BONDING Let’s get together… Barbara A. Gage PGCC CHM 1010

Why Do Atoms Bond? Chemical bonds form because they lower the potential energy between the charged particles that compose atoms A chemical bond forms when the potential energy of the bonded atoms is less than the potential energy of the separate atoms Barbara A. Gage PGCC CHM 1010

Types of Bonds We can classify bonds based on the kinds of atoms that are bonded together Types of Atoms Type of Bond Bond Characteristic metals to nonmetals Ionic electrons transferred nonmetals to Covalent shared metals Metallic pooled Barbara A. Gage PGCC CHM 1010

Types of Bonding Barbara A. Gage PGCC CHM 1010

Ionic Bonds When a metal atom loses electrons it becomes a cation metals have low ionization energy, making it relatively easy to remove electrons from them When a nonmetal atom gains electrons it becomes an anion nonmetals have high electron affinities, making it advantageous to add electrons to these atoms The oppositely charged ions are then attracted to each other, resulting in an ionic bond Barbara A. Gage PGCC CHM 1010

Covalent Bonds Nonmetal atoms have relatively high ionization energies, so it is difficult to remove electrons from them When nonmetals bond together, it is better in terms of potential energy for the atoms to share valence electrons potential energy lowest when the electrons are between the nuclei Shared electrons hold the atoms together by attracting nuclei of both atoms Barbara A. Gage PGCC CHM 1010

an organization of metal cation islands in a sea of electrons Metallic Bonds The relatively low ionization energy of metals allows them to lose electrons easily The simplest theory of metallic bonding involves the metal atoms releasing their valence electrons to be shared as a pool by all the atoms/ions in the metal an organization of metal cation islands in a sea of electrons electrons delocalized throughout the metal structure Bonding results from attraction of cation for the delocalized electrons Barbara A. Gage PGCC CHM 1010

Lewis Electron-Dot Symbols For main group elements - The A group number gives the number of valence electrons. Place one dot per valence electron on each of the four sides of the element symbol. Pair the dots (electrons) until all of the valence electrons are used. Example: Nitrogen, N, is in Group 5A and therefore has 5 valence electrons. N : . . N : : N . : N . Barbara A. Gage PGCC CHM 1010

Determining the Number of Valence Electrons in an Atom The column number on the Periodic Table will tell you how many valence electrons a main group atom has Transition Elements all have two valence electrons. Why? Barbara A. Gage PGCC CHM 1010

Lewis Structures of Ions Cations have Lewis symbols without valence electrons lost in the cation formation Anions have Lewis symbols with eight valence electrons electrons gained in the formation of the anion Barbara A. Gage PGCC CHM 1010

Ionic Bonding & the Crystal Lattice Ionically bonded substances form a structure in which every cation is surrounded by anions, and vice versa This structure is called a crystal lattice The crystal lattice is held together by the electrostatic attraction of the cations for all the surrounding anions The crystal lattice maximizes the attractions between cations and anions, leading to the most stable arrangement Barbara A. Gage PGCC CHM 1010

Crystal Lattice Electrostatic attraction is nondirectional!! no direct anion–cation pair Therefore, there is no ionic molecule the chemical formula is an empirical formula, simply giving the ratio of ions based on charge balance One unit of the empirical formula is called a formula unit Barbara A. Gage PGCC CHM 1010

Ionic compound dissolved in water Electrical conductance and ion mobility. Solid ionic compound Molten ionic compound Ionic compound dissolved in water Barbara A. Gage PGCC CHM 1010

Lewis Theory of Covalent Bonding Lewis theory implies that another way atoms can achieve an octet of valence electrons is to share their valence electrons with other atoms The shared electrons would then count toward each atom’s octet The sharing of valence electrons is called covalent bonding Barbara A. Gage PGCC CHM 1010

Covalent Bonding Atoms with incomplete octets can share rather than transfer electrons. Each pair of shared electrons = 1 bond Shared electrons move around the nuclei of both atoms in the bond so both atoms have possession of the shared electrons. Barbara A. Gage PGCC CHM 1010

Lewis Dot Structures for Covalent Compounds Sum the valence electrons of all atoms. Determine the central atom. Position the central atom and place the additional atoms equally around it. Place the required number of electrons around the outside atoms first and then around the central atom to be each one meets the octet rule (or the number needed if it is an exception). Barbara A. Gage PGCC CHM 1010

Lewis Dot Structures for Covalent Compounds CCl4 Total electrons = 1(4) + 4(7) = 32 Cl Cl C Cl Barbara A. Gage PGCC CHM 1010

Lewis Dot Structures for Covalent Compounds SO3 Total electrons = 1(6) + 3(6) = 24 O O S O O O O S O O S O Barbara A. Gage PGCC CHM 1010

Covalent Bonding: Bonding and Lone Pair Electrons Electrons that are shared by atoms are called bonding pairs Electrons that are not shared by atoms but belong to a particular atom are called lone pairs aka nonbonding pairs . O S Lone pairs Bonding pairs Barbara A. Gage PGCC CHM 1010

Single Covalent Bonds •• •• • • •• •• O H F O H F When two atoms share one pair of electrons it is called a single covalent bond 2 electrons One atom may use more than one single bond to fulfill its octet to different atoms H only duet H • O •• F •• • H O •• F •• F Barbara A. Gage PGCC CHM 1010

Double Covalent Bond •• • •• • •• When two atoms share two pairs of electrons the result is called a double covalent bond four electrons O •• • O •• • O •• Barbara A. Gage PGCC CHM 1010

Octet Rule Exceptions Some elements are stable with fewer or more than 8 e-. H 2e- Be 4e- B 6e- P, Cl, Br (and more) 10e- S, Se, Xe (and more) 12e- Barbara A. Gage PGCC CHM 1010

Resonance: Delocalized Electron-Pair Bonding O3 can be drawn in 2 ways - Neither structure is actually correct but can be drawn to represent a structure which is a hybrid of the two - a resonance structure. Resonance structures have the same relative atom placement but a difference in the locations of bonding and nonbonding electron pairs. is used to indicate that resonance occurs. Barbara A. Gage PGCC CHM 1010

Writing Resonance Structures PROBLEM: Write resonance structures for the nitrate ion, NO3-. SOLUTION: Nitrate has 1(5) + 3(6) + 1 = 24 valence e- N does not have an octet; a pair of e- will move in to form a double bond. Barbara A. Gage PGCC CHM 1010

Bond Lengths The distance between the nuclei of bonded atoms is called the bond length Because the actual bond length depends on the other atoms around the bond we often use the average bond length averaged for similar bonds from many compounds Barbara A. Gage PGCC CHM 1010

Bond Energies Chemical reactions involve breaking bonds in reactant molecules and making new bonds to create the products The DH°reaction can be estimated by comparing the cost of breaking old bonds to the income from making new bonds The amount of energy it takes to break one mole of a bond in a compound is called the bond energy in the gas state homolytically – each atom gets ½ bonding electrons Barbara A. Gage PGCC CHM 1010

Barbara A. Gage PGCC CHM 1010

What is the relationship between bond order and bond length for bonds Silberberg, Principles of Chemistry What is the relationship between bond order and bond length for bonds between the same two elements? What is the relationship between bond length and bond energy for bonds Barbara A. Gage PGCC CHM 1010

Comparing Bond Length and Bond Strength PROBLEM: Using the periodic table, rank the bonds in each set in order of decreasing bond length and bond strength: (a) S - F, S - Br, S - Cl (b) C = O, C - O, C O (a) The bond order is one for all and sulfur is bonded to halogens; bond length should increase and bond strength should decrease with increasing atomic radius. (b) The same two atoms are bonded but the bond order changes; bond length decreases as bond order increases while bond strength increases as bond order increases. SOLUTION: (a) Atomic size increases going down a group. (b) Using bond orders we get Bond length: S - Br > S - Cl > S - F Bond length: C - O > C = O > C O Bond strength: S - F > S - Cl > S - Br Bond strength: C O > C = O > C - O Barbara A. Gage PGCC CHM 1010

Break 1 mol C─H +414 kJ Make 1 mol Cl─Cl +243 kJ 1 mol C─Cl −339 kJ 1 mol H─Cl −431 kJ Barbara A. Gage PGCC CHM 1010

Electron Distribution in a Covalent Bond Are electrons shared equally in a covalent bond? If not, why not? Distance of electrons from nucleus and number of protons in the nucleus Electronegativity – attraction of one atom in a bond for the electrons in that bond Barbara A. Gage PGCC CHM 1010

Polar Covalent Bonding Covalent bonding between unlike atoms results in unequal sharing of the electrons one atom pulls the electrons in the bond closer to its side one end of the bond has larger electron density than the other The result is a polar covalent bond bond polarity the end with the larger electron density gets a partial negative charge the end that is electron deficient gets a partial positive charge Barbara A. Gage PGCC CHM 1010

The Pauling electronegativity (EN) scale. Barbara A. Gage PGCC CHM 1010

Polarity When atoms in a bond have different electronegativities, the electron sharing is unequal. As the ΔEN increases, the electron distribution becomes more uneven and the molecule becomes polar. Barbara A. Gage PGCC CHM 1010

Polarity HCl ENH = 2.1 ENCl = 3.0 ΔEN = 0.9 The end with the higher EN will be slightly negative and the other will be slightly positive δ+H – Clδ- H – Cl Barbara A. Gage PGCC CHM 1010

Electronegativity Difference and Bond Type If difference in electronegativity between bonded atoms is 0, the bond is pure covalent equal sharing If difference in electronegativity between bonded atoms is 0.1 to 0.4, the bond is nonpolar covalent If difference in electronegativity between bonded atoms is 0.5 to 1.9, the bond is polar covalent If difference in electronegativity between bonded atoms is larger than or equal to 2.0, the bond is ionic 0.4 2.0 4.0 4% 51% Percent Ionic Character Electronegativity Difference “100%” Barbara A. Gage PGCC CHM 1010

Bond Dipole Moments Dipole moment, m, is a measure of bond polarity a dipole is a material with a + and − end it is directly proportional to the size of the partial charges and directly proportional to the distance between them m = (q)(r) not Coulomb’s Law measured in Debyes, D Generally, the more electrons two atoms share and the larger the atoms are, the larger the dipole moment Barbara A. Gage PGCC CHM 1010

Determining Bond Polarity from EN Values PROBLEM: (a) Use a polar arrow to indicate the polarity of each bond: N-H, F-N, I-Cl. (b) Rank the following bonds in order of increasing polarity: H-N, H-O, H-C. (a) Find EN values; the arrow should point toward the negative end. (b) Polarity increases across a period. SOLUTION: (a) The EN of N = 3.0, H = 2.1; F = 4.0; I = 2.5, Cl = 3.0 N - H F - N I - Cl (b) The order of increasing EN is C < N < O; all have an EN larger than that of H. H-C < H-N < H-O Barbara A. Gage PGCC CHM 1010

Percent Ionic Character The percent ionic character is the percentage of a bond’s measured dipole moment compared to what it would be if the electrons were completely transferred The percent ionic character indicates the degree to which the electron is transferred Barbara A. Gage PGCC CHM 1010