Final answer:
Valence electrons are the electrons in an atom's outermost energy level, crucial for chemical bonding and reactivity. Their number often matches the group number on the periodic table, while their energy level corresponds to the element's period.
Step-by-step explanation:
Valence electrons are the electrons located in the highest occupied principal energy level of an atom and are important for determining an atom's chemical properties. These electrons are responsible for an atom's ability to form bonds with other atoms and are depicted using electron dot diagrams or Lewis dot structures. The number of valence electrons an element has can usually be determined by its group number on the periodic table. For representative elements, which exclude the transition metals, the number of valence electrons matches the group number, with the exception that for the p block elements the number is the group number minus ten. For example, elements in Group 1 have one valence electron, while those in Group 17 have seven valence electrons.
An element's valence electron energy level corresponds to its period on the periodic table. The energy level of an atom's valence electrons is indicated by its period number, which is the horizontal row in which it appears. For instance, elements in period 1 have their valence electrons in the first energy level, whereas elements in period 3, like sodium and magnesium, have their valence electrons in the third energy level.