy.

Explanation:

To determine the correct description of nuclear fission, recall:

• Nuclear fission involves a heavy nucleus (e.g., uranium - 235) being bombarded by a neutron. This causes the nucleus to split into smaller nuclei (fission fragments) and release a large amount of energy (along with more neutrons, which can sustain a chain reaction).
• Analyze each option:
• First option: Fission releases energy (does not absorb), and it is not spontaneous in the sense of “just splitting” without a neutron trigger for most cases (like in power plants). Eliminate.
• Second option: Combining nuclei is fusion, not fission. Eliminate.
• Third option: Matches the definition—neutron collision causes splitting and energy release.
• Fourth option: Describes nuclear fusion (combining to form heavier nuclei), not fission. Eliminate.

Answer:

C. A nucleus collides with a neutron and splits, releasing energy.