Determining Molecular shape

[Tyler Fidler 4J]

I am slightly confused when determining the shape of atoms from there Lewis structures. Also when it comes to writing out the Lewis structure I get confused as to whether I should draw it in a straight line, make it at angles or what...any explanations?

[Chloe 3D]

From the lewis structure, you can determine molecular shape by determining the number of regions of electron density surrounding the central atom. Regions of electron density include both bonded atoms and lone pairs. Remember that lone pairs, however, experience a stronger repulsion; they therefore affect molecular geometry differently than bonded atoms. For example, in CH4, the central atom C is surrounded by four H atoms (four regions of electron density) and has a tetrahedral shape. In NH3, the central atom N is surrounded by three H atoms and one lone pair. While this molecule still has four regions of electron density, its shape is trigonal pyramidal. You can review page 109 of the textbook for diagrams of the various shapes.

When writing out the Lewis structure, you can draw the bonds at angles to more accurately depict the shape of the actual molecule.

[CenCen]

It helps understanding why certain molecules take a shape. But something i think of when determining their shape is considering the bonds and lone pairs in relevance to the central atom. I consider the regions of density and go from there. Its a simple process ive noticed. As you progress through regions of density your shape changes from linear to trigonal to tetrahedral then to octahedral. And from those categories youre going to have to take into account whether you have any lone pairs that will cause your shapes to change.

For example, if we have a molecule that is linear we know its because its region of density it sp. But we also know that if the central atom has 3 lone pair and two bonds it falls under a density region of sp4 and the lone pairs cause the shape to be categorized as linear because of the strength in repulsion. Hope this helps so sorry if i confused you! But this is how i process this and it hasnt failed me yet!

[BriannaWillingham_1G]

What is the correct shape for (PO4)3-??
Does it have one double bond, or no double bond at all? Why?

[Roy Hsieh 1C]

for (PO4)3-, it should be a see-saw shape. There is a total of 34 electrons - 3 from the charge, 7 from P, and 6 from each of the 4 O, making a total of 34 electrons. Drawing a P in the middle, attach Os to the P with single bonds, then by calculating the charges, there is one extra lone pair, causing a see-saw shape. There is no double bonds.