Question 8
An element has an atomic number of 6. What can you say about it?
The right answer is the one marked above.
It is carbon, and it can be nothing else. No other element has an atomic number of 6.
Every atom of one element has the same number of protons, and different elements have different numbers — so two different elements can never share an atomic number. That makes Z a unique fingerprint. The mass number can vary between atoms of carbon (those are its isotopes) without the element changing at all.