Planar vs Pyramidal

[Phoebe Chen 4I]

Trigonal planar and trigonal pyramidal molecules both have three atoms connected to the central atom. How can you tell if a molecule should be planar or pyramidal? I know that if there is a lone pair it will be pyramidal.However, if the three atoms attached to the central atom are all different but there is no lone pair, would that be planar or pyramidal? Would the different atom's electronegativity affect the molecules shape? I am wondering the same thing for tetrahedral molecules vs square planar molecules. Thanks in advance!

[almaochoa2D]

If the central atom has three bonds and no lone pairs then it will be trigonal planar it doesn't matter if the elements are different.

[Tiffany_Cacy_3D]

So when considering the shape of a molecule it helps to first look at how many electrons are going to be connected to the central atom. The difference between a trigonal planar and a trigonal pyramidal is the planar has a total of three things connected to the central atom while the pyramidal has four. There are three other atoms connected to both but the pyramidal has a lone pair with also needs to be separated from the other electrons and therefore changes the shape. The lone pairs always contribute to the shape when connected to the central atom. The same concept applies to the tetrahedral and the square planar molecules. The tetrahedral has a total of four things connected to the central atom while the square planar has six, four in the same plane and one lone pair of electrons on either side of the plane.

[sarahartzell1A]

Even if the atoms are different, it will still be planar.