Aim: What are polyatomic ions?

General Rules1. Select the central atom (atom in the middle); the least

electronegative atom (H and halogens will not be central atoms)

2. Count the total number of valence electrons there should be.3. Put one bond between each atom. Put the rest of the valence

electrons on the outer atoms. 4. Check if every atom fulfills octet. If not move paired electrons

to fulfill octet.5. Recount the number of total shared valence electrons.

Exceptions to Octet Rule

• The octet rule cannot be satisfied whose total number of valence electrons is an odd number. There are also molecules in which an atom has fewer, or more, than a complete octet in the valence electrons.

Example 1

• Boron has a tendency to form compounds in which it has an incomplete octet. For example BF3

Example 2

• Atoms of elements in the third or higher periods can exceed their octet as they have d orbitals available for bonding. For example SF6

Polyatomic Ions

• A polyatomic ion is a tightly bound group of atoms that has a positive or negative charge and behave as a unit.• A group of atoms that are covalently bound and as a whole

have a charge.

Which compound contains both ionic and covalent bonds?1. HBr2. CBr4

3. NaBr4. NaOH

Practice

1. Draw the electron dot structure for the polyatomic boron tetrafluoride anion (BF4

-)

Practice

2. Draw the electron dot structure for the hydrogen carbonate ion (HCO3-). Carbon is the central atom, and hydrogen is attached to one oxygen in this polyatomic anion.

• http://www.kentchemistry.com/links/bonding/covalentlewisdot.htm