Unit 6: Molecular Geometry Molecular Geometry [6-5] the polarity of each bond, along with the geometry of the molecule determines Molecular Polarity. To predict the geometries of more complicated molecules, the location of all electron pairs must be considered. VSEPR Theory valence-shell-electron-pair-repulsion theory States that repulsion between the sets of valence level electrons surrounding an atom causes these sets to be oriented as far apart as possible. the shared pairs are oriented as far away from each other as possible. postulates that the lone pair occupies space just as the bonding pairs do. A - represents the central atom B or - represents atoms bonded to the central atom E - represents the unshared electron pair (lone pairs) unshared electron pairs repel more strongly than bonding electron pairs. Flow chart of molecular shapes: The chart is used to determine the shape of any covalent molecule. To use this chart, you need to look at the middle atom and figure out how many electron pairs an atom has ( dots from the dot diagrams).
Flow chart of molecular shapes: How many atoms are Attached to the central atom? One Two Three Four Linear Molecule Tetrahedral molecule How many lone electron pairs are on the central atom? How many lone electron pairs are on the central atom? zero two zero one linear bent trigonal trigonal molecule molecule planar pyramidal molecule molecule if your molecule has only two atoms, either can be considered the central atom, making the model linear. Molecules with two atoms are always linear.
# of e` pairs VSEPR formul a VSEPR and Molecular Geometry Molecular shape Bond angle Molecular polarity 1 A Linear A 180 Polar 1 A 2 Linear A A 180 Nonpolar 2 A 2 Linear A 180 Unipolar 2 AE Linear :A 180 Polar 3 A 3 3 A 2 E 3 AE 2 4 A 4 4 A 3 E Trigonal planar Angular / bent Linear A : A : A Tetrahedral A Trigonal pyramidal A 120 Unipolar +120 Polar 180 Polar 109.5 Unipolar 107.5 Polar 4 A 2 E 2 Angular / bent : A 104.5 Polar 4 AE 3 Linear A 180 Polar
Hybridization Defined as the mixing of 2 or more atomic orbitals of similar energies on the same atom to produce new orbitals of equal energies. used to explain how the orbitals of an atom become rearranged, when forming covalent bonds. Hybrid orbitals - are orbitals of equal energy produced by the combination of two or more orbitals on the same atom. SP = 1-s orbital + 1-p orbital SP 2 = 1-s orbital + 2-p orbitals SP 3 = 1-s orbital + 3-p orbitals
Beryllium [4] - [ ] [ ] Hybrid Beryllium [4] - 1s 1s 2s [ ] [ ][ ] -2sp- Boron [5] - [ ] [ ] [ ][ ][ ] 1s 2s -- 2p -- Hybrid Boron [5] - [ ] [ ][ ][ ] 1s -- 2sp 2 -- Carbon [6] - [ ] [ ] [ ][ ][ ] 1s 2s -- 2p -- Hybrid Carbon [6] - [ ] [ ][ ][ ][ ] 1s --- 2sp 3 ---
Intermolecular Forces they vary in strength but are generally weaker than bonds that join atoms in molecules, ions, or metals atoms (intramolecular). Boiling point is a good measure of the force of attraction between particles of a substance. Ionic compounds and metals have values that are much higher than those for molecular substances see pg 190 strongest intermolecular forces exist between polar molecules A dipole - is created by equal but opposite charges that are separated by a short distance. It is another name for a polar molecule. These molecules are said to have a... dipole moment depends upon the size of the partial charges along with the distances between them. + H O H + partial charges are indicated by a delta [ ] direction of dipole is from [ + pole ] to its [ - pole ]. Weak Forces Van der Waals Forces are weak attractive forces between the protons of one atom and the electrons of another atom. 1. dipole-dipole 2. dipole-induced dipole 3. dispersion forces many molecules will exhibit 2 or all 3 of these interactions Non-Van der Waals Forces 4. Hydrogen bonding
dipole-dipole forces when two permanent dipoles are attracted to one another. Polar to polar. dipole [ + ] [ + ] dipole dipole-induced dipole forces - when a polar molecule induces a nonpolar molecule to turn into a dipole,,,the dipole causes the nonpolar molecule to distort it s shape, thereby causing the nonpolar molecule to be a temporary dipole. dipole [ + ] [ ] nonpolar dipole [ + ] [ + ] induced dipole London dispersion forces - resulting from the constant motion of electrons and the creation of instantaneous dipoles. The slight uneven charge creates a temporary dipole, (asymmetrical electron distribution ) induces an adjacent nonpolar atom or molecule. nonpolar [ ] [ ] nonpolar temporary dipole [ + ] [ ] nonpolar temporary dipole [ + ] [ + ] temporary dipole Hydrogen bonding An unusually strong dipole-dipole attraction It occurs when hydrogen is bound to fluorine, oxygen and nitrogen the most electronegative elements. The small sizes of the elements involved and the large electronegativity differences result in large + and values. These bonds are usually represented by using a dashed line. dipole-dipole and dipole-induced dipole are weaker then hydrogen bonding;
dispersion forces are the weakest.