State University of New York at Brockport Chemical Bonds Mark P. Heitz State University of New York at Brockport © 2005, Prentice Hall, Inc.

Chemical Bonds: Electrostatics Forces called chemical bonds hold atoms together in molecules and keep ions fixed in ionic solids Chemical bonds are electrical forces that balance attraction and repulsion between electrically charged particles EOS Chapter 9: Chemical Bonds

Chapter 9: Chemical Bonds Energy of Interaction Scientists can determine the internuclear distances that correspond to the lowest energy states of molecules EOS – Quantum mechanical calculations can then be used to develop theoretical models Chapter 9: Chemical Bonds

The Lewis Theory of Chemical Bonding: An Overview Electrons, particularly valence electrons, play a fundamental role in chemical bonding EOS In losing, gaining, or sharing electrons to form chemical bonds, atoms tend to acquire the electron configurations of noble gases Chapter 9: Chemical Bonds

Chemical Bonding: An Overview When metals and nonmetals combine, valence electrons usually are transferred from the metal to the nonmetal atoms giving rise to ionic bonds EOS In combinations involving only nonmetals, one or more pairs of valence electrons are shared between the bonded atoms, producing covalent bonds Chapter 9: Chemical Bonds

Chapter 9: Chemical Bonds Lewis Symbols Valence electrons are shown by dots around the element symbol EOS Use rules of electron configurations when forming dot structures … e.g., electrons remain unpaired if possible Chapter 9: Chemical Bonds

Lewis Structures of Simple Molecules A Lewis structure shows the bonded atoms with the electron configuration of a noble gas; that is, the atoms obey the octet rule EOS By double-counting the shared electrons in a Lewis structure, each atom appears to have a noble gas configuration Chapter 9: Chemical Bonds

Chapter 9: Chemical Bonds Lewis Structures The shared pairs of electrons in a molecule are called bonding pairs In common practice, the bonding pair is represented by a dash (—): e.g., H—Cl EOS The other electron pairs, which are not shared, are called nonbonding pairs, or lone pairs Chapter 9: Chemical Bonds

Second Period Compounds EOS Chapter 9: Chemical Bonds

Coordinate Covalent Bonds In some cases, one atom provides both electrons of the shared pair to form a bond called a coordinate covalent bond H+ EOS Chapter 9: Chemical Bonds

Multiple Covalent Bonds The covalent bond in which one pair of electrons is shared is called a single bond e.g., H : Cl or H—Cl Double bonds have two shared pairs of electrons EOS Triple bonds have three shared pairs of electrons Chapter 9: Chemical Bonds

Chapter 9: Chemical Bonds Electronegativity Electronegativity (EN) is a measure of the ability of an atom to attract bonding electrons to itself EOS The greater the electronegativity of an atom in a molecule, the more strongly it attracts the electrons in a covalent bond Chapter 9: Chemical Bonds

Pauling’s Electronegativities Electronegativity Illustrated EOS Chapter 9: Chemical Bonds

Electronegativity Difference and Bond Type Two identical atoms have the same electronegativity and share a bonding electron pair equally. This is called a nonpolar covalent bond Example: chlorine gas EOS All homonuclear diatomic molecules have nonpolar covalent bonds: H2, N2, O2, F2, Cl2, Br2, I2 Chapter 9: Chemical Bonds

Electronegativity Difference and Bond Type In covalent bonds between atoms with somewhat larger electronegativity differences, electron pairs are shared unequally. This is called a polar covalent bond Example: hydrogen chloride gas, HCl The electrons are drawn closer to the atom of higher electronegativity, Cl EOS Chapter 9: Chemical Bonds

Electronegativity Difference and Bond Type With still larger differences in electronegativity, electrons may be completely transferred from metal to nonmetal atoms to form ionic bonds Example: sodium chloride, NaCl EOS Chapter 9: Chemical Bonds

Electronegativity Differences EOS Chapter 9: Chemical Bonds

Electron Distributions and Covalent Bonds EOS Asymmetric Distribution Symmetric distribution Chapter 9: Chemical Bonds

Writing Lewis Structures Hydrogen atoms are terminal atoms (bonded to only one other atom) EOS The central atom of a structure usually has the lowest electronegativity and the terminal atoms (except H) generally have higher electronegativities Chapter 9: Chemical Bonds

Writing Lewis Structures In oxoacids, hydrogen atoms are usually bonded to oxygen atoms EOS Molecules and polyatomic ions usually have compact, symmetrical structures Chapter 9: Chemical Bonds

Writing Lewis Structures Determine the total number of valence electrons Write a plausible skeletal structure and connect the atoms by single dashes (covalent bonds) Place pairs of electrons as lone pairs around the terminal atoms to give each terminal atom (except H) an octet Assign any remaining electrons as lone pairs around the central atom If necessary, move one or more lone pairs of electrons from a terminal atom to form a multiple bond to the central atom. EOS Chapter 9: Chemical Bonds

Resonance: Delocalized Bonding Resonance theory states that whenever a molecule or ion can be represented by two or more plausible Lewis structures that differ only in the distribution of electrons, the true structure is a composite, or hybrid, of them Resonance structures EOS Chapter 9: Chemical Bonds

Molecules that Don’t Follow the Octet Rule Molecules with an odd number of valence electrons have at least one of them unpaired and are called free radicals EOS Chapter 9: Chemical Bonds

Molecules that Don’t Follow the Octet Rule Some molecules have incomplete octets. These are usually compounds of Be, B, and Al, generally have some unusual bonding characteristics, and are often quite reactive EOS Chapter 9: Chemical Bonds

Molecules that Don’t Follow the Octet Rule Some compounds have expanded valence shells, which means that the central atom has more than eight electrons around it EOS Chapter 9: Chemical Bonds

Molecules that Don’t Follow the Octet Rule An expanded valence shell may also need to accommodate lone-pair electrons as well as bonding pairs EOS Chapter 9: Chemical Bonds

Chapter 9: Chemical Bonds Bond Lengths Bond order indicates whether a covalent bond is single (b.o. = 1), double (b.o. = 2), or triple (b.o. = 3) Bond length is the distance (in pm) between the nuclei of two atoms joined by a covalent bond EOS Bond length depends on the particular atoms in the bond and on the bond order BondFormation Chapter 9: Chemical Bonds

Representative Bond Lengths EOS Chapter 9: Chemical Bonds

Chapter 9: Chemical Bonds Bond Energies (BE) Bond-dissociation energy (D) is the quantity of energy required to break one mole of covalent bonds between atoms in a molecule in the gas phase EOS An average bond energy is the average of the bond-dissociation energies for a number of different molecules containing the particular bond Chapter 9: Chemical Bonds

Calculations Using BEs The sum of the enthalpy changes for breaking the old bonds and forming the new bonds is the enthalpy change for the reaction EOS Chapter 9: Chemical Bonds

Lewis Structure and Hydrocarbons For saturated hydrocarbons the Lewis structure = molecular structure Hydrocarbons with double or triple bonds between carbon atoms are called unsaturated hydrocarbons alkenes EOS alkynes Chapter 9: Chemical Bonds

Lewis Structure and Hydrocarbons alkenes EOS alkynes Chapter 9: Chemical Bonds

Chapter 9: Chemical Bonds Polymers Polymers are compounds in which many identical molecules have been joined together EOS Nylon Chapter 9: Chemical Bonds