Lewis Dot Diagrams
Fall 2010
Electron Dot Formula Worksheet
Background: Covalent bonding is found between nonmetals. A covalent bond is a shared pair of electrons. Atoms share electrons to obtain a stable octet (except for hydrogen and helium which form a stable duet). The stable octet is also called the noble gas configuration. Molecules can be represented by means of diagrams in which VALENCE electrons are shown as dots around the chemical symbols of the atoms and bonds are represented as lines.
Historical Note: The dot formulas are sometimes referred to as Lewis Dot Structures after the chemist who first introduced them in 1916----G.N. Lewis
Rules for Molecules:
1. Determine the total number of valence electrons in the formula
a. Sum of valance electrons for each atom in the molecule.
2. Central atom will be element with smallest# of valence electrons (excluding H and halogens:
a. Aim for symmetry.
b. Be sure to include all elements in the formula!!
c. Do not string oxygens, or make boxes.
d. Always string carbons together
e. Nitrogen atoms can be bonded together
3. Hydrogen and halogens always bond on the outside.
4. All atoms want to have 8 valence electrons. Can be any combinations of unshared pairs or bonding pairs. Exceptions:
a. Hydrogen – 2 valence electrons (1bond)
b. Beryllium (Be) – 4 valence electrons (2 bonds)
c. Boron (B) – 6 valence electrons(3 bonds)
d. Carbon (C) – almost always has 4 bonds, no unshared pairs
5. Carbon atoms, oxygen atoms and nitrogen atoms can make multiple bonds as needed to satisfy bonding requirements.
a. Same applies for elements in their respective families
6. Bonding pairs and unshared pairs will try to get as far apart as possible (this determines molecular shape).
a. When in doubt, write the Electron Dot Structure for the central element
b. Then attach any remaining elements
c. Count valence electrons around each atom (see step 4)
d. Add double or triple bonds as needed to satisfy valence electron requirements
e. Electron arrangement and molecular shape may be different due to unshared pars of electrons
7. Count total # of valence electrons and compare to original number for the formula (step 1).
a. IF TOTAL # VALENCE ELECTRONS IS CORRECT AND ALL BONDING REQUIREMENTS HAVE BEEN MET, THEN THE ELECTRON DOT STRUCTURE IS CORRECT.
Examples:
1. Nitrogen is a group 5 element, so it has 5 valence electrons.
Electron Dot Formula
2. Cl2. Chlorine is a group 7element, so each atom has 7 valence electrons for a total of 14 valence electrons in the molecule.
3. H2O H is a group 1 element, so each atom has 1 valence electron for a total of 2.
O is a group 6 element, so it has 6 valence electrons. Total number of valence
electrons for the molecule is 8.
Electron Dot Formula
1. Name the compound, draw the electron dot structures, predict the arrangement of electrons, and the molecular shape:
a) Br2 b) HCl c) CH4
d) NH3 e) H2S f) SiF4
g) PF3 h) CO2 i) SO3
j) NH4+ k) SO4-2 l) HCN