How to find valence electrons on the periodic table?

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To find valence electrons on the periodic table , look at where the element is located: its group determines the count. For main-group elements, groups 1 and 2 have 1 and 2 valence electrons, and groups 13 to 18 have the group number minus 10: group 13 has 3, group 14 has 4, group 15 has 5, group 16 has 6, group 17 has 7 and group 18 has 8, except helium, which has 2. Sodium (group 1) has 1, carbon (group 14) has 4 and chlorine (group 17) has 7. Valence electrons are the electrons in the outermost shell, and they decide bonding, ionic charge and reactivity, which is why elements in the same column behave alike. Transition metals (groups 3 to 12) are less simple, because d electrons can take part, so they usually show 1 or 2 valence electrons in the outer s subshell and variable oxidation states. Valence electrons explain many of the periodic table's trends. Atoms tend to reach a full outer shell of eight electrons (the octet rule, or two for hydrogen and helium), so sodium loses its one valence electron to form Na + , chlorine gains one to form Cl − , and carbon, with four valence electrons, shares them in covalent bonds. Elements in the same group have the same number of valence electrons, which is why lithium, sodium and potassium all react in the same way. The number of valence electrons also matches the common charge of simple ions: group 1 forms +1 ions, group 2 forms +2, group 16 forms −2 and group 17 forms −1. Lewis dot diagrams draw valence electrons as dots around the element symbol; for example, oxygen in group 16 has six dots, nitrogen in group 15 has five and neon has eight. Helium is the exception in group 18 with only two, because the first shell holds only two electrons. You can check the result against the electron configuration by counting the electrons in the highest energy level.