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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.
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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 . .. .. . . .. . .. · ·· ··
Basic Shapes • Linear: all diatomics, CO, H2, N2, O2, etc. and also some triatomics: CO2 • Bent: some triatomics: H2O, H2S, H2Se, H2Te, OF2 • Pyramidal: NH3, PH3, NI3 • Tetrahedral: CH4, CCl4, CBr4, CH3F, 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.
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. 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: AX3 The shape you are most likely to run into.
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.
CO2 .. .. Lewis structure = O :: C :: O .. .. 4 bonding pairs. But only 2 regions of electron density 180 apart. CO2 is linear.
BF3: Trigonal Planar • B has 3 valence electrons. It’s a very small atom. Each F has 7 valence electrons. Total = 3 X 7 + 3 = 24. : F : B .. .. These molecules don’t obey the octet rule, so you aren’t likely to see them. 3 regions FBF = 120 F .. .. .. .. .. .. F .. ..
CH4 H .. Lewis structure = H : C : H .. H 4 bonding pairs 4 regions of electron density Electron pairs are 109 apart.
NH3 H .. Lewis Structure = H : N : H .. 3 bonding pairs 1 nonbonding pair 4 regions. 109 apart.
4-atom molecule. Shape = trigonal pyramid HNH = a bit less than 109
H2O .. Lewis Structure of H2O = H:O:H .. Count up the electron pairs: 2 bonding pairs between the O and the H’s 2 nonbonding pairs on the O 4 different regions of electron density
H2O • 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.
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).
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!
Nonpolar Symmetric Molecules Nonpolar
Asymmetric Molecules Polar
