Question

in summary, there are a couple of different ways to determine the number of valence electrons for main group elements. identify those ways below: add up all the electrons from the entire electron configuration, and that will be your number of valence electrons. use the electron configuration. add up the total number of electrons in the highest principal energy level/valence shell. use the group number. the group number will be equal to the number of valence electrons (with the exception of helium). the group number minus one will give you the number of valence electrons. the periodic table with lewis symbols for the main group elements is pictured above. lewis symbols for the transition metals are not usually written because they could have up to 10d electrons, which would make the symbols cumbersome. there are also some odd exceptions in those transition metals, and their chemistries are rather unique due to expanded octets.

Explanation:

• The first option is incorrect. Valence electrons are only those in the outermost shell, not all electrons in the electron configuration.
• The second option is correct. By using the electron configuration and adding up electrons in the highest principal energy level (valence shell), we can find valence electrons.
• The third option is correct. For main - group elements (except helium), the group number (in the old - style Roman numeral group numbering where groups are 1 - 8) is equal to the number of valence electrons. For example, group I elements (e.g., Li, Na) have 1 valence electron.
• The fourth option is incorrect. The group number (in the correct main - group context) itself (except for helium) gives the valence electron number, not group number minus one.

Answer:

• Second option: Use the electron configuration. Add up the total number of electrons in the highest principal energy level/valence shell.
• Third option: Use the group number. The group number will be equal to the number of valence electrons (with the exception of helium).