How do you determine the geometric shape of molecules?

Question

Two main things generally determine whether a bond is polar or nonpolar. One is the electronegativity of the elements, the other is the geometric shape of the molecules formed. How do we determine the geometric shape? For example, if you have three atoms, it can either be linear, like H-Be-H, or bent, like H/O\H. If it's linear, there's a good chance of it being, nonpolar, if it's bent, it's probably polar. So it's important. Or you could have 4 atoms, and they could either be Planar, Triangular, or Pyramidal. What determines which geometric shape it is?

Answer 1

It all depends on whether or not you have lone pairs of electrons around the central atom. The reason water (H2O)is bent is because it has 2 lone pairs of electrons and 2 hydrogens.

This would be tetragonal in shape if there were 4 hydrogens, but instead, the hydrogens have more room to spread out because of the lone pairs of electrons. The goal is to stagger themselves as far away as possible.

The molecule will determine its own shape depending on what is most energetically favorable.

Answer 2

1. Number atoms + electrons 2. Number of lone electron pairs IE. NH3 molecule has 3 atoms around N, and 1 lone electron pair. This means that NH3 will adopt a tetrahedral shape. You have 1 lone pair. Because lone pairs need more space than atoms do (basis of VSPER), the tetrahedral arrangement will be distorted. This gives you its trigonal pyramidal shape.

Answer 3

Shape can be determined from the type of molecular orbitals that are used in forming bonds. HOH is formed from an oxygen with sp3 orbitals, of which 2 are used to bond, and two are filled with unshared electron pairs. These pairs repel each other and push the HOH bond inwards from its expected angle of 109 degrees to 92 degrees if I remember correctly. Bonds of sp2 orbitals lead to atoms at a 120 angle and in a plane, such as benzene.