Question

Which of the following has a central atom that exceeds the octet rule? PCl5 BeF2 H2S CO2

Answer 1

Schwarzschild Radius here.

So,

The octet rule really only holds up for the second period elements, and only certain elements in that period, for that matter.  Elements in the third periods and beyond can have expanded octets.

Draw Lewis dot diagrams for each molecule.

$PCl_5$ :

P is the central atom bonded individually to each Cl.  The only logical diagram is that with three lone pairs surrounding each Cl and the P single-bonded to each Cl atom.  If you count the bonds, you will find that P has five pairs, or 10 electrons.  This exceeds the octet rule.

$BeF_2$ :

The Lewis dot diagram for this molecule is a linear molecule with the Be in the center, singly bonded to both F's, each of which have three lone pairs.  This means Be has only two pairs, or four electrons.  While not an example of a central atom that exceeds the octet rule, Be is one of the special elements that can stably have an incomplete octet.

$H_{2}S$ :

The most stable diagram for this molecule is with S in the center with two lone pairs and singly bonded to each H atom.  With four pairs, or 8 electrons, this molecule obeys the octet rule.

$CO_2$ :

The diagram for this molecule is C in the center doubly bonded to each O atom.  Each O atom has 2 lone pairs.  C has four pairs, or 8 electrons.  This molecule also obeys the octet rule.

Hope this helps!