2 The Covalent Bond Section 9.1 Why do atoms bond?To achieve full outer electron shellsOctet Rule- atoms gain, lose, or share electrons to achieve the electron configuration of noble gasesGain and Lose IONIC BONDINGShare COVALENT BONDING
3 What is a covalent bond?A chemical bond that results in the SHARING of valence electronsOccurs between 2 or more nonmetals
4 A molecule is formed when two or more atoms bond covalently Examples: sugars, DNA, proteins, fats, carbohydrates, cotton, synthetic fibers
5 Formation of a covalent bond REMEMBER: Hydrogen (H2), Nitrogen (N2), Oxygen (O2), Fluorine (F2),Chlorine (Cl2) Bromine (Br2) and Iodine (I2) occur in nature as diatomic molecules
6 Covalent BondingAn attractive force occurs between the protons of one atom and the electrons of the other atomWhen a single pair of electrons is shared, such as in the hydrogen molecule, a single covalent bond forms
7 Lewis StructuresUse electron-dot diagrams to show how electrons are arranged in molecules
8 Group 7A (Halogens) have 7 VE One more VE is necessaryA single covalent bond will formGroup 6A have 6 VETwo more VE are necessaryTwo covalent bonds will formGroup 5A have 5 VEThree more VE are necessaryThree covalent bonds will formGroup 4A have 4 VEFour more VE are necessaryFour covalent bonds will form
9 Sigma Bonds- single covalent bonds- when electron pairs are centered between two atoms Multiple BondsIn many molecules, atoms attain noble gas configuration by sharing more than one pair of electrons between two atomsCarbon, Nitrogen, Oxygen, and Sulfur most often form multiple bonds
10 Strength of Covalent Bonds The strength of covalent bonds depends on how much distance separates both nucleiThe distance between the two bonding nuclei at the position of maximum attraction is called bond lengthDetermined by the size of the atoms and how many electron pairs are sharedBond length decreases as the number of bonds increases (triple bond has a shorter bond length than a single bond)
11 Energy ChangesAn energy change accompanies the forming or breaking of a bond between atoms in a molecule.Energy is released when a bond formsEnergy must be added to break the bonds of a moleculeThe amount of energy required to break a specific covalent bond is called bond dissociation energy
12 Bond dissociation energy indicates the strength of a chemical bond because a direct relationship exists between bond energy and bond lengthIn chemical reactions, bonds in reactant molecules are broken and new bonds are formed as product molecules form
13 Endothermic reactions occur when a greater amount of energy is required to break the existing bonds in the reactants than is released when the new bonds from in the product moleculesExothermic reactions occur when more energy is released forming new bonds than is required to break bonds in the initial reactants
14 CheckpointWhat is a covalent bond? How does it differ from an ionic bond?What type of elements form covalent bonds?Draw the Lewis Structures for each of these molecules:PH3H2SCCl4
15 Naming Molecules Section 9.2 Naming Binary Molecular CompoundsThe first element in the formula is always named first, using the entire element name.The second element in the formula is named using the root of the element and adding the suffix – ide.Prefixes are used to indicate the number of atoms of each type that are present in the compound.
16 Prefixes in Covalent Compounds Number of AtomsPrefix1Mono-6Hexa-2Di-7Hepta-3Tri-8Octa-4Tetra-9Nona-5Penta-10Deca-
18 Naming Acids Binary Acids Use the prefix hydro- to name the hydrogen part of the compoundThe rest of the name consists of a form of the root of the second element plus the suffix –ic, followed by the word acid.Examples:HCl Hydrochloric AcidHCN Hydrocyanic acid (even though there are more than 2 elements present, if no oxygen is present- the acid is named as a binary)
19 OxyacidsAcids that contain an oxyanion (polyatomic ion that contains oxygen)The name of the oxyacids consists of a form of the root of the anion, a suffix, and the word acidIf the anion suffix is –ate, it is replaced with the suffix –icIf the anion suffix is –ite, it is replaced with the suffix –ous.
21 Formulas and Names of Covalent Compounds CheckpointWrite the molecular formula for each of the following compoundsDisulfur trioxideIodic acidDinitrogen monoxideHydrofluoric acidPhosphorus pentachlorideWhat is the difference between a binary acid and oxyacid?Complete the following table Formulas and Names of Covalent CompoundsFormulaNamePCl5Hydrobromic acidH3PO4Oxygen difluorideSO2
22 Molecular Structures Section 9.3 Structural Formula- uses letter symbols and bonds to show relative positions of atomsLewis Structure Procedure1. Predict the location of certain atomsHydrogen is always a terminal, or end, atom. Because it can share only one pair of electrons, hydrogen can be connected to only one other atomThe atom with the least attraction of shared electrons in the molecule is the central atom. This element usually is the one closer to the left on the periodic table. The central atom is located in the center of the molecule, and all other atoms become terminal atoms.
23 2. Find the total number of electrons available for bonding 2. Find the total number of electrons available for bonding. This total is the number of valence electrons in the atoms in the molecule.3. Determine the number of bonding pairs by dividing the number of electrons available for bonding by two.4. Place one bonding pair (single bond) between the central atom and each of the terminal atoms.
24 5. Subtract the number of pairs you used in step 4 from the number of bonding pairs you determined in step 3. The remaining electron pairs include lone pairs as well as pairs used in double and triple bonds. Place lone pairs around each terminal atom bonded to the central atom to satisfy the octet rule. Any remaining pairs are assigned to the central atom.
25 6. If the central atom is not surrounded by 4 electron pairs, it does not have an octet. You must convert one or two of the lone pairs on the terminal atoms to a double bond or a triple bond between the terminal atom and the central atom. These pairs are still associated with the terminal atom as well as with the central atom. Remember that, in general, carbon, nitrogen, oxygen, and sulfur can form double or triple bonds with the same element or with another element.
26 Space-filling Structure Ball-and-stick molecular model Lewis StructureBall-and-stick molecular model
28 Resonance StructuresResonance is a condition that occurs when more than one valid Lewis structure can be written for a molecule or ionNitrate Ion Resonance Structures:
29 Exceptions to the Octet Rule A small group of molecules has an odd number of valence electrons and cannot form an octet around each atom. (i.e. NO2 , ClO2, and NO)Some compounds form with fewer than eight electrons present around an atom. (Example: Boron)When one atom donates a pair of electrons to be shared with an atom or ion that needs two electrons to become stable, a coordinate covalent bond forms
30 3. Some central atoms contain more than eight valence electrons (expanded octet) Examples: PCl5, SF6, and XeF4
31 Molecular Shape Section 9.4 The shape of the molecule determines many of its physical and chemical properties.Molecular shape is determined by the overlap of orbitals that share electronsValence Shell Electron Pair Repulsion model or VSEPR modelBased on an arrangement that minimizes the repulsion of shared and unshared pairs of electrons around the central atom
32 VSEPR ModelThe angle formed by any two terminal atoms and the central atom is a bond angle.Shared electron pairs repel one anotherLone pairs of electrons occupy a slightly larger orbital than shared electronsShared bonding orbitals are pushed slightly together by lone pairs
34 HybridizationA hybrid results from combining two of the same type of object, and it has characteristics of both.Hybridization- a process in which atomic orbitals are mixed to form new, identical hybrid orbitals.
35 Practice ProblemsDetermine the molecular geometry and bond angle for the following:BF3NH4+OCl2CF4
36 Electronegativity and Polarity Section 9.5 Electron affinity is a measure of the tendency of an atom to accept an electron
37 The character and type of a chemical bond can be predicted using the electronegativity difference of the elements that are bondedPolar Covalent- Unequal sharingNonpolar Covalent- Equal sharingGenerally- Ionic bond form when the electronegativity difference is greater than 1.70
38 Polar Covalent BondsPolar covalent bonds form because not all atoms that share electrons attract them equallyThe shared pair of electrons is pulled toward one of the atomsPartial charges occur at the ends of the bondPartially negativePartially positive The resulting polar bond is referred to as a dipole (two poles)
39 Molecular PolarityMolecules are either polar or nonpolar, depending on the location and nature of the covalent bonds they contain.A polar molecule has a partial negative charge on one side, while the other side of the molecule has a partial positive chargeNote: symmetric molecules are usual nonpolar and molecules that are asymmetric are usually polar
41 Solubility of polar molecules The ability of a substance to dissolve in another substance is known as the physical property solubilityThe bond type and the shape of the molecules present determine solubility“Likes dissolve likes”Polar compounds are usually soluble in polar substancesNonpolar molecules only dissolve in nonpolar substances
42 Properties of Covalent Compounds Lower melting and boiling points (indicating weak bond strength)Many are liquids or gases at room temperatureDo not conduct electricityMany do not dissolve in water (polar)
43 Practice ProblemsDecide whether each of the following molecules is polar or nonpolarSCl2H2SCF4CS2
44 Vocabulary on Test Structural formula molecule VSEPR model Coordinate covalent bondhybridizationoxyacidelectronegativitypolar covalentcovalent bondresonanceendothermicexothermicterminal atomSigma bond