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MR. SURRETTE VAN NUYS HIGH SCHOOL CHAPTER 17: COVALENT BONDING AND BIOCHEMISTRY COVALENT BONDING CLASS NOTES COVALENT COMPOUNDS Covalent compounds form when pairs of electrons are shared between atoms. Covalent compounds are usually called molecules. COVALENT BONDS Hydrogen and helium (atomic numbers 1 and 2) follow the duet rule of bonding because they only hold two electrons. Elements 3 – 18 follow the octet rule because they share electrons to obtain eight valence electrons. Example 1. Two hydrogen and one oxygen atom combine to form water. Complete the following steps to construct an LDS diagram for the water molecule. 1a. Draw LDS diagrams for hydrogen and oxygen. A. 1b. Show the sharing of electrons to form the water molecule. A. 1c. Draw the LDS diagram for water. A. 1|Page CHEMISTRY MR. SURRETTE VAN NUYS HIGH SCHOOL Example 2. Four hydrogen and one carbon combine to form methane (CH4). Complete the following steps to construct an LDS diagram for the methane molecule. 2a. Draw LDS diagrams for carbon and hydrogen. A. 2b. Show the sharing of electrons to form the methane molecule. A. 2c. Draw the LDS diagram for methane. A. STRUCTURAL DIAGRAMS Lines can be drawn instead of dots between atoms to represent covalent bonds. These are called structural diagrams. Example 3. Draw the structural diagram for methane. 3A. As seen in Example 2, the LDS diagram for methane is: 2|Page CHEMISTRY MR. SURRETTE VAN NUYS HIGH SCHOOL 3A. (continued…) Since the central carbon atom shares one pair of electrons with each of the hydrogen atoms, the structural diagram for methane is: Example 4. Two hydrogen atoms combine to form the hydrogen molecule. Draw the structural diagram for H2. 4A. Both hydrogen atoms have a single electron. When they combine to form the hydrogen molecule, they share these two electrons. This creates a single covalent bond between them. The structural diagram for H2 is: COVALENT BOND STRUCTURE Atoms that share a pair of electrons create a single covalent bond. Atoms can also combine to form multiple bonds. For example, atoms that share four electrons create double bonds, and atoms that share six electrons create triple bonds. Example 5. Two oxygen and one silicon atom combine to form silicon dioxide. Complete the following steps to construct a structural diagram for SiO2. 5a. Draw LDS diagrams for silicon and oxygen. 5b. Show the sharing of electrons to form SiO2. A. Notice the two lines drawn around each oxygen atom shown below. These lines represent valence electrons already paired up and unavailable for bonding. 3|Page CHEMISTRY MR. SURRETTE VAN NUYS HIGH SCHOOL 5b. (continued…) The green circles represent orbital spaces that can accept electrons: 5c. Draw the LDS diagram for SiO2. A. 5d. Draw the structural diagram for SiO2. A. Example 6. Two nitrogen atoms combine to form the nitrogen molecule. Complete the following steps to construct a structural diagram for N2. 6a. Draw the LDS diagram for nitrogen. A. 6b. Show the sharing of electrons to form N2. A. 4|Page CHEMISTRY MR. SURRETTE VAN NUYS HIGH SCHOOL 6c. Draw the LDS diagram for N2. A. 6d. Draw the structural diagram for N2. A. Notice the triple bonds between the nitrogen atoms: COVALENT BOND LENGTHS The distance between atoms in a compound is called bond length. In covalent compounds, the bond length is proportional to the number of bonds between atoms. Single covalent bonds have the greatest bond lengths. Double covalent bonds are a little shorter, and triple covalent bonds are the shortest. METALS (re-visited) The outer electrons of metal atoms are weakly held to the atomic nucleus. As a result, these outer electrons are easily removed, leaving behind positively charged metal ions. METALLIC BONDS Electrons removed from a large group of metal atoms flow freely through the metal ions. This “fluid” of electrons holds the positively charged metal ions together. This is the metallic bond. METALLIC BONDS 5|Page CHEMISTRY MR. SURRETTE VAN NUYS HIGH SCHOOL CHAPTER 17: COVALENT BONDING AND BIOCHEMISTRY BIOCHEMISTRY CLASS NOTES THE CARBON ATOM The fundamental atom composing organic compounds is carbon. Carbon can form large molecules in limitless varieties because carbon has the ability to bond covalently in four directions to other atoms. CARBON BONDING Carbon may form single, double, or even triple bonds. A single bond occurs when two electrons are shared. One line is used in the structural formula. Two lines indicate the sharing of two pairs of electrons, a double bond. Three lines represent the sharing of three pairs of electrons, a triple bond. CARBON ATOM STRUCTURE Carbon, like all other atoms, exists in three-dimensional space. Visualize the carbon atom sitting in the middle of a tetrahedron (pyramid-shape). The four corners of the tetrahedron are the positions of its electrons. Lines drawn from the central carbon to the four corners are its bonding electrons. CARBON-BASED MOLECULES CARBOHYDRATES Carbohydrates are made of carbon, oxygen, and hydrogen. They include sugars, starches, cellulose, chitin, and glycoproteins. SUGARS Monosaccharides – like glucose (C6H12O6) look like pentagonal or hexagonal rings. Two monosaccharide rings may join in a reaction called dehydration synthesis. In this joining, a certain hydroxide (HO-) group from one ring approaches a hydroxide group in another ring. DEHYDRATION SYNTHESIS One of the hydroxides and a hydrogen from the other hydroxide break off and join to form water. The oxygen that is left attached to one ring bonds covalently with the exposed carbon of the other ring. 6|Page CHEMISTRY MR. SURRETTE VAN NUYS HIGH SCHOOL SUGAR FORMATION SUGARS The bonding of two monosaccharides makes a disaccharide like sucrose (C12H22O11). These bonded rings may go through a reverse reaction called hydrolysis where water is put back in and the rings separate. Polysaccharides are formed as more and more rings add together. LIPIDS Lipids, which include oils, fats, and waxes, are large molecules used for energy storage and to coat a surface to make it waterproof. A large part of the lipid structure consists of long chains of nonpolar, covalently bonded carbons. They are hydrophobic and do not attract polar water molecules. FATS Organic acids have a carboxyl group (-COOH) at one end. Long organic acids are called fatty acids. These acids are commonly chains of sixteen to eighteen carbons. A glycerol molecule and three fatty acids combine by dehydration synthesis to make a fat molecule. Much more energy can be stored in a gram of fat than in a gram of starch. FORMATION OF FAT 7|Page CHEMISTRY MR. SURRETTE VAN NUYS HIGH SCHOOL PROTEINS Proteins, polymers of amino acids, make up hair, muscle, connective tissue, enzymes, and fingernails. An amino acid is a small molecule that has a central carbon covalently bonded to an amine group, a carboxyl (acid) group, a hydrogen atom and an R-group. PROTEINS About twenty R-groups exist. Dehydration synthesis creates a peptide bond that holds two amino acids together, making a dipeptide. A longer chain of amino acids is a polypeptide. A protein is the total molecule. PROTEIN FORMATION 8|Page CHEMISTRY