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Chapter 8: Molecular Compounds
Chemistry
Molecules and Molecular Compounds
• Molecule: neutral group of atoms joined by covalent bonds (neutral) – Diatomic Molecules: Molecule consis:ng of two atoms (H2, N2, O2, and Halogens2)
– Molecular Compounds: Compound composed of molecules
• Low MP and BP • Gas and Liquid at room temp • Nonmetal + Nonmetal etc…
– Ex: C + O à CO – Pg 214 good contrast between Ionic Compound and Molecular – NaCl and H2O
Molecules and Molecular Compounds
• Monatomic: elements existing in singular form. – Ex: Noble Gases (He, Ne, Ar, Kr…)
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)
Covalent Bonding
• Covalent Bond: atoms held together by sharing e • Neither atom has the electronega:vity 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
Prac:ce • 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
• 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 informa:on 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-‐ configura:on – Covalent bonds – atoms SHARE to ajain a noble gas configura:on
Covalent Bonding
• Single Covalent Bonds: 2 atoms held together by sharing a pair of electrons – Ex: H2 H• + H• à H••H 1s1 + 1s1 à 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-‐ à SO32-‐
Covalent Bonding
• Bond DissociaBon Energies: the energy required to break the bond between two covalently bonded atoms – A large bond dissocia:on 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 (protec:ve layer that absorbs UV radia:on from sun; @ lower eleva:ons it is a pollutant contribu:ng to smog)
Covalent Bonding
• Octet Rule ExcepBons: cannot be sa:sfied 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 dissocia:on energy?
• How are oxygen atoms bonded in ozone? • What are some excep:ons 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
• Structures – A = central atom – X = surrounding atoms
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
Polarity and Molecules
• Nonpolar Covalent Bond: – Bond is pulled equally – Bonding e-‐ are equally shared – H2, O2… Diatomic
Polarity and Molecules
• Polar Covalent Bond: – Bond is unequal – Bonding e-‐ are unequally shared – The more electronega:ve atom ajracts electrons more strongly and gains a slightly nega:ve charge
•
Polarity and Molecules
• Polar molecules: one end is slightly nega:ve and other is slightly posi:ve
• Dipole: molecule that has two poles (dipolar) – When polar molecules are placed between opp. Charged plates, they orient with respect to posi:ve and nega:ve plates pg. 239
Polarity and Molecules
• Intermolecular Ajrac:ons: • Weaker than ionic or covalent bonds
– 1)Van der Waals Forces: • Dipole interac:ons • Dispersion forces • WEAKEST
– 2) Hydrogen Bonds • Ajrac:ve forces in which a H covalently bonded to a very electronega:ve atom is weakly bonded also to an unshared e-‐ pair.