Atoms That Share: Covalent Bonds A water molecule is an example of the single covalent bond between oxygen and hydrogen. Don't look now, but you're surrounded by covalent bonds. They're in the air you breathe and in the water you drink. You even make them yourself when you exhale. What are these bonds? What do they want? Take a look at water (H2O). H2O is a MOLECULE, a discrete unit of atoms that are bonded together. A molecule of water consists of an oxygen atom that is bonded to two hydrogen atoms. Atoms That Share: Covalent Bonds (Continued) A hydrogen atom and the oxygen atom each donate one valence electron to form a chemical bond. These two valence electrons forming the bond are shared by both atoms, resulting in a SINGLE COVALENT BOND. Think of this in terms of two pieces of wood that are nailed together. The pieces of wood are the atoms, and the Each piece of wood shares a portion of the nails. Atoms That Share: Covalent Bonds (Continued) A water molecule is an example of the single covalent bond between oxygen and hydrogen. Atoms That Share: Covalent Bonds (Continued) Air, too, contains oxygen. Oxygen does not exist as a single oxygen atom, but as a molecule of two oxygen atoms. These two oxygen atoms share two pairs of valence electrons (four valence electrons total) between them, forming a DOUBLE COVALENT BOND. This is true of any double covalent bond; four valence electrons are shared between two atoms. Atoms That Share: Covalent Bonds (Continued) Another component of air is nitrogen. Like oxygen, nitrogen does not exist as a single nitrogen atom, but as a molecule made up of two nitrogen atoms. The two nitrogen atoms in a molecule of nitrogen share three pairs of valence electrons (six valence electrons total) to form a TRIPLE COVALENT BOND. Atoms That Share: Covalent Bonds (Continued) Is it possible to predict whether bonds are covalent or not? A good rule of thumb is that bonds between nonmetals (remember that hydrogen is considered a nonmetal) are usually covalent bonds. For example, the carbon dioxide (CO2) molecules you exhale are bonded together covalently. Atoms That Share: Covalent Bonds (Continued) Oxygen molecules shares two electrons to make a double covalent bond. Atoms That Share: Covalent Bonds (Continued) In a molecule of nitrogen, two atoms share three electrons -- a triple covalent bond Atoms That Share: Covalent Bonds (Continued) Polar Covalent Bonds 1. Sometimes in a covalent bond the electrons are not shared equally between the two atoms. 2. On average, one of the atoms partially "pulls" the bonding electrons toward itself, creating an unequal sharing of those bonding electrons. This is called a POLAR COVALENT BOND. 3. In order to determine whether a covalent bond is polar or not, it's necessary to understand electronegativity. Atoms That Share: Covalent Bonds (Continued) Recall that electronegativity is a measure of an atom's ability to draw its bonding electrons to itself. Each element has a numeric value corresponding to its electronegativity. The values used here were devised by Linus Pauling, though there are a few other scales of electronegativity values. Atoms That Share: Covalent Bonds (Continued) Atoms That Share: Covalent Bonds (Continued) Fluorine was determined to be the most electronegative element and has an electronegativity value of 4.0. Francium, the least electronegative element, has a value of 0.7. It is important to remember a general trend in the periodic table: electronegativity increases from left to right going across a period, and it increases from the bottom to the top of a group. Atoms That Share: Covalent Bonds (Continued) For example, in period 2, nitrogen (group 5A) has an electronegativity value of 3.0, compared to 2.5 for carbon (group 4A). In group 5A, phosphorous (period 3) has an electronegativity value of 2.1, compared to 3.0 for nitrogen (period 2). Atoms That Share: Covalent Bonds (Continued) In an action that resembles toddlers tugging on a toy, a polar covalent bond occurs when one atom with a higher electronegativity draws the bonding electrons toward itself, pulling those electrons away from the atom with the lower electronegativity value. This creates an unequal sharing of electrons known as UNEQUAL CHARGE DISTRIBUTION, or charge separation. The charge separation makes the bond polar because the more electronegative atom becomes partially negatively charged and the atom with the lower electronegativity becomes partially positively charged. Atoms That Share: Covalent Bonds (Continued) Consider H2 and HBr. In H2, the atoms have an equal "pull" on the bonding electrons, making the bond NONPOLAR. Bonded hydrogen atoms showing equal charge distribution Atoms That Share: Covalent Bonds (Continued) In HBr, however, Br has an electronegativity of 2.8, compared to 2.1 for hydrogen. The Br atom pulls the bonding electrons toward itself, creating a partial negative charge on itself and a partial positive charge on the hydrogen atom. Atoms That Share: Covalent Bonds (Continued) The larger the absolute difference in electronegativity between two atoms, the more polar that bond is. For example, the electronegativity difference of a carbon-oxygen bond is -1.0, the result of 2.5 (the electronegativity value for carbon) minus 3.5 (the value for oxygen). The absolute value for the difference in electronegativity is the value without the minus sign (1.0 for a carbon-oxygen bond). Atoms That Share: Covalent Bonds (Continued) For a carbon-chlorine bond, the difference in electronegativity is 0.5 (2.5 - 3.0 = -0.5). Thus, a carbon-oxygen bond is more polar (1.0) than a carbonchlorine bond (0.5). Atoms That Share: Covalent Bonds (Continued) Hydrogen and bromide bond with an unequal charge distribution.