CHEMICAL BONDS Covalent Bonding MOLECULAR COMPOUNDS neutral group of atoms that are held together by covalent bonds chemical compound whose simplest units are molecules indicates the relative numbers of atoms of each kind in a chemical compound by using atomic symbols and numerical subscripts shows the types and numbers of atoms combined in a single moleculee of a molecular compound DIATOMIC MOLECULES Moleculee containing only two atoms Note that the shape formed by six of these elements in the periodic table is like a 7, starting at Atomic # 7 FORMATION OF COVALENT BOND The nucleus of one atom attracts the electron cloudd of the other atoms, and vice versa Distance between nuclei is reached at which: Attractive forces equals Repulsive forces Potential Energy is at minimum Stability is greatest BOND LENGTH Averagee distance between two bonded atoms at their minimum Potential Energy BOND ENERGY The energy required to break a chemical bond and isolated atoms Indirect measure of bond strength form neutral PROPERTIES OF COVALENT COMPOUNDS Low melting and boiling points Do not conduct electricity Solids are brittle Can be solids, liquids, or gas EXCEPTIONS TO OCTET RULE Molecules with an odd number of electrons One or more unpaired electrons Also called free radicals Molecules in which an atom has less than an octet Hydrogen: 2 e- Beryllium: 4 e- Boron: 6 e- Molecules in which an atom has more than an octet Expanded Valence - Bonding involves electrons inn the d-orbitals as well as s and p orbitals CHEMISTRY OSHIKIRI Page 1 of 9
LEWIS STRUCTURES Formulas in which atomic symbols represent nuclei and inner-shell electrons Dot-pairs or dashes between two atomic symbols represent electron pairs in covalent bonds Dots ( ) adjacent to only one atomic symbol represent unshared electrons pair of electrons that are not involved in bonding and that belongs exclusively to only one atom ( ) pair of electrons involved in covalent bonding; indicated by a dash ( ) The second-row elements C,N,O and F should always be assumed to obey octet rule They never exceed the octet since their valence orbitals (2s and 2p) can accommodate only 8 e- The second row elements B and Be have fewer than 8 electrons, making these electrondeficient compounds very reactive Third-row and heavier elements often satisfy the octet, but can exceed the octet rule by using their empty valence d-orbitals If electrons remain after the octet rule has been satisfied, place them on elements having available d-orbitals (elements in Period 3 or beyond) SINGLE COVALENT BOND Covalent bond produced by the sharing of one pair of electrons between two atoms MULTIPLE COVALENT BONDS Double bond two pairs of electrons are shared between atoms Triple bond three pairs of electrons are shared between atoms Multiple bonds are most often formed by C, N, O, P and S atoms. Remember C-NOPS SIGMA BONDING Sigma Bond (σ) end to end overlapping of S and P x orbitals; P x and P x orbitals Single bond = 1 sigma bond Sigma bonds are symmetrical along the axis between the 2 nuclei Electrons are held tightly in this region PI BONDING Pi bond (π) side to side overlapping of parallel P y and P z orbitals Bonding e- are found in sausage shaped regions above and below the bond axis Not held as tightly by nuclei Double = 1 sigma and 1 pi bond Triple = 1 sigma and 2 pi bonds CHEMISTRY OSHIKIRI Page 2 of 9
COORDINATE COVALENT BOND Covalent bond formed when only one atom contributes both bonding electrons EX: NH 3 + BF 3 d H 3 N-BF 3 POLYATOMIC IONS A charged group of covalently bonded atoms Charge is due to excess or deficit of electrons Structure is written in brackets with the charge of the ion as an outside superscript WRITING LEWIS STRUCTURES Central atom occurs least in the formula It is the least electronegative atom with the exception of Hydrogen Bonding pairs should always be placed between the atoms Each atom should have 8 electrons except hydrogen which has 2 WRITING LEWIS STRUCTURES N-A-S-B-L METHOD N is the total number of valence electrons NEEDED by all atoms or ion to become stable N = 8 x # atoms that are not H, plus 2 x # H atoms A is the number of electrons AVAILABLE in the valence shell of the atoms Equal to the sum of their group numbers Adjust A for ionic charges Add electrons to account for negative charges Subtract electrons to account for positive charges S is the total number of electrons SHARED in the molecule or polyatomic ion S = N A B Number of BONDS shared by central atom = S/2 L number of LONE PAIR electrons or simply dots Place the dots to fill the octet of atoms attached to the central atom except for Hydrogen Remember that hydrogen has only one bond and NO DOTS! F 2 CO 2 H 2 SO 4 PO 4 3- CHEMISTRY OSHIKIRI Page 3 of 9
PRACTICE PROBLEMS: Write Lewis diagrams for the following: S 2 Cl 2 CH 3 OH C 2 F 4 C 2 Cl 2 HCHO SO 2 NH 4 + SO 4 2- CHEMISTRY OSHIKIRI Page 4 of 9
RESONANCE Refers to bonding in molecules or ions that cannot be correctly represented by a single Lewis structure Hybrid structures are formed that are an average between all possible Lewis structures A double headed arrow is placed between the structures to indicate resonance EX: ozone, O 3 MOLECULAR COVALENT BONDING Consist of many identical molecules bound together by intermolecular forces acting between the molecules EX: NH 3, H 2 O, CCl 4, CH 4 NETWORK COVALENT BONDING Consist of atoms covalently bonded together to form three-dimensional network EX: diamond, graphite, buckminsterfullerene MOLECULAR GEOMETRY Lewis structures show possible arrangements of electrons to determine how atoms bond together Structural formulas show how atoms link together, but they don t represent the 3-D structure of a molecule Structure determines function so the 3-D shape is VERY IMPORTANT!!!!!!!! VSEPR THEORY Valence-Shell Electron Pair Repulsion Developed by Ronald James Gillespie and Dr. Ronald Nyholm around 1956-1957 States that repulsion between the sets of valence-level electrons surrounding an atom causes these sets to be oriented as far apart as possible Electron dot structures and structural formulas fail to reflect 3-dimensional shapes of molecules Electron pairs spread as far apart as possible to minimize repulsive forces ELECTRON PAIR REPULSIONS LONE PAIRS An unshared electron pair is acted upon by only one nucleus Repulsion between two unshared pairs is greatest because they occupy the most space BOND PAIRS A shared pair moves within the field of two nuclei, and electron movement is restricted Repulsion between two shared pairs is least because they occupy the least space CHEMISTRY OSHIKIRI Page 5 of 9
TO PREDICT MOLECULAR GEOMETRY Treat multiple bonds as single bonds. Lone pairs of electrons occupy more space than bonding pairs Consider only the directional character of the bond The number of Bond pairs and Lone pairs within a molecule determine the shape of the molecule EFFECT ON LONE PAIRS ON BOND ANGLES lone pair-lone pair repulsion > lone pair-bond pair > bond pair-bond > pair repulsion repulsion VSEPR AND UNSHARED ELECTRON PAIRS Electron Pair Shape Bond Angle Repulsion a. CH 4 b. NH 3 c. H 2 O MOLECULAR GEOMETRY (SHAPES) MOLECULAR GEOMETRY # OF BONDS # OF LONE PAIRS MOLECULE TYPE Linear 2 0 AB 2 ELECTRON PAIR GEOMETRY Linear triatomic EXAMPLES: LEWIS STRUCTURES BeF 2 Trigonal Planar 3 0 AB 3 Trigonal Planar Bent 2 1 AB 2 E Trigonal Planar CHEMISTRY OSHIKIRI Page 6 of 9
MOLECULAR GEOMETRY # OF BONDS # OF LONE PAIRS MOLECULE TYPE Tetrahedral 4 0 AB 4 ELECTRON PAIR GEOMETRY Tetrahedral EXAMPLES: LEWIS STRUCTURES Trigonal Pyramidal 3 1 AB 3 E Tetrahedral Bent 2 2 AB 2 E 2 Tetrahedral Trigonal Bipyramidal 5 0 AB 5 Trigonal Bipyramidal Octahedral 6 0 AB 6 Octahedron CHEMISTRY OSHIKIRI Page 7 of 9
MOLECULAR POLARITY The uneven distribution of molecular charge Bonding electrons shifted to more electronegative atom Molecular polarity is determined by Polarity of each bond Geometry of the molecule POLAR MOLECULES Contain at least one pair of bonded atoms with different electronegativity Central atom contains lone pairs Shapes do not allow cancellation of bond dipoles Trigonal pyramidal, bent always POLAR because they contain lone pairs NONPOLAR MOLECULES All atoms have the same electronegativity Molecular shapes permit cancellation of bond dipoles No NET Molecular dipole Tetrahedral, trigonal-planar, linear, trigonal bipyramidal & octahedral Considered NONPOLAR as long as they have the same atoms bonded to central atom These molecular shapes will be NONPOLAR as long as they have the same terminal atoms (T) attached to the central atom (C) Becomes POLAR when the atoms bonded to central atom are different from each other CHEMISTRY OSHIKIRI Page 8 of 9
HYBRIDIZATION Mixing of two or more atomic orbitals of similar energies on the same atom to produce new orbitals of equal energies HYBRID ORBITALS Orbitals of equal energy produced by the combination of two or more orbitals of the same atom Explains the bonding and geometry of many molecules formed by Group 15 and 16 elements sp 3, sp 2, sp sp 3 HYBRID s, p, p, p 4 hybrid orbitals Characterized by single bonds Sigma bonds Bond angle 109.5 Tetrahedral sp 2 HYBRID s,p,p 3 hybrid orbitals Plus 1 unhybridized orbital Characterized by double bonds 1 sigma bond 1 pi bond Bond angle 120 Trigonal Planar sp HYBRID s,p 2 hybrid orbitals Plus 2 unhybridized orbitals Characterized by triple bonds 1 sigma bond 2 pi bonds Bond angle 180 Linear PRACTICE PROBLEMS: H 3 C-NtCtO H 2 CtCH 2 CH 4 NoC-CoN CHEMISTRY OSHIKIRI Page 9 of 9