Molecular Geometry & Polarity

Transcription

1 Molecular Geometry & Polarity Learn Shapes you will Because the physical and chemical properties of compounds are tied to their structures, the importance of molecular geometry can not be overstated. Localized Electron Model According to the Localized Electron Model, the arrangement of atoms, bonds and nonbonding electron pairs to form an octet can be predicted for most molecules. One important reason for drawing these Lewis Structures is to be able to predict the three-dimensional geometry of molecules and molecular ions. By noting the number of bonding and nonbonding electron pairs according to the Localized Electron Model, we can easily predict the shape of the molecule. What Determines the Shape of a Molecule? Simply put, electron pairs, whether they be bonding or nonbonding, repel each other. By assuming the electron pairs are placed as far as possible from each other, we can predict the shape of the molecule. Valence Shell Electron Pair Repulsion Theory (VSEPR) Developed by Ronald Nyholm (left) and Ronald Gillespie (right) in 1957 The best arrangement of a given number of electron domains is the one that minimizes the repulsions among them. Meaning, bonded and non bonding pairs of electrons around a central (ABn) atom will be as far apart as possible. Or, have the greatest possible bond angle. Electron-pair geometry: The arrangement of electron pairs, or electron domains, around a central atom. Electron Domains This molecule has four electron domains around the central atom. We can refer to the electron pairs as electron domains. In a double or triple bond, all electrons shared between those two atoms are on the same side of the central atom; therefore, they count as one electron domain. 1

2 Electron-Pair Geometries or (Electron-Domain Geometries) These are the electron-domain geometries for two through six electron domains around a central atom. Electron-Domain Geometries All one must do is count the number of electron domains in the Lewis structure. The geometry will be that which corresponds to that number of electron domains. # e- pairs Electron-pair Example Bond geometry angle 2 Liner BeCl o 3 Triagonal Planar BF o 4 Tetrahedral CH o 5 Triagonal Bipyramidal PCl o, 90 o 6 Octahedral SF 6 90 o, 90 o Remember, this is only electron pair geometry To determine the electron-pair geometry: 1. Draw the Lewis structure(s) 2. Count & total the number of electron pairs around the central atom. (double and triple bonds are only counted as one pair) 3. Describe the geometry in terms of greatest possible bond angle between electron-pairs 1. Determine the electron domain geometry for all molecules on the Lewis Structure worksheet. * Any resonance structure can be used to predict geometry. Molecular Geometries The electron-domain geometry is often not the shape of the molecule, however. The molecular geometry refers to the positions of the atoms in the molecules as they are affected by the nonbonding pairs. 2

3 Electron-pair geometry is excellent for predicting the general arrangement; however, it is not the same as the actual molecular geometry. Compare: CH 4 and NH o o Molecular geometry describes not only the geometry of the electron pairs, but it also attempts to describe the influence of any unbonded electron pairs on the total molecule. Experimentation has shown that un-bonded electron pairs occupy more space than bonded pairs, because they are not localized between two atoms; Therefore, the angles between them and other electron pairs, bonded or not, is increased. Molecular Geometries Within each electron domain, then, there might be more than one molecular geometry. 2. Identify the molecular geometry for all molecules on the Lewis Structure worksheet. Reference In larger molecules, it makes more sense to talk about the geometry about a particular atom rather than the geometry of the molecule as a whole. Larger Molecules Polar Covalent Bonds Although atoms often form compounds by sharing electrons, the electrons are not always shared equally. Fluorine pulls harder on the electrons it shares with hydrogen than hydrogen does. Therefore, the fluorine end of the molecule has more electron density than the hydrogen end. 3

4 Electronegativity Electronegativity and Bond Types is the relative ability of atoms to attract shared electrons is higher for nonmetals, with fluorine as the highest with a value of 4.0 is lower for metals, with cesium and francium as the lowest with a value of 0.7 increases from left to right going across a period on the periodic table decreases going down a group on the periodic table Nonpolar Covalent Bonds A nonpolar covalent bond occurs between nonmetal atoms consists of an equal (or almost equal) sharing of electrons has a zero (or close to zero) electronegativity difference of 0.0 to 0.4 Examples: Atoms Electronegativity Type of Bond Difference N N = 0.0 Nonpolar covalent Cl Br = 0.2 Nonpolar covalent H Si = 0.3 Nonpolar covalent 21 Polar Covalent Bonds A polar covalent bond occurs between nonmetal atoms consists of atoms that share electrons unequally has an electronegativity difference range of 0.5 to 1.7 Examples: 22 Atoms Electronegativity Type of Bond Difference O Cl = 0.5 Polar covalent Cl C = 0.5 Polar covalent O S = 1.0 Polar covalent Dipoles and Bond Polarity Bonds become more polar as the difference in electronegativity increases. A polar covalent bond that has a separation of charges is called a dipole. The positive and negative ends are represented by the Greek letter delta with a + or charge. Arrows can also be used to represent dipoles. 3. Identify any polar bonds in each molecule on the Lewis Structure worksheet by drawing dipole moments. 4

5 Ionic Bonds An ionic bond occurs between metal and nonmetal ions is a result of electron transfer has a large electronegativity difference (1.8 or more) Examples: Atoms Electronegativity Type of Bond Difference Cl K = 2.2 Ionic N Na = 2.1 Ionic S Cs = 1.8 Ionic Molecular Polarity But just because a molecule possesses polar bonds does not mean the molecule as a whole will be polar. 25 Molecular Polarity describes the charge distribution along the entire molecule as the vector sum of the individual dipole moments. Compare the following for molecular polarity: Nonpolar Molecules A nonpolar molecule may contain identical atoms (nonpolar bonds) may have a symmetrical arrangement of polar bonds that cancel dipoles 28 Polar Molecules A polar molecule contains polar bonds has a separation of positive and negative charge called a dipole indicated by a dipole arrow has dipoles that do not cancel Determining Molecular Polarity The polarity of a molecule is determined from its electron-dot formula shape polarity of the bonds dipole cancellation

6 Molecular Polarity By adding the individual bond dipoles, one can determine the overall dipole moment for the molecule. 4. Label each molecule on the Lewis Structure worksheet as polar or nonpolar. 6