Keller Test 3

Drawing Lewis Structures 1
Write the skeletal structure (the arrangement of atoms within the molecule):

Central atom = the atom with the lowest electronegativity (usually).

Hydrogen is always a terminal atom (on the end).

Drawing Lewis Structures 2
Count the total number of valence electrons:

Group number for each element = # valence electrons.

Add electrons for negatively charged ions.

Subtract electrons for positively charged ions.

Drawing Lewis Structures 3
Draw a bond between the central atom and each surrounding atom.

Single bond = 1 pair of electrons

Drawing Lewis Structures 4
Place lone pairs of electrons about each terminal atom to complete their octets.

Octet = 4 electrons pairs around an atom (eight electrons)

Hydrogen can only have 2 electrons.

Drawing Lewis Structures 5
If there are more electrons left, place them as lone pairs on the central atom.

This will sometimes lead to an “expanded octet” around the central atom.

Expanded octet = five or six electron pairs around an atom.

Only central atoms from the third period and above can have expanded octets.

Drawing Lewis Structures 6
If the central atom has an incomplete octet, use the electrons from surrounding atoms to make double or triple bonds.

Do not add electrons. “Borrow” them from surrounding atoms.

Double bond = 2 pairs of electrons

Triple bond = 3 pairs of electrons

Only C, N, O, P, and S form multiple bonds.

F and Cl do not form multiple bonds.

Drawing Lewis Structures 7
Sometimes you just can’t complete the octet for a central atom.

If there is an odd number of electrons, give the central atom 7 electrons instead of 8.

Boron and Beryllium just don’t have enough electrons to go around and often have incomplete octets.

Aromatic hydrocarbons
characterized by having ring systems in which the bonding can be described as alternating single and double bonds.
aliphatic hydrocarbons
subdivided into alkanes, alkenes and alkynes.
Hydrocarbons
compounds that contain only the elements H and C. They can be divided into two classes: aliphatic and aromatic.
alkanes
each C atom is bonded to four other atoms and all bonds are single bonds.
alkene
contains a carbon-carbon double bond.
Alkyne
carbon-carbon triple bond.
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