Lewis+diagrams

Lewis diagrams can be drawn for atoms, ions, molecules, and polyatomic ions.

They represent the number of //valence electrons// that atom has. Most atoms follow the //octet rule//, which means that the most valence electrons an atom will have is eight.

The number of valence electrons in a neutral atom is equal to its group number.

Go here to see what Lewis diagrams will look like for the first 20 elements.


 * Here are the Lewis diagrams for the main group elements' atoms:**



Lewis diagrams can be used to **predict common oxidation states** (ionic charges) for most main group elements.

Recall that metals will lose their valence electrons to form positively charged cations. This means that the number of 'dots' around a metal's Lewis diagram will correspond to the positive charge its ion will have.

For example, a calcium atom has a Lewis diagram that has two dots: ●Ca●, which means it will lose both electrons, and form the Ca 2+ ion.

Nonmetals will gain electrons until they fulfill the octet rule. The number of electrons a nonmetal gains will correspond to the charge of that atom's anion.

Oxygen has six dots (see the chart above), and will need two more to get to eight dots. Therefore, the oxygen ion (oxide ion) will be O 2-.

Here are some more examples:



Lewis Diagrams for Compounds

 * To draw a Lewis diagram for a compound, follow these steps. **

1. Add up the valence electrons (from group number) for the elements in the compound. If it is an anion, add the charge to the total; subtract if it is a cation.

2. Determine which atom or atoms will be the center of the molecule. Starting from the noble gases, the lower the valence number, the more likely the atom will be the center atom, for it can form more bonds.

3. Using single bonds, attach all other atoms in the compound to the center atom(s). Use the chemical formula to figure out how the atoms are arranged. Count up all of the bonds that you drew, multiply by two, and subtract this value from the total from step 1. Each bond is a covalent bond, which means it will contain two electrons.

4. With the exception of H, the octets of the outer atoms should be filled using lone pairs (two dots). Add up the number of dots you drew and subtract from the total. Hydrogen can only support two electrons (one bond) due to its small size. If you use up all of the electrons (your total is zero), your Lewis diagram is complete.

5. Any remaining electrons must be drawn about the center atom(s) as lone pairs, even if the octet rule is not satisfied. Subtract these from the total as well. If any center atoms are shy an octet, then share a lone pair (or two) from an adjacent atom to form a double (or triple) bond. Once you use up all of the electrons (your total is zero), your Lewis diagram is complete.