Chemical Bonding: The Covalent Bond Model Chapter Five Chemical Bonding: The Covalent Bond Model

Chemical Bonding: The Covalent Bond Model cont’d Ionic Molecules Covalent Molecules Formed between a metal and a nonmetal Formed between two or more nonmetals Formed when valence electrons are _____________ Formed when valence electrons are __________ Do not exist as discrete molecules (formula unit) Exist as discrete molecules Their solutions conduct electricity Their solutions usually don’t conduct electricity

Chemical Bonding: The Covalent Bond Model cont’d Fig. 5.1 Electron sharing can occur only when electron orbitals from two different atoms overlap.

Chemical Bonding: The Covalent Bond Model cont’d Fig. 5.3 (a) A “regular” covalent single bond is the result of overlap of two half-filled orbitals. (b) A coordinate covalent single bond is the result of overlap of a filled and a vacant orbital.

Chemical Bonding: The Covalent Bond Model cont’d Fig. 5.10 Linus Pauling received the Nobel Prize in chemistry in 1954 for his work on the nature of the chemical bond. © Bettman/CORBIS

Chemical Bonding: The Covalent Bond Model cont’d Fig. 5.2 The number of covalent bonds formed by a nonmetallic element is directly correlated with the number of electrons it must share in order to obtain an octet of electrons.

Chemical Bonding: The Covalent Bond Model cont’d Formed when two (or more) nonmetals ________ valence electrons in order to obey the ________ rule. Single covalent bond: two atoms share ___ pair of electrons between them (__ shared electrons = _ bond) Example: Cl2 Double covalent bond: two atoms share ___ pairs of electrons between them (_ shared electrons = _ bonds) in order to obey the ________ rule Example: CO2

Chemical Bonding: The Covalent Bond Model cont’d Triple covalent bond: two atoms share __ pairs of electrons between them (6 shared electrons = ___ bonds) Example: CO

Chemical Bonding: The Covalent Bond Model cont’d Lewis structures show us how atoms are bonded to each other. They can be used to predict molecular shapes. Conventions: A _____represents one non-bonding electron A ______ represents two shared (bonding) electrons (1 bond) Examples: NH3 O2 C2H2

Chemical Bonding: The Covalent Bond Model cont’d Fig. 5.4 The sulfur dioxide molecule. One Sulfur atom and two oxygen atoms

Chemical Bonding: The Covalent Bond Model cont’d Example: SO2 1. Add together the _______ electrons for all of the atoms in the compound. Add or subtract ________ as necessary if you have an _____. 2. Determine which atom is your ________ atom (usually the first one in the formula unless the first atom is H). Draw a ______ bond between the central atom and each of the other atoms. How many electrons do you have left?

Chemical Bonding: The Covalent Bond Model cont’d 3. Put electrons around the outer (non-central) atoms until each of them has ____ electrons around it. Remember: H only gets ___ electrons around it. How many electrons do you have left? 4. If you have leftover electrons, put them on the central atom. (Sometimes the central atom will have more than 8 electrons around it; that atom will not be obeying the octet rule)

Chemical Bonding: The Covalent Bond Model cont’d SO42- Note: When drawing the structures of ions, write the structure in square brackets with the charge in the upper right-hand corner.

Chemical Bonding: The Covalent Bond Model cont’d Fig. 5.7 The sulfate ion.

Chemical Bonding: The Covalent Bond Model cont’d Fig. 5.5 The phosphorus trifluoride molecule.

Chemical Bonding: The Covalent Bond Model cont’d Fig. 5.6 The hydrogen cyanide molecule. HCN

Chemical Bonding: The Covalent Bond Model cont’d Lewis Structures tell us how atoms are connected in a covalent molecule or ion They can also be used to predict molecular shape Molecular shape determines how molecules ________. Molecular shape is essential to drug design and other biochemical processes.

Chemical Bonding: The Covalent Bond Model cont’d In order to predict molecular shape, we assume that electron groups, i.e, bonding and nonbonding electrons, _____ each other (because they are all negatively charged). Therefore, the molecule adopts whichever 3D geometry minimizes this __________. We call this process Valence Shell Electron Pair Repulsion (VSEPR) theory.

Chemical Bonding: The Covalent Bond Model cont’d Molecules adopt shapes that minimize the repulsion between electron groups/domains What is an electron group/domain? A collection ________ _________ present in a localized region about the ________ atom in a molecule There are two kinds of electron groups/domains: A pair of ________________ electrons An area where there are _________ electrons (it doesn’t matter if the bond is a single, double or triple bond)

Chemical Bonding: The Covalent Bond Model cont’d Fig. 5.8 Arrangement of valence electron pairs about a central atom that minimize repulsions between the pairs.

Chemical Bonding: The Covalent Bond Model cont’d Determining Molecular Shape First, draw the Lewis Structure for the compound There are two important features of Lewis Structures that you must recognize a. The number of ATOMS bonded to the central atom (this is the number of bonding electron groups/domains) b. The number of PAIRS of nonbonding electrons on the central atom (this is the number of nonbonding electron groups/domains) Example: CO2

Chemical Bonding: The Covalent Bond Model cont’d NH3 H2CO CH4 H2O

Chemical Bonding: The Covalent Bond Model cont’d Molecular shapes are considered around EACH central atom Examples: C2H2 H2O2 HN3

Chemical Bonding: The Covalent Bond Model cont’d Fig. 5.9 (a) Acetylene molecule. (b) Hydrogen peroxide molecule. (c) Hydrogen azide molecule.

Chemical Bonding: The Covalent Bond Model cont’d Electronegativity A measure of the relative _________ that an atom has for the _______ electrons in a bond Electrons are _______ ________ to the atom with the higher electronegativity Trends in electronegativity Electronegativity values increase from left to right on the periodic table Electronegativity values increase from the bottom to the top of the periodic table

Chemical Bonding: The Covalent Bond Model cont’d Fig. 5.11 Abbreviated periodic table showing Pauling electronegativity values for selected representative elements.

Chemical Bonding: The Covalent Bond Model cont’d Polar covalent bonds A covalent bond in which there is _________ sharing of electrons between two atoms Nonpolar covalent bonds

Chemical Bonding: The Covalent Bond Model cont’d Fig. 5.12 (a) In the nonpolar covalent bond present, there is a symmetrical distribution of electron density. (b) In the polar covalent bond present, electron density is displaced because of its electronegativity.

Chemical Bonding: The Covalent Bond Model cont’d Bond Polarity: a measure of the degree of ____________ in the sharing of electrons between two atoms in a chemical bond There is no sharp distinction between bonding types. The positive end (or pole) in a polar bond is represented _____ and the negative pole _____. The negative end is toward the atom with the higher electronegativity Example: HCl

Chemical Bonding: The Covalent Bond Model cont’d Polar Bonds? Example: NaCl Example: O2 Example: NH3 Example: HCl

Chemical Bonding: The Covalent Bond Model cont’d Fig. 5.13 (a) Methane is a nonpolar tetrahedral molecule. (b) Methyl chloride is a polar tetrahedral molecule.

Chemical Bonding: The Covalent Bond Model cont’d Molecular Polarity A measure of the degree of inequality in the attraction of bonding electrons to various locations within a molecule Polar Molecule: A molecule in which there is an unsymmetrical distribution of charge

Chemical Bonding: The Covalent Bond Model cont’d Why do we care if the molecule is polar? Polar molecules dissolve in polar solvents (like water or blood) Nonpolar molecules dissolve in nonpolar solvents (like hexane or cell membranes) For a molecule to be polar: 1. It must contain polar bonds 2. The molecular geometry must not cancel out the effect of the polar bonds (through vector addition)

Chemical Bonding: The Covalent Bond Model cont’d

Chemical Bonding: The Covalent Bond Model cont’d Rules of Thumb Most covalent molecules are somewhat polar. It is easier to remember the exceptions. The exceptions for this class: Molecules in which two or more of the same atom are bonded to the central atom AND in which there are no nonbonding pairs of electrons on the central atom

Chemical Bonding: The Covalent Bond Model cont’d H2O PF3 SO2 CH3Cl H2 HCl

CAG 5.2

Chemical Bonding: The Covalent Bond Model cont’d Naming Binary Molecular Compounds (Non-Acid) Binary molecular compounds have two different elements. The ______ electronegative element is written first Exception: NH3. Greek prefixes are used to indicate the number of atoms.

Chemical Bonding: The Covalent Bond Model cont’d Table 5.1

Chemical Bonding: The Covalent Bond Model cont’d First word of name: Greek prefix + name of first element Note: if the prefix would be “mono” for the first element, the prefix is not included Note: when an element name begins with a vowel, an a or o at the end of the Greek prefix is dropped for phonetic reasons Second word of name Greek prefix + stem of name of second element with “-ide” ending

Chemical Bonding: The Covalent Bond Model cont’d Table 5.2 Compounds in which hydrogen is the first element listed are named without prefixes. Some compounds have common names. Last

Name the following binary compounds. SF4 P4O6 ClO2 H2S

Name the following binary compounds. Cl2O CO PI3 HI

Write formulas for the following binary compounds. Iodine monochloride Dinitrogen monoxide Nitrogen trichloride Hydrogen bromide

Write formulas for the following binary compounds. Bromine monochloride Tetrasulfur dinitride Sulfur trioxide Dioxygen difluoride