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determining molecular shape?

Posted: Fri Nov 24, 2017 7:53 pm
by Paula Dowdell 1F
Hi I'm a little confused on when to know if the question is asking for a shape that ignores or includes lone pairs in its answers.
4.11 asks to give the "VSEPR formula and predict its shape" and by judge of it's answers they are taking lone pairs into consideration since the answers are a)seesaw b)t-shaped etc.
On the other hand. 4.13 asks for also the VSEPR formula and "molecular shape", however the answers are a)linear b)tetrahedral etc.

How do I know if the question is asking me to take lone pairs into account or not?

Re: determining molecular shape?

Posted: Fri Nov 24, 2017 9:07 pm
by Vincent Grospe 3C
One always takes into account the number of lone pairs when trying to determine the VSEPR formula (“AXE” system and the molecular shape.

Regarding VSEPR formula (AXE), A represents the number of central atoms (usually one), X represents the number of bonded atoms, and E represents the number of lone pairs. So, TeCl4 would have a VSEPR formula of AX4E. The VSEPR formula can help you determine the molecular shape — vice versa. So, the molecular shape of TeCl4 — a compound with four bonded CL atoms and one lone pair — would be see-saw.

However, what can be confusing is that some compounds with different VSEPR formulas have the same molecular shape (name). For instance, molecules with AX2E3 and AX2 formulas have the shape of “linear”.