No, the atomic number of an antimatter element is not a negative number. The concept of atomic number applies to both matter and antimatter elements.
The atomic number represents the number of protons in the nucleus of an atom, which determines the element's identity. For example, hydrogen has an atomic number of 1 because it has one proton in its nucleus, while helium has an atomic number of 2 because it has two protons.
Antimatter elements have the same atomic number as their corresponding matter elements. The key difference is that antimatter particles have opposite charges compared to their matter counterparts. For instance, an antielectron (positron) has a positive charge, while an electron has a negative charge, but both still have an atomic number of 1.
Therefore, the atomic number of an antimatter element is a positive integer, just like its matter counterpart.