Purpose of Experiment 12

become familiar with:

1. Lewis structures

2. principles of the VSEPR model

3. the three-dimensional structures of covalent molecules

chemical bond

when atoms or ions are strongly attached to one another

Three types of chemical bonds

1. ionic

2. covalent

3. metallic

Ionic bond

- electrostatic forces btwn ions of opposite charge

- formed by transfer of one or more electrons

- from atom of low ionization energy to atom of high ionization energy

usu metals with nonmetals (except noble gases)

Covalent bond

sharing of electrons between two atoms

More familiar exs= bonds among nonmetallic elements

Metallic bonds

found in metals like gold, iron, magnesium

each atom is bonded to several neighboring atoms

give rise to high electrical and thermal conductivity

b/c in m bonds electrons are relatively free to move throughout the 3D shapes of mc's

Which electrons are involved in bonding

valence electrons (reside in incomplete outer shell of an atom)

creator of Lewis Dot structure

G.N. Lewis, American chemist

Lewis symbol consists of:

chemical abbreviation for the element

and a dot for each valence electron

each side accommodates up to two electrons

What can atoms do to achieve noble gas config?

gain, lose, or share electrons

octet rule

atoms tend to gain, lose, or share electrons until they are surrounded by eight valence electrons

octet of electrons= full s and p subshells

Properties of ionic substances

(most substances do not have these characteristics)

usually brittle

high melting point

crystalline solids w/ well-formed faces

- characteristics result from electrostatic forces that maintain ions in a rigid, well-defined, 3D arrangement

Example of how H₂ forms a bond

- the nuclei and electrons cause electron density to concentrate btwn the nuclei

- the shared pair of electrons acts as a kind of "glue" holding the atoms together

single bond

double bond

sharing of one pair of electrons

sharing of two pairs of electrons

- triple bond is the sharing of three pairs of electrons

Steps for drawing Lewis dot structures

1. Sum the number of valence e^-

a. add electrons for negative charges

b. subtract electrons for positive charges

2. Decide the central atom (least electronegative excluding H)

3. Assign leftover electrons to the terminal atoms until all octet

4. Assign any leftover e^- to the central atom (if central atom doesn't have octet, create multiple bonds)

Why formal charge?

sometimes can draw several diff Lewis structures that all obey the octet rule

formal charge helps decide the structure that is the most reasonable

formal charge (of an atom)

the charge that an atom in a molecule or ion would have if all atoms had the same elcectronegativity

equation for formal charge

(regular valence e^-) - (valence e^- found in the molecule)

* remember to count only one of the two bonded molecules for each bonded atom

Most bonds btwn two diff kinds of atoms are usually...

polar

Why are most bonds of two diff atoms usu polar?

b/c diff atoms have diff electronegativities (so electrons are not shared equally by the two bonded atoms)