Hybridization, Polarity, & Electronegativity Lots of multisyllabic words, I know 

Taking a Step back: Polarity Covalent Bonding atoms “bonded” through sharing of electrons Draw H2, Cl2, and H2O H2 and Cl2 – have an equal sharing of electrons Non Polar Covalent H2O– unequal sharing of electrons Polar Covalent

Polar Covalent F2 vs. HF The only time a bond is completely nonpolar is when identical elements are bonded together. In the above example, Fluorine pulls harder on the electrons than the Hydrogen… the Fluorine end of the molecule has more electron density than the hydrogen end. Fluorine is δ- and Hydrogen is δ+

Electronegativity Is the ability of an atom in a molecule to draw electrons to itself Is a combination of Ionization Energy and Electron Affinity Is a MADE UP NUMBER

Electronegativity On the periodic chart, electronegativity increases as you go… …from left to right across a row. …from the bottom to the top of a column.

Electronegativity and Polarity You can use the difference in the electronegativities of two elements to determine whether the bond they form will be nonpolar, polar, or ionic If the difference in electronegativities is > 1.7, the bond is considered ionic. Bond Electronegativity Difference Type of Bond H and H 2.1 – 2.1 = 0 Non-polar covalent N and H 3.0 – 2.1 = 0.9 Polar Covalent Na and Cl 0.9 – 3.0 = 2.1 Ionic

Polar Covalent Bond The greater the difference in electronegativities, the more polar the bond is

Polar Covalent Bond When two atoms share electrons unequally, a bond dipole results. The dipole moment, , produced by two equal but opposite charges separated by a distance, r, is calculated: = Qr

Polar Molecules Predicting whether or not a molecule will be polar or nonpolar depends on both the bonds and the geometry To determine whether or not a molecule will be nonpolar: All terminal atoms are identical All of the terminal atoms are arranged symmetrically

Polar Molecules Draw CH4 , BF3, Cl2CO, NH3 Which ones have a net dipole moment and which ones have no net dipole? CH4 and BF3 = no net dipole Cl2CO and NH3 = net dipole

Overlap and Bonding We think of covalent bonds forming through the sharing of electrons by adjacent atoms. In such an approach this can only occur when orbitals on the two atoms overlap.

Hybridization Atomic orbitals (s, p, d, f) cannot adequately explain the bonding in molecules Consider CH4 … how do we get the tetrahedral shape out of the above orbitals

Hybridization Consider beryllium: An averaging of atomic orbitals into a new set of “hybrid orbitals” Consider beryllium: In its ground electronic state, it would not be able to form bonds because it has no singly-occupied orbitals.

Hybridization Consider beryllium: if it absorbs the small amount of energy needed to promote an electron from the 2s to the 2p orbital, it can form two bonds.

Hybrid Orbitals Mixing the s and p orbitals yields two degenerate orbitals that are hybrids of the two orbitals. These sp hybrid orbitals have two lobes like a p orbital. One of the lobes is larger and more rounded as is the s orbital.

Hybrid Orbitals These two degenerate orbitals would align themselves 180 from each other. This is consistent with the observed geometry of beryllium compounds: linear.

Hybrid Orbitals With hybrid orbitals the orbital diagram for beryllium would look like this. The sp orbitals are higher in energy than the 1s orbital but lower than the 2p.

Hybrid Orbitals Think about Boron What’s its electron configuration? How many bonds does it normally form? How?

Hybrid Orbitals …three degenerate sp2 orbitals.

Hybrid Orbitals For Carbon:

Hybrid Orbitals …four degenerate sp3 orbitals.

Hybrid Orbitals For geometries involving expanded octets on the central atom, we must use d orbitals in our hybrids.

Hybrid Orbitals This leads to five degenerate sp3d orbitals… …or six degenerate sp3d2 orbitals.

Hybrid Orbitals Once you know the electron-domain geometry, you know the hybridization state of the atom. Short cut… count the bonded electron domains… that’s the number of hybridizations.

# Hybrid Orbitals Hybridization Bond Angles Electron Pair Geometry 2 sp 180 Linear 3 sp2 120 Trigonal planar 4 sp3 109 Tetrahedral 5 sp3d 120,90 Trigonal bipyramidal 6 sp3d2 90 octahedral

Practice Problems Specify the electron-pair and molecular geometry for each underlined atom in the following list. Describe the hybrid orbital set used by this atom in each molecule or ion. BBr3 CO2 CH2Cl2 CO3-2

Practice Problems Answers Electron-Pair Molecular Hybrid Geometry Geometry Orbital Set Trigonal Planar Trigonal Planar sp2 Linear Linear sp Tetrahedral tetrahedral sp3 Trigonal planar trigonal planar sp2