Lewis Dot Structure

This article is part of the ChemHelp Tutoring Wiki

To draw the Lewis structure for a molecule or poly atomic ion:

1. Count the valence electrons in the molecule or polyatomic ion

2. If a polyatomic ion has a negative charge, add electrons to the total (1). If a polyatomic ion has a positive charge, subtract electrons from total in (1)

3. Draw the skeletal structure of the molecule and place two electrons (or a single bond) between each pair of bonded atoms. If the molecule contains three or more atoms, the least electronegative atom will usually occupy the central position.

4. Add electrons to the surrounding atoms until each has a complete outer shell.

5. add the remaining electrons to the central atom

6. Look at the central atom (a) If the central atom has fewer than eight electrons, remove an electron pair from an outer atom and add another bond between that outer atom and the central atom. (b) If the central atom has a complete octet, you are finished. (c) If the central atom has more than eight electrons, that's okay too.

Resonance Structure

All resonance strucutre forms are considered to exist simultaneously, and the strength and lengths of all three bonds are the same; somewhere between the strength and length of a single bond and a double bond

Incomplete Octets

Some atoms can have a complete outer shell with less than eight electrons; for example, hydrogen can have a maximum of two electrons, and beryllium can be stable with only four valence electrons as in BeH2.

Expanded Octets

In molecules that have d subshells available, the central atom can have more than eight valence electrons.

Molecular geometry

Electrons repel one another, so when atoms come together to form a molecule, the moelcule will assume the shape that keeps its different. When we predict the geometries of moelcules using this idea, we are using the valence shell electron-pair repulsion (VSEPR) Model.

In a molecule with more than two atoms, the shape of the moelcule is determined by the number of electron pairs on the central atom. The central atom forms hybrid orbitals, each of which has a and lone pairs of electrons on the central atom. Here are some things you should remember when dealing with the VSEPR model.

Here are some things you should remember when dealing with the VASEPR model

- DOuble and triple bonds are treated int he same way as single bonds in terms of predicting voerall geometry for a moelcule; however, multiple bonds have slightly more repulsive strength and wiull therefore occupy a little mores space than single bonds

-lone electron pairs have a little more repulsive strength than bonding pairs, so lone pairs will occupy a little more space than bonding pairs.

Note:

• IF the central atom has 2 electron pairs, then it has sp hybridization and its basic shape is linear.

• IF the central atom has 3 electron piars then it has sp2 hybridization and its basic shape is trignoal planer; its bond and gle are about 120.

• IF the cnetral atom has 5 electron piars then it has sp3d hybridization and its basic shape is trignoal bipyramidal.

• IF the central atom has 6 electron pairs, then it sh as sp3d2 hybridization and its basic shpae is octahedral

• IF the central atom has 4 electron pairs, then it has sp3 hybridization and its basic shape is tetrahedral ; its bond angles are about 109.5