An atom has the following electron configuration.

1s22s22p63s23p3

How many valence electrons does this atom have?

3

5

8

15

5 electrons

Explanation:

Valence electrons are electrons located in the outermost energy shell, so you want to count the number of electrons in the last energy shell.

You can divide the configuration into 1s2 / 2s22p6 / 3s23p3 to see the energy shells in this atom. There are 3 shells occupied by the atom's electrons, so you need to count the electrons in the third shell as those are its valence electrons.

2 + 3 = 5 valence electrons total

Note: you don't count the 3 before the letter because that only indicates the shell level, not the number of electrons. Count only the exponents.