Final answer:
To calculate valence electrons, add the electrons in the outermost shell for each element in a molecule or ion. Adjust the total by adding one electron for each negative charge or subtracting one for each positive charge in polyatomic ions.
Step-by-step explanation:
How to Calculate Valence Electrons
To calculate the number of valence electrons in an atom, you need to refer to the periodic table to determine the number of electrons in the outermost shell, which are considered the valence electrons. For molecules or ions, you must add together the valence electrons from each atom within the molecule or ion. For instance, in a molecule like HCN, the calculation would be (1 × 1) + (4 × 1) + (5 × 1), which equals 10 valence electrons.
When dealing with polyatomic ions, such as CO3²⁻, you need to adjust the total count of valence electrons based on the ion's charge. For a negative charge, you add one electron per negative charge to the total, while for a positive charge, you subtract one electron per positive charge. Therefore, for CO3²⁻, which has a -2 charge, you would add two electrons to the sum of valence electrons from the constituent atoms of carbon and oxygen.