Which of the following is a chemical change? A. Boiling water using fire
B. Breathing in oxygen and breathing out carbon dioxide
C. Ripping a piece of paper in half
D. Melting gold to make jewelry

B is a physical change, A involves no chemical change, C is a physical change. So your answer is D.

The answer is D.

The atomic number stands for the number of protons in each element. The number of protons is what is unique to each element. The number of neutrons varies; this is what isotopes are. The number of protons being the atomic number explains why the atomic number is an identifying characteristic of each element.