Final answer:
An aluminum (Al) atom has three valence electrons, as it belongs to group 13 (3A) of the periodic table, which aligns with its electron configuration of 1s²2s²2p¶3s²3p¹.
Step-by-step explanation:
The number of valence electrons in an aluminum (Al) atom can be determined by its position in the periodic table. Aluminum belongs to the 3A group (also known as group 13), and according to periodic trends, all elements in a group have the same number of valence electrons as the first digit of their group number. Thus, aluminum has three valence electrons. This is further confirmed by aluminum's electron configuration, which is 1s²2s²2p¶3s²3p¹. The electrons that are in the outermost energy level, which are the 3s² and 3p¹, make up the valence electrons. When aluminum forms a cation (Al³+), it loses these three valence electrons, resulting in a noble gas configuration similar to neon (1s²2s²2p¶). The correct answer to the number of valence electrons in an Al atom is B) 3.