Chemical Bonding polarity & Dipole Moments Chapter 8 Part III
Exercise Arrange the following bonds from most to least polar: a) N F O F C F b) C F N O Si F c) Cl Cl B Cl S Cl
Exercise a) C F, N F, O F b) Si F, C F, N O c) B Cl, S Cl, Cl Cl
Dipole Moments A molecule with a center of negative charge and a center of positive charge is dipolar (two poles), or has a dipole moment. Center of charge doesn t have to be on an atom. Will line up in the presence of an electric field.
Bond Polarity and Dipole Moments Molecules with a charge distribution of a positive end and a negative end are dipolar, or said to have a dipole moment. But what about molecules made up of more than one atom?
Bond Polarity and Dipole Moments For a molecule with more than 2 atoms, the dipole moment depends on both the polarities of the individual bonds and the geometry of the molecule. The overall dipole moment of a polyatomic molecule is the sum of its bond dipoles.
Bond Polarity Bond Polarity is a measure of how equally the electrons in a bond are shared between the two atoms of the bond. Polarity is used when talking about covalently bonded molecules. If the molecule has only two different atoms, such as, HF or CCl 4 you can calculate the electronegativity difference and determine the type of covalent bond (polar or non-polar).
Polarity and Bond Type Electronegativity Difference Bonding Type <0.5 Non-polar covalent 0.5 1.9 Polar covalent > 2.0 ionic
Polar Covalent Bonds The greater the difference in electronegativity, the more polar is the bond.
Which Molecules Have Dipoles? Any two atom molecule with a polar bond. With three or more atoms there are two considerations. 1) There must be a polar bond. 2) Geometry can t cancel it out.
Geometry and polarity Three shapes will cancel them out. Linear
Geometry and polarity Three shapes will cancel them out. Planar triangles 120º
Geometry and polarity Three shapes will cancel them out. Tetrahedral
Geometry and polarity Others don t cancel Bent
Geometry and polarity Others don t cancel Trigonal Pyramidal
Electron Configurations Electron configurations help us understand about compound formation. Elements in ionic compounds combine to achieve a noble gas configuration in order to become stable. Covalent compounds combine to complete each other s valence electron configuration
Ionic Compounds We mean the solid crystal. Ions align themselves to maximize attractions between opposite charges, and to minimize repulsion between like ions. Can stabilize ions that would be unstable as a gas. React to achieve noble gas configuration
Size and Charge Ion size is important in determining the structure and stability of ionic solids. What determines the Size? Look first at relative size of ion and its parent atom.
Ion Size Positively charged ions have lost outer shell electrons and are smaller than their parent atom. Negatively charged ions have gained electrons and are larger than their parent atom.
Ion Size
Isoelectronic ions These are ions of different elements with the same number of electrons. For example O 2-, F -, Na +, Mg 2+ and Al 3+ All have the electron configuration of neon.
Arrange the ions in order of decreasing size. Se 2-, Br -, Rb + and Sr 2+ Se 2- Br - Rb + Sr 2+ Examples Choose the largest in each group. Li+, Na+, K+, Rb+, Cs+ Answer: Cs+ Ba 2+, Cs +, I -, Te 2- Te 2-