Molecular Shape

Things to remember The shape of a molecule is determined by where the nuclei are located. But the nuclei go to certain locations because of the electron pairs.

Use the Lewis Structure Lewis structure is 2-D, but it can help you figure out the 3-D shape. Learn a few basic shapes and you’ll be ok.

Bonding Capacity Atom Lewis Structure # Unpaired Electrons Bonding Capacity HH11 F, Cl, Br, I ·F:71 C, Si ·C·44 N, P ·N·53 O, S ·O:62 Ne, Ar, Kr :Ne: · ·· ··

Basic Shapes Linear: all diatomics, CO, H 2, N 2, O 2, etc. and also some triatomics: CO 2 Bent: some triatomics: H 2 O, H 2 S, H 2 Se, H 2 Te, OF 2 Pyramidal: NH 3, PH 3, NI 3 Tetrahedral: CH 4, CCl 4, CBr 4, CH 3 F, etc.

VSEPR Model Draw the Lewis structure. Count the regions of electron density on the central atom. (How many sides have electron density?) Each single, double, &/or triple bond counts as 1 region of high electron density. Each nonbonding electron pair counts as 1 region of high electron density. Count the number of atoms bonded to the central atom.

Orientation of Regions of ED Total Number of Regions of ED Orientation 1 or 2Linear: 2 electron pairs can be 180  apart 3Planar Triangular: 3 electron pairs  apart 4Tetrahedral: 3-D, 4 electron pairs  apart

Summary of Molecular Shapes # of regions of electron density # of bonded atoms shape 22Linear 33Trigonal planar* 44Tetrahedral 43Trigonal pyramid 42Bent

2-Atom Molecules Atoms are right next to each other. 2 points make a line – these are linear!

3-Atom Molecules Most likely possibilities are linear and bent.Most likely possibilities are linear and bent. Linear  Bent 

3-Atom Molecules Triangular? Well, isn’t that another way of saying bent?

4-Atom Molecules Two possibilities: –Trigonal Planar – in 1 plane –Trigonal Pyramidal

4-Atom Molecules: Trigonal Planar Bond angles = 120 . All 4 atoms lie in the same plane You aren’t likely to see this. These molecules don’t obey the octet rule!

4-Atom Molecules: AX 3 The shape you are most likely to run into.

5-Atom Molecules: AX 4

5 Atoms & Tetrahedrons Tetrahedral means 4 faces. 1 atom is at the center & 4 are at the points.

Molecular Shape Determined by overlap of orbitals. Shape is determined by two factors: –The total number of atoms and –The number of electron pairs in different locations on the central atom. –Classify electron pairs as bonding or nonbonding.

Molecular Shape & VSEPR Electron pairs repel each other. They want to be as far apart from each other as they can. Nonbonding pairs take up a little more room than bonding pairs.

Orientation of Electron Pairs Total Number of Electron Pairs Orientation 1 or 2Linear: 2 electron pairs can be 180  apart 3Planar Triangular: 3 electron pairs  apart 4Tetrahedral: 3-D, 4 electron pairs  apart

CO 2 Lewis structure = O :: C :: O bonding pairs. But only 2 regions of electron density  180  apart. CO 2 is linear.

BF 3 : Trigonal Planar B has 3 valence electrons. It’s a very small atom. Each F has 7 valence electrons. Total = 3 X = 24. : F : B F F These molecules don’t obey the octet rule, so you aren’t likely to see them. 3 regions FBF = 120 

CH 4 Lewis structure = H : C : H H H bonding pairs 4 regions of electron density Electron pairs are 109  apart.

CH 4 = a 5-atom molecule

NH 3 Lewis Structure = H : N : H H bonding pairs 1 nonbonding pair 4 regions. 109  apart.

4-atom molecule. Shape = trigonal pyramid HNH = a bit less than 109 

Looks like a pyramid with N at the top & a triangular base.

Lewis Structure of H 2 O = H:O:H.... H2OH2O 2 bonding pairs between the O and the H’s 2 nonbonding pairs on the O 4 different regions of electron density Count up the electron pairs:

H2OH2O 4 electron pairs are 109  apart, but the nonbonding pairs spread out a bit more and squeeze the bonding pairs together.

Summary of Molecular Shapes Start with Lewis Structure! Look at number of regions of electron density on central atom. Look at number of atoms bonded to central atom.

Summary of Molecular Shapes # of regions of electron density # of bonded atoms shape 22Linear 33Trigonal planar* 44Tetrahedral 43Trigonal pyramid 42Bent

Molecular Polarity Look at the type of bonds in the molecule. Look at the shape of the molecule. A polar molecular must contain polar bonds & it must be asymmetric (NOT symmetric).A polar molecular must contain polar bonds & it must be asymmetric (NOT symmetric).

Molecular Polarity If molecule is symmetric, the electrical charge at any point on 1 side = electrical charge at matching point on opposite side. the “pull” of one polar bond is offset by the “pull” of another polar bond. –It’s a tug-of-war that no one can win!

  SymmetricMolecules  Nonpolar Nonpolar

AsymmetricMolecules  Polar  Polar