Chapter 8: Molecular Compounds Chemistry

Covalent Bonding Covalent Bond: atoms held together by sharing e Neither atom has the electronegativity to over power the other atom and take its electrons “tug of war” between electrons Co: with Valent: refers to outermost electrons Covalent: sharing of the outermost electrons between two atoms

Nomenclature - Covalent Covalent bonds usually occur between two or more NONMETALS. Named using Greek prefixes

Nomenclature - Covalent In naming binary covalent compounds: The first element retains its name and a prefix is used ONLY if there is more than one in the compound The second element drops its ending and adds “-ide”; The second element ALWAYS gets a prefix Ex: CO2 = Carbon dioxide

Practice Draw electron dot structures for each diatomic molecule: Chlorine molecule Bromine molecule Iodine molecule Oxygen Fluorine molecule Hydrogen Nitrogen H2O2 PCl3 NH3

Molecules and Molecular Compounds Monatomic: elements existing in singular form. Ex: Noble Gases (He, Ne, Ar, Kr…)

Molecules and Molecular Compounds Molecule: neutral group of atoms joined by covalent bonds (neutral) Diatomic Molecules: Molecule consisting of two atoms Molecular Compounds: Compound composed of molecules Low MP and BP Gas and Liquid at room temp Nonmetal + Nonmetal etc… Ex: C + O  CO Pg 214 good contrast between Ionic Compound and Molecular NaCl and H2O

Molecules and Molecular Compounds Molecular Formulas: chemical formula for a molecular compound Shows # atoms of each element Ex: H2O (2-H’s and 1-O) Ex: C6H12O6 (6-C’s and 12-H’s and 6-O’s)

Molecules and Molecular Compounds Diagrams and Models Structural Formula represents the covalent bonds by dashes

Molecules and Molecular Compounds Diagrams and Models Ball and stick Model

Molecules and Molecular Compounds Diagrams and Models Perspective Drawing

Molecules and Molecular Compounds Diagrams and Models Space Filling Molecular Model

8.1 - Key Concepts How are the MP’s and BP’s of molecular compounds different from those of ionic compounds? What information does a molecular formula provide?

The Nature of Covalent Bonding The Octet Rule: electrons tend to be transferred or shared so that each ion or atom acquires a noble gas e- configuration Covalent bonds – atoms SHARE to attain a noble gas configuration

Covalent Bonding Single Covalent Bonds: 2 atoms held together by sharing a pair of electrons Ex: H2 H• + H•  H••H 1s1 + 1s1  1s2 (both look like He) Halogens form SINGLE covalent bonds and DIATOMIC molecules

Covalent Bonding Unshared pair: pair of valence e- not shared between atoms AKA: lone pair or nonbonding pair Ex: H2O contains 2 lone pairs Ex: Ammonia NH3 contains 1 lone pair Ex: Methane CH4 contains 0 lone pairs PRACTICE: Draw e- Dot Structures for a)Chlorine b)Bromine c)Iodine PRACTICE: Draw e- dot structure for compounds a)H2O2 b)PCl3

Covalent Bonding Double Covalent Bonds: 2 atoms held together by 2 shared pairs of electrons Ex: Oxygen O2

Covalent Bonding Triple Covalent Bonds: 2 atoms held together by 3 shared pairs of electrons Ex: Nitrogen N2

Covalent Bonding Coordinate Covalent Bonds: a covalent bond in which one atom contributes both bonding electrons The shard e- pair comes from one bonding atom Ex: CO Polyatomic Ion Ex: NH4+ Ex: SO3 + 2e-  SO32-

Covalent Bonding Bond Dissociation Energies: the energy required to break the bond between two covalently bonded atoms A large bond dissociation energy corresponds to a strong covalent bond BIG ENERGY = STRONG BOND

Covalent Bonding Resonance: structure that occurs when it is possible to draw two or more valid e- dot structures that have the same # of e- pairs Ex: Ozone O3 (protective layer that absorbs UV radiation from sun; @ lower elevations it is a pollutant contributing to smog)

Covalent Bonding Octet Rule Exceptions: cannot be satisfied in molecules whose: Total # of Valence e- is an odd # An atom has fewer or more than a complete octet of valence e- NO2 PCl5 SF6

8.2 - Key Concepts What is the result of electron sharing in covalent bonds? How do electron dot structures represent shared electrons? How do atoms from double or triple covalent bonds? How are coordinate covalent bonds different from other covalent bonds? How is the strength of a covalent bond related to its bond dissociation energy? How are oxygen atoms bonded in ozone? What are some exceptions to the octet rule?

Bonding Theories Molecular Orbitals: orbitals that apply to the entire molecule. Molecular orbital belongs to the whole molecule Bonding Orbital: molecular orbital that can be occupied by 2 e- of a covalent bond Where the bonding occurs** Sigma Bond: 2 atomic orbitals combine to form a molecular orbital that is symmetrical around the axis connecting 2 atomic nuclei (σ) ‘s’ orbitals or ‘p’ orbitals

Bonding Theories Sigma Bond

Bonding Theories Pi Bond: bonding electrons are most likely found above/below axis Weaker than sigma Pi overlap less than Sigma

VSEPR Theory VSEPR Theory: explains 3D shapes Repulsion between e- pairs causes molecular shapes to adjust so that the valence e- pairs stay as far apart as possible. Unshared Pairs – important predictors of the molecular shapes

VSEPR Theory Linear Triatomic AX2 Trigonal Planar AX3 Tetrahedral AX4

VSEPR Theory Pyramidal AX3 Trigonal bipyramidal AX5 Octahedral AX6

VSEPR Theory T-shaped AX3 Bent triatomic AX2 Square planar AX4

Polar Bonds and Molecules Nonpolar Covalent Bond: Bond is pulled equally Bonding e- are equally shared H2, O2… Diatomic

Polar Bonds and Molecules Polar Covalent Bond: Bond is unequal Bonding e- are unequally shared The more electronegative atom attracts electrons more strongly and gains a slightly negative charge

Polar Bonds and Molecules Polar molecules: one end is slightly negative and other is slightly positive Dipole: molecule that has two poles (dipolar) When polar molecules are placed between opp. Charged plates, they orient with respect to positive and negative plates pg. 239

Polar Bonds and Molecules Intermolecular Attractions: Weaker than ionic or covalent bonds 1)Van der Waals Forces: Dipole interactions Dispersion forces WEAKEST 2) Hydrogen Bonds Attractive forces in which a H covalently bonded to a very electronegative atom is weakly bonded also to an unshared e- pair.