Final answer:
To determine the number of valence electrons in an atom, refer to the periodic table and count the number of valence electrons based on the element's position. To determine the total number of valence electrons in a molecule or ion, add together the valence electrons from each atom.
Step-by-step explanation:
To determine the number of valence electrons in an atom, you can refer to the periodic table. The number of valence electrons is indicated by the element's position in the periodic table.
For example, elements in Group 1 (alkali metals) have 1 valence electron, elements in Group 2 (alkaline earth metals) have 2 valence electrons, and so on. To count the total number of valence electrons in a molecule or ion, add together the valence electrons from each atom. If the species is a polyatomic ion, adjust the number of valence electrons to account for the charge on the ion.