Bonding Unit Part B) Structures and Shapes VSEPR-Shapes Notes Bonding Unit Part B) Structures and Shapes

EXCEPTIONS TO THE OCTET RULE Octet rule can be used to predict the structural formals for millions of molecules.. But there are exceptions! Atoms with less than an octet Atoms with more than an octet Molecules with an odd # of electrons Some of the geometric shapes represent these exceptions

EXCEPTION: having less than octet Compounds with BORON Has 6 valence electrons Ex. BF3, BBr3 Write this one down… Shape: TRIGONAL PLANAR SHAPE

EXCEPTION: having more than octet Compounds with Sulfur and Phosphorous can form bonds that give more than an octet Usually 10 or 12 electrons Ex. SF4 SF6 PCl5 Trigonal Bipyramidal Octahedral

EXCEPTION: having an odd number of electrons If odd # of valence electrons, octet is impossible Ex. NO Total of 11 valence electrons ….it is an unstable molecule

VSEPR Model The shape of a molecule determines physical & chemical properties Electron densities created by the overlap of the orbitals of shared electrons determine molecular shape The molecular geometry (shape) can be determined once a lewis dot structure is drawn.

VSEPR Model The model used to determine molecular shape Valence Shell Electron Pair Repulsion model Central principle: electrons repel each other Based on an arrangement that minimizes the repulsion of shared and unshared electron pairs around the central atom

Shapes The shape of a molecule depends a number of factors. These include: Atoms forming the bonds Bond distance Bond angles

Types of electrons Bonding electrons: the valence electrons are shared between atoms to form covalent bonds. Nonbonding electrons: valence electrons may not be shared with other atoms Also called lone pairs

Shape: Linear Atoms connected in straight line All molecules with only 2 atoms and also many with 3 atoms Bond angle is 180° Draw:

Shape: Trigonal Planar Central atom is bonded to 3 other atoms AND the central atom has no unshared pairs of electrons. Bond angle is 120° Draw:

Shape: Tetrahedral Draw: Central atom has 4 pairs of bonding electrons / 4 regions of electron density It is like a pyramid with a flagpole on the top. Bond angle is 109.5° Draw:

Shape: Pyramidal Draw: Central atom bonded to 3 other atoms and an unshared pair of valence electrons Bond angle is 107° Draw:

Shape: Bent Draw: 3 or more regions of electron density Typically two regions of bonding electrons and two regions of nonbonding electrons Bond angle is 105° Draw:

Practice Draw the Lewis Structure and identify the shape: CCl4 BF3 BeCl2 H2S PCl3

Answer CCl4, Tetrahedral BF3, Trigonal Planar BeCl2, Linear H2S, Bent PCl3, Pyramidal

Molecule Polarity To determine whether a molecule is polar or nonpolar, you need to look at: Polarity of the bonds Shape of the molecule You already know how to determine the polarity of bonds….

Molecule Polarity Have polar bonds If a molecule has only non-polar bonds, it is a non-polar molecule. If a molecule contains polar bonds, it is not necessarily polar. If symmetry cancels out the polarity, the molecule is non-polar. To be a polar molecule: Have polar bonds Have the polar bonds arranged in such a way that their polarity is not cancelled out HF and H2O are both polar molecules. CCl4 is non-polar

Determining Polar VS Nonpolar Molecule Draw lewis structure/shape Look at the electronegativity difference between each atom involved in a bond. Identify each bond as nonpolar or polar. If all bonds are nonpolar, it is a nonpolar molecule. If bonds are polar…. Draw arrows in the direction of highest E.N. on each bond. If the arrows do not cancel each other out, it is polar. There is a positive and negative end. If symmetry of the molecule cancels out the arrows, it is nonpolar.

Example: Cl2 Polar or nonpolar molecule?

Example: H2O Polar or nonpolar molecule?

Example: CO2 Polar or nonpolar molecule?