Which of the following can never expand its valence shell C

P

Br

S

I

C

Explanation:

Key Points

The octet rule states that atoms with an atomic number below 20 tend to combine so that they each have eight electrons in their valence shells, which gives them the same electronic configuration as a noble gas.

The two elements that most commonly fail to complete an octet are boron and aluminum; they both readily form compounds in which they have six valence electrons, rather than the usual eight predicted by the octet rule.

While molecules exist that contain atoms with fewer than eight valence electrons, these compounds are often reactive and can react to form species with eight valence electrons. For example, BF3 will readily bind a fluoride anion to form the BF4– anion, in which boron follows the octet rule.

Key Terms

atomic number: The number, equal to the number of protons in an atom, that determines its chemical properties. Symbol: Z.

valence electrons: The electrons in the outermost (valence) principal energy level of an atom that can participate in the formation of chemical bonds with other atoms.

octet rule: Atoms gain, lose or share electrons with other atoms to fill their valence level with eight electrons.