# Aluminum has an atomic number of 13. How many protons does an atom of aluminum have? How many electrons does it have?

The atomic no., $Z$, defines the number of protons, and hence the identity of the atom.
Aluminum has (by definition) 13 protons, 13 positively charged particles. Because aluminum is neutral (as is all matter), it therefore has 13 negatively charged particles, electrons. Aluminum has an atomic mass of about $27$ amu; how many neutrons are present in the aluminum nucleus?