Final answer:
The valence electrons of an atom are determined from the atom's electron configuration; for example, aluminum with electron configuration 3s²3p¹ has three valence electrons.
Step-by-step explanation:
The number of valence electrons an atom has can be determined from its electron configuration. Specifically, the valence electrons are the electrons in the outermost shell, which is involved in chemical bonding. For example, in the case of the aluminum atom (Z = 13), using the given information, the electron configuration is 3s²3p¹, indicating that aluminum has a total of three valence electrons. Two electrons fill the 3s orbital, and one electron is in the 3p orbital. These are the electrons available for bonding and would be represented in a Lewis dot structure with three dots around the symbol for aluminum.
To achieve a stable configuration similar to noble gases, elements in various groups may lose or gain electrons. Group 1 elements need to lose one electron to achieve stability. Group 14 elements need to gain four electrons, and Group 17 elements need to gain one electron for a stable configuration due to their electron configurations reflecting their position in the periodic table.