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Molecular shape is primarily determined by the positions of atomic nuclei, influenced by electron pairs as indicated in Lewis structures. While the Lewis structure presents a 2-D view, it aids in visualizing the 3-D arrangement of molecules. Molecular geometry is classified based on the total number of atoms and electron densities around the central atom. The VSEPR theory helps predict geometric shapes like linear, bent, trigonal planar, trigonal pyramidal, and tetrahedral based on electron pair repulsion. It is also crucial in determining molecular polarity, which is affected by the symmetry and bond types within the molecule.
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Things to remember shape of molecule: • determined by location of nuclei • nuclei go to certain locations because of its electron pairs
Use the Lewis Structure • Lewis structure is 2-D, but can help figure out 3-D shape
Atom Lewis Structure # Unpaired Electrons Bonding Capacity . H H 1 1 .. F, Cl, Br, I ·F: 1 1 .. . C, Si ·C· 4 4 . .. N, P ·N· 3 3 . O, S ·O: 2 2 · ·· Ne, Ar, Kr :Ne: 0 0 ·· Bonding Capacity ..
Molecular Shape • Determined by overlap of orbitals • Shape determined by two factors: • total # atoms & • # e- pairsin different locationson central atom Classify electron pairs as bonding or nonbonding
Molecular Shape & VSEPR • Electron pairs repel each other: • want to be as far apart from each other as can be • Nonbonding pairs take up a little more room than bonding pairs
Total Number of Electron Pairs Orientation 1 or 2 Linear: 2 electron pairs can be 180 apart 3 Planar Triangular: 3 electron pairs - 120 apart 4 Tetrahedral: 3-D, 4 electron pairs - 109 apart Orientation of Electron Pairs
2-Atom Molecules • Atoms located right next to each other • linear molecules!
Linear Bent 3-Atom Molecules • linear or bent
.. .. .. .. CO2 Lewis structure: O :: C :: O 4 bonding pairs But only2 regionsof electron density: 180 apart so CO2 is linear
Lewis Structure of H2O = H:O:H .. Count up the electron pairs: H2O .. • 2 bonding pairs between O and the H’s • 2 nonbonding pairs on the O • 4 different regions of electron density
H2O • 4 electron pairs are 109 apart • nonbonding pairs spread out a bit more • squeeze the bonding pairs together • bent shape H – O H
3-Atom Molecules • Triangular? Yes, ozone (O3) is triangular
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 because thesemolecules don’t obey the octet rule!
.. .. F .. .. .. .. .. F .. .. BF3: Trigonal Planar • B has 3 valence electrons • very small atom e • each F has 7 valence electrons • total = 3 X 7 + 3 = 24 e- : F : B These molecules don’t obey octet rule - so aren’t likely to see them - 3 regions - FBF = 120 ..
4-Atom Molecules: AX3 shape most likely to see
.. Lewis Structure: H : N : H .. NH3 H • 3 bonding pairs • 1 nonbonding pair • 4 regions • 109 apart
4-atom molecule • Shape = trigonal pyramid • A bit less than 109
5 Atoms & Tetrahedrons Tetrahedral means 4 faces: - 1 atom in center - 4 atoms at the points
H Lewis structure = H : C : H .. H CH4 .. • 4 bonding pairs • 4 regions of electron density • electron pairs are 109 apart
Summary of Molecular Shapes • Start with Lewis Structure! • Look at # regions of electron density on central atom • Look at # atoms bonded to central atom
Molecular Polarity • Look at type bonds in molecule • Look at shape of molecule • nonpolar molecular must be symmetrical
Molecular Polarity • molecule is symmetric if: • electrical charge on 1 side = electrical charge at matching point on opposite side • “pull” of one polar bond is offset by “pull” of another polar bond • tug-of-war that can't be won!
Nonpolar Symmetrical Molecules Nonpolar
Asymmetric Molecules Polar
