When atoms in a substance gain more thermal energy, which of the following best describes what happens to them?

• A. They decrease in size and become more tightly packed.
• B. Their average kinetic energy increases, causing them to move faster.
• C. Their chemical identity changes to form new elements.
• D. They lose electrons and become ions.

Answer: (B)

Verified step by step guidance

1. Understand that thermal energy is related to the energy of motion of atoms or molecules in a substance.
2. Recall that when atoms gain thermal energy, their average kinetic energy increases, which means they move faster.
3. Recognize that increasing kinetic energy typically causes atoms or molecules to move more vigorously, not to decrease in size or become more tightly packed.
4. Note that gaining thermal energy does not change the chemical identity of atoms; elements remain the same unless nuclear reactions occur, which is not the case here.
5. Also, gaining thermal energy alone does not cause atoms to lose electrons and become ions; ionization requires specific energy input beyond just thermal energy.