Final answer:
An atom with completely filled s and p outermost energy levels has eight valence electrons, as seen in the noble gases which are known for their stability due to following the octet rule.
Step-by-step explanation:
An atom with completely filled s and p outermost energy levels has eight valence electrons. The second and third energy levels can hold a total of eight electrons, with the s sublevel containing two and the p sublevel containing six.
For example, the noble gases have eight valence electrons, which gives them a special stability. This configuration satisfies the octet rule, which is a principle of chemical bonding that atoms tend to combine in such a way that they each have eight electrons in their valence shells, giving them the same electronic configuration as a noble gas.