Erin D. is correct--but why? Make sure you learn the "why" behind this so you can answer other similar questions.
Atoms of all elements are made up of three primary particles: protons, neutrons, and electrons. The "sub-atomic" particles each have specific properties that such as size, electrical charge, etc. that are crucial for the stability of the atom. The particles break down like this:
Particle Relative size Charge Location How to determine
Proton 1 + in the nucleus # of protons= element's atomic # (use periodic table)
Electron 0 - outside nucleus # of electrons=# of protons(in neutral, "normal" atom)
Neutron 1 0 in the nucleus Must be told or given (ie. can vary--isotopes--
for small elements most common form often
consists of isotope where #protons=#neutrons)
So Aluminum has 13 electrons because it needs 13 negative charges to balance the 13 postively-charged protons (which is what the periodic table told us it contains). This will make the Aluminum atom electrically neutral.
What if I had Nitrogen (N) or Uranium (U) atoms?
Well, these atoms would have 7 and 92 electrons respectively. Why? See if you can use the above material and figure out the "why" and you will be set for any similar questions.