Ch 12 Chemical Bonding A Types of Chemical

Ch 12 Chemical Bonding A. Types of Chemical Bonds • Bond – force that holds groups of atoms together and makes them function as a unit • Bond energy – energy required to break a chemical bond

Ionic Bonding (a) (b) • Ionic compound results when a metal reacts with a nonmetal

Covalent Bonding • A covalent bond results when electrons are shared by nuclei

Covalent Bonding (continued) • A polar covalent bond results when electrons are shared unequally by nuclei – One atom attracts the electrons more than the other atom

B. Electronegativity • Electronegativity – the relative ability of an atom in a molecule to attract shared electrons to itself – Increases from left to right across a period – Decreases down a group of representative elements

• The polarity of a bond depends on the difference between the electronegativity values of the atoms forming the bond

C. Bond Polarity and Dipole Moments • A dipole moment results when a polar molecule has a center for positive charge separate from a center for negative charge

• Water molecule dipole moment

• The polarity of water affects its properties – Permits ionic compounds to dissolve in it – Causes water to remain liquid at higher temperature

D. Stable Electron Configurations and Charges on Ions

• Atoms in stable compounds usually have a noble gas electron configuration – Metals lose electrons to reach noble gas configuration – Nonmetals gain electrons to reach noble gas configuration

Predicting Formulas of Ionic compounds • Chemical compounds are always electrically neutral

E. Ionic bonding and Structures of Ionic Compounds • Ions are packed together to maximize the attractions between ions

Structures of Ionic Compounds • Cations are always smaller than the parent atom • Anions are always larger than the parent atom

Ionic Compounds Containing Polyatomic Ions • Polyatomic ions work in the same way as simple ions – The covalent bonds hold the polyatomic ion together so it behaves as a unit

F. Writing Lewis Structures • In writing Lewis structures we include only the valence electrons • Most important requirement – Atoms achieve noble gas electron configuration (octet rule, duet rule)

• Bonding pairs are shared between 2 atoms • Unshared pairs (lone pairs) are not shared and not involved in bonding

Rules for Covalent Bonds 1. Determine the total number of valence electrons 2. Write the skeleton structure and join the atoms in this structure by single covalent bonds 3. For each single bond formed, subtract 2 from the total number of valence electrons 4. With the remaining valence electrons first complete the octet (or duet) of the outside atoms, then complete the octet of inner atoms 5. If the inner atom lacks an octet, form multiple bonds by sharing more electrons from outer atoms to inner atoms Double bond 2 pair e. Triple bond 3 pair e-

G. Lewis Structures of Molecules with Multiple Bonds • Single bond – covalent bond in which 1 pair of electrons is shared by 2 atoms • Double bond – covalent bond in which 2 pairs of electrons are shared by 2 atoms • Triple bond – covalent bond in which 3 pairs of electrons are shared by 2 atoms

– A molecule shows resonance when more than one Lewis structure can be drawn for the molecule

H. Molecular Structure • Three dimensional arrangement of the atoms in a molecule – Water - bent

• Linear structure – atoms in a line – Carbon dioxide

• Trigonal planar – atoms in a triangle – BF 3

• Tetrahedral structure – methane

I. The VSEPR Model • Valence shell electron pair repulsion (VSEPR) model – Molecular structure is determined by minimizing repulsions between electron pairs

Two Pairs of Electrons • Be. Cl 2 – 180 o - linear

Three Pairs of Electrons • BF 3 – 120 o – trigonal planar

Four Pairs of Electrons • CH 4 – 120 o – tetrahedral

J. Molecules with Double Bonds When using VSEPR model to predict molecular geometry of a molecule • a double bond is counted the same as a single electron pair