AlCl3 has 6 covalent bonds and 2 coordinate bonds. Chlorine has 17 electrons. 10 electrons are occupied in these orbitals (1S,2S,2P orbitals). The other seven are distributed in the 3S and the three 3P-orbitals, but upon forming (Covalent) bonds these orbitals hybridise into SP-orbitals. Chlorine thus has 7 electrons in the 4 SP3-orbitals: 3 orbitals are filled with 2 e− each, the fourth has only one.
Aluminium has only 3 electrons: 2 in the 3S and one in one of the 3P-orbitals. However, upon hybridization these 3 electrons are spread over 4 SP3-orbitals. Chlorine has a high Electronegativity , which means that it pulls rather hard at electrons from other atoms