Molecular Shapes (geometry) and Forces of Attraction Molecular Shapes (geometry) and Forces of Attraction.

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Presentation transcript:

Molecular Shapes (geometry) and Forces of Attraction Molecular Shapes (geometry) and Forces of Attraction

VSEPR Theory Valence Shell Electron Pair Repulsion Valence Shell Electron Pair Repulsion theory predicts molecular shapes pairs of valence electrons repel predicts molecular shapes based on the idea that pairs of valence electrons surrounding an atom repel each other or

shape valence electrons central The shape of a molecule is determined by the valence electrons surrounding the central atom. Unshared pairs of electrons (on the central atom) bend. Unshared pairs of electrons (on the central atom) will cause atoms to bend. Five basic shapes linear, bent, tetrahedral, trigonal pyramidal, and trigonal planar. Five basic shapes : linear, bent, tetrahedral, trigonal pyramidal, and trigonal planar.

Molecular Shapes (geometry)

1.LINEAR example: CO 2 central atom central atom ALL shared pairs NO unshared pairs bond angles bond angles 180° no bending!

2. BENT example: H 2 O central atom central atom 2 shared 2 shared pairs 2 unshared 2 unshared pairs bond angles bond angles104.5°bends!

3. TETRAHEDRAL example: CH 4 central atom central atom 4 shared 4 shared pairs 0 unshared 0 unshared pairs bond angles bond angles 109.5° “tetra” = four!

4. TRIGONAL PYRAMIDAL example: NH 3 central atom central atom 3 shared 3 shared pairs 1 unshared 1 unshared pair bond angles bond angles107.3°bends!

5. TRIGONAL PLANAR example: BH 3 central atom 3 shared pairs 0 unshared pairs bond angles120°triangle-shaped!

remember… polaroppositely charged ends  all covalent compounds are either polar or nonpolar…and that polar means oppositely charged ends! dipolepolar bond or molecule  dipole = a polar bond or molecule

Shape Affects Polarity. 1. POLAR Molecules benttrigonal pyramidal all bent, trigonal pyramidal molecules unshared pairs any molecule that has unshared pairs linear, tetrahedral, and trigonal planar ONLY IF : dipolescancel dipoles do not cancel each other out no symmetry no symmetry within the molecule 2. NONPOLAR Molecules linear, tetrahedral, and trigonal planar ONLY IF: cancel dipoles cancel out symmetry there is symmetry within the molecule

Molecular Shape Bond Angle (°) # Shared Pairs # Unsha red Pairs Polar or Nonpola r Examp les Linear180ALL0 depends HF, CO 2 Bent polar SCl 2, H 2 O Trigonal Pyramidal polar NH 3 Trigonal Planar depends BF 3 Tetrahedr al depends CH 4

Forces of Attraction

Dispersion forces (one type of  Dispersion forces (one type of van der Waals forces) weakest van der Waals forces) – the weakest kind of intermolecular attraction (caused by the motion of electrons) Dipole interactions (a second  Dipole interactions (a second type of van der Waals forces) weak type of van der Waals forces) – a weak intermolecular force resulting from the attraction of oppositely charged regions of polar molecules Hydrogen Bonds strongest  Hydrogen Bonds – the strongest intermolecular force; hydrogen is bonded FON to FON (fluorine, oxygen and nitrogen).