AAMC Ques on shapes

[Neplina94]

Which of the following compounds has the same geometry as methane?
A. H2S
B. CO2
C. XeF4
D. SiCL4

It says the right ans is D. Im confused as to why not C. When I looked up the shape of XeF4, it showed an extra two lone pairs on Xe. How was I supposed to know that?

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[Doogie.Howser]

VSEPR.

Xe has 8 electrons, F has 7 x 4 = 28 electrons. The molecule has a total of 36. The 4 Xe-F bonds take 8, leaving 28 left over. The four Fluorines each have 6 lone electrons = 24, leaving 4 left over. The four electrons must be placed on the Xe, giving it a square planar structure.

 

[AdaptPrep]

Hi, neppywise--

Doogie is correct in that VSEPR holds the key. However, an alternative way to solve it, in order to save time, is too note that Si and C are in the same group. Therefore, CH4 and SiCl4 should have the same geometry. Xe is in a different group, and VSEPR would indicate that there is a different arrangement /geometry around the central atom.