Culver City H.S. Chemistry Honors Name _________________________________ Period ___ Date ___/___/___ 9 Covalent Bonding LEWIS STRUCTURES 1. A bond forms when two orbitals ____________ allowing electrons to be close to two nuclei at the same time. When two atoms bond, they overlap electrons in their __________ orbitals. 2. As two atoms approach each other, the potential energy ____________ because of the electronproton attractions. The potential energy __________ because of electron-electron and proton-proton repulsions. See Figure 9-1 on p.242. 3. When a covalent bond forms, two ______ __________ orbitals overlap. Hydrogen only uses one orbital, the _____ orbital. Draw and label oxygen’s valence orbitals: Oxygen: Consider the two H atoms and the O atom in a water molecule. How many bonds are shown? _____ For each bond, the hydrogen orbital uses its ____ orbital. The oxygen atom uses its two half-filled ____ orbitals to overlap each of the hydrogen’s half-filled orbitals. 4. Draw the Lewis dot symbol for H2O. The oxygen atom is surrounded by ____ e-‘s. 5. The Lewis symbol for O can be drawn two ways, so the Lewis symbol for H2O can be drawn two ways. Draw them: Which one is correct? ________________ (Consider the picture of water.) 6. Not all atoms follow the octet rule. Elements ___ through ___ are too small. Families ___, ___, and ___ also do not follow the octet rule when forming covalent bonds. However, these families are USUALLY involved in ionic bonds. Some atoms have MORE than an octet, too, this is called the “extended valence shell”. 7. Each of the diatomic elements involves a covalent bond. Draw the Lewis symbol for each one. Which one does not follow the octet rule? ___ Diatomic Lewis Molecule Symbol H2 H H N2 N N O2 O O F2 F F Cl2 Cl Cl Br2 Br Br I2 I I 8. The Lewis Structure of a molecule shows how the valence electrons are arranged among the atoms in the molecule. Steps for Writing Lewis Structures: a. Sum the valence electrons from all the atoms. Do not worry about keeping track of which electrons come from which atoms. It is the total number of electrons that is important. b. Use a pair of electrons to form a bond between each pair of bound atoms. c. Arrange the remaining electrons to satisfy the duet rule for hydrogen and the octet rule for the second-row elements. Example: Give the Lewis structure for each of the following: a. HF 9. The rules we have used for Lewis structures apply to most molecules, but there are exceptions. Incomplete Octet: Boron, for example, tend to form compounds in which the boron atom as fewer than 8 electrons. BF3 Expanded Octet: Some atoms exceed the octet rule. This behavior is observed only for those elements in Period 3 of the periodic table and beyond. SF6 b. N2 c. NH3 d. CH4 e. CF4 f. NO+ Comments about the Octet Rule: 1. The second-row elements C,N,O, and F should always be assumed to obey the octet rule. 2. The second-row elements B and Be often have fewer than 8 electrons around them in their compounds. These electron-deficient compounds are very reactive. 3. The second-row elements never exceed the octet rule, since their valence orbitals (2s and 2p) can accommodate only 8 electrons. 4. Third-row and heavier elements often satisfy the octet rule but can exceed the octet rule by using their empty valence d orbitals. 5. When writing the Lewis structure for a molecule, satisfy the octet rule for the atoms first. If electrons remain after the octet rule has been satisfied, then place them on the elements having available d orbitals (elements in Period 3 or beyond).