Writing/drawing Chemical Structures Reference Sheet (Page 3 of 4) in pdf

Writing/drawing Chemical Structures Reference Sheet Page 3

Curved Arrow Convention

Curved arrows show the movement (flow) of electron during bond breaking and/or bond

making processes. The foot of the arrow indicates where the electron or electron pair

originates, the head of the arrow shows where the electron or electron pair ends up. .

A. The movement of a single electron is denoted by a curved single headed arrow

(fishhook or hook).

The movement of an electron pair is denoted by a curved double headed arrow.

If an electron pair moves in on a new atom, another electron pair must leave so that the

atom does not exceed a full valance of eight electrons. There are two common exceptions:

A. When an atom already has an incomplete valance (R

C+).

With second row (or below) elements the octet rule may be violated.

The arrows completely dictate the Lewis structure of the product.

Drawing and Interpreting Resonance Forms

No one resonance forms accurately depicts the structure of the molecule.

The real structure is a composite or hybrid of all resonance forms

Resonance forms differ only by the placement of p- or non-bonding electrons.

Neither the position or hybridization of the atoms changes.

Resonance forms are not necessarily equivalent. While all resonance forms contribute

to the actual structure (resonance hybrid), some forms may contribute more.

All resonance forms must be proper Lewis structures.

The actual resonance hybrid is more stable than any single resonance form.

The greater the number of resonance forms, the more stable the resonance hybrid.

Writing/drawing Chemical Structures Reference Sheet Page 3