Chapter 16 Notes, part IV Polarity and IMFs
Types of Bonds Up until now, we have assumed that there are two types of bonds: Covalent and Ionic. This is true, but covalent bonds can be broken into two categories: This is true, but covalent bonds can be broken into two categories:
Nonpolar Covalent In nonpolar covalent bonding electrons are shared equally. Electrons spend an equal amount of time with both elements in the bond. Typical in diatomic elements: H 2, N 2, O 2, F 2, Cl 2, Br 2, I 2
Polar Covalent In polar covalent bonding, electrons are still shared, but they are shared unequally. This is due to one nucleus pulling the shared pair harder than the other. This creates a dipole—a bond where one side is slightly positive and the other is slightly negative.
A dipole is caused because the electron spends more time on one side than the other. The polarity of the bond is shown like: Polar Covalent H—Cl + - OR
Ionic Bonds In an ionic bond, electrons are transferred. The nucleus of one element pulls hard enough to take electrons away completely.
How can you tell what kind of bond there is? By looking at the difference in electronegativity! Remember, electronegativity is the tendency of an atom to attract an electron when bonding. The more electronegative, the more it will pull electrons.
Nonpolar Bonds: EN Diff ≤ 0.4 Polar Bonds: 0.4 < EN Diff ≤ 1.7 Ionic Bonds: EN Diff > 1.7
What type of bond is between: H and Cl Li and Cl C and S F and O
What type of bond is between: N and Br Na and F C and O
Polar Molecules If the slightly positive and slightly negative ends of polar bonds can collect on two different sides of a molecule, it can make an entire molecule polar.
If bonds are nonpolar, a molecule will always be nonpolar.
If bonds are polar and the shape of a molecule is symmetrical, the molecule will be nonpolar because the charges cancel out. (Linear, trigonal planar, tetrahedral and trigonal bipyramidal are the symmetrical shapes we talked about.)
If bonds are polar and the molecule is asymmetrical, the molecule will be polar! (Bent and pyramidal molecules are asymmetrical)
Practice Problem Are the following polar or nonpolar molecules? CO 2 H 2 O
Attractions between molecules They are what make solid and liquid molecular compounds possible. They are what make solid and liquid molecular compounds possible. The weakest are called van der Waal’s forces - there are two kinds: The weakest are called van der Waal’s forces - there are two kinds:
1. Dispersion forces weakest of all, caused by motion of e - increases as # e - increases halogens start as gases; bromine is liquid; iodine is solid
2. Dipole interactions Occur when polar molecules are attracted to each other. Occur when polar molecules are attracted to each other. Slightly stronger than dispersion forces. Slightly stronger than dispersion forces. Opposites attract but not completely hooked like in ionic solids. Opposites attract but not completely hooked like in ionic solids. HFHF HFHF
Dipole Interactions
Hydrogen bonding The strongest of the intermolecular forces. The strongest of the intermolecular forces. When a hydrogen is covalently bonded to a highly electronegative atom, AND is also weakly attracted to an unshared electron pair of another EN atom. When a hydrogen is covalently bonded to a highly electronegative atom, AND is also weakly attracted to an unshared electron pair of another EN atom. The hydrogen is left very electron deficient, thus positive, so it attracts with any nearby negative charge The hydrogen is left very electron deficient, thus positive, so it attracts with any nearby negative charge
Hydrogen bonding F, O, or N, are very electronegative. F, O, or N, are very electronegative. A strong dipole forms when Hydrogen is bonded with one these, and the hydrogen partially share with the lone pair in the molecule next to it. A strong dipole forms when Hydrogen is bonded with one these, and the hydrogen partially share with the lone pair in the molecule next to it.
Hydrogen bonding H H O H H O H H O H H O H H O H H O H H O