Which of the following statements is true? a. some chemical reactions release energy, and other chemical reactions absorb energy. b. chemical reactions, by definition, cannot absorb or release energy. c. some chemical reactions release energy, but no chemical reaction can absorb energy. d. some chemical reactions absorb energy, but no chemical reaction can release energy.

Option (A) is the correct answer.

Explanation:

A reaction that brings changes in chemical composition of a substance is known as a chemical reaction.

When energy is absorbed in a chemical reaction then it is known as an endothermic reaction. Whereas when energy is released in a chemical reaction then it is known as an exothermic reaction.

Therefore, we can conclude that the statement some chemical reactions release energy, and other chemical reactions absorb energy, is true.

Electron cloud, nucleus.  Atoms are actually mostly empty space due to the fact that the electron cloud takes up a lot of space compared to the nucleus which is very small and dense.  The nucleus does however contain the protons and neutrons which give atoms their mass.  Electrons have such a small mass that we usually don't even take their mass into consideration saying that both protons and neutrons are 1amu in mass while electrons are 0amu in mass.

I hope this helps.  Let me know if anything is unclear.