Drawing Electron Dot Structures

Procedure

1. Calculate total # of valence electrons
(Group #)(# of elements of that type) for all elements
add 1 for each "-" charge on an ion
subtract 1 for each "+" charge on an ion
Example:
SO4= (6)(1) + (6)(4) +2 = 6+24+2 = 32
NH4+ is 5(1) + (1)(4) -1 = 5+4-1 = 8

2. Place elements in a skeletal structure using either line notation or electron dots between elements.

3. Place octet of electrons about all outer elements except H and 2 about an H

4. Compute remaining electron = total electrons - assigned electrons and assign remaining electrons, in pairs, to the central element).

5. If central atom has fewer than 8 electrons, then use multiple bonds (use electron pairs on outer elements to form multiple bond(s) between that element and the central element.
2 fewer = one double bond
4 fewer = 1 triple bond or 2 double bonds


EXCEPTIONS TO THE OCTET RULE
1. Free Radicals (odd number of electrons)
If an odd # electrons exist then one cannot have an octet or duet for each element (requires an even # of e). Then (usually) the central element has one less electron (an odd number)

Examples:
NO (11e) NO2 (17e) ClO2 (19e)

2. Molecules with incomplete octets

Examples:
BH3 (6e) BF3 (24e) BeCl2 (16e)

3. Molecules with expanded octets
Third period elements (and beyond) may have expanded octets since their energy level may hold 18 electrons

Examples:
PCl5 (40e) SF6 (48e)