Aluminium has an atomic number of 13 which means it has 13 protons (p+) an 13 electrons (e-) and an average atomic mass of 28.98 due to the 14 neutrons (n0) of its most stable isotope. Aluminium is a relatively small atom, with an atomic radius of 118 pm (picometer), which is smaller than most metallic elements.
Diagram of an aluminium atom
Its smaller atomic radius is due to the fact that it has few electrons (e-) as there is a small shielding effects and also because the electrons are very close to the nucleus which decreases its atomic radius. The 1st, 2nd, and 3rd ionization energy of aluminium is 577.539 KJ/mol , 1816.679 KJ/mol , and 2744.781 KJ/mol respectively. This increase in ionization energy is due to the atom being positively charged each time an electron is removed causing them to be attracted to the nucleus more and at the same time there are less electrons then before meaning less of a shielding effect. these two factors make removing the next electron harder thus requiring more energy.