## how to know which shape it is

(Polar molecules, Non-polar molecules, etc.)

Samantha Hoegl Roy 2C
Posts: 81
Joined: Fri Sep 28, 2018 12:15 am

### how to know which shape it is

trigonal planar, trigonal pyramidal and p shaped all have three atoms; how do you tell them apart?

Posts: 31
Joined: Fri Sep 28, 2018 12:27 am

### Re: how to know which shape it is

The difference is in the number of valence electron lone pairs. Trigonal planar has zero lone pairs, trigonal pyramidal has one lone pair, and t-shaped has two lone pairs.

michelle
Posts: 62
Joined: Fri Sep 28, 2018 12:23 am

### Re: how to know which shape it is

The first step is drawing Lewis structure and see how the bonded pairs electrons repel each other, according to VSEPR. For example, BeCl2 is linear because there are two bonded pairs of electrons around the central atom Be, while BF3 is trigonal planar because there are 3 bonded pairs around the central atom B. There are examples from the lecture and you can explore more from homework questions.

chris_tsai_4H
Posts: 52
Joined: Fri Sep 28, 2018 12:26 am

### Re: how to know which shape it is

The VSEPR model for Lewis structures is based on electron pairs so you have to count them to determine which one they correspond to.

ryanhon2H
Posts: 60
Joined: Fri Sep 28, 2018 12:28 am

### Re: how to know which shape it is

The way to figure this out is using VSEPR. Basically you can use the Lewis structure to figure out the molecular geometry, using the VSEPR notation of AXnEy, where A represents the central atom, X represents the atoms bonded to the central atom, n is the number of bonds to the central atom (double and triple bonds count as only one bond), E represents lone pairs and y is the number of lone pairs. Different notations correspond to different geometries.

For example, BF3. The Lewis structure as B as the central atom, 3 bonds to B, and no lone pairs. Thus, the VSEPR notation of BF3 would be AX3, which is trigonal planar.

NH3 is an example for trigonal pyramidal. The Lewis structure has 3 bonds to the central atom N, and one lone pair, for a notation of AX3E. Still 3 molecules, but now there is also a lone pair, which makes the geometry trigonal pyramidal.

I'm sorry if this wasn't that clear, but the way I would try and study this is to basically just memorize the relations between VSEPR notation and the corresponding molecular geometry.

TLDR; you can tell them apart using the # of lone pairs.
in general use # of bonds and # of lone pairs to figure out VSEPR notation and corresponding molecular geometry

I am Sodium Funny
Posts: 65
Joined: Fri Sep 28, 2018 12:26 am

### Re: how to know which shape it is

Have to account for lone electron pair

Chloe Likwong 2K
Posts: 70
Joined: Fri Sep 28, 2018 12:23 am

### Re: how to know which shape it is

Hi!

The preceding comments are helpful, but I believe that looking at a VSEPR chart would solidify what they're saying.

This link may provide you with a better idea: https://goo.gl/images/bkB9yu

Hope this helps!

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