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Mrs Emery's Science Pages
Lewis Dots and VSEPR

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Lewis Dot Structures

atoms

 Draw only outer electrons that fill in the A groups. Maximize the number of unpaired electrons. Examples

simple ions

 Atoms gain or lose electrons to reach a noble gas configuration. Thus, there are no dots in these structures. Examples (note that the questions and answers are the same):

The dot structure of Na+1 is Na+1 . The dot structure of O-2 is O-2.

Note that Na is in group 1 and should lose 1 electron while O is in group 6 and should gain 2 electrons.

ionic compounds
  1. Determine the charges on the ions from the group in the periodic table.

    group

    1

    2

    3

    4

    5

    6

    7

    8

    charge

    +1

    +2

    +3

    -4

    -3

    -2

    -1

    0

  2. Make sure that the sum of the charges of all of the ions add up to zero.
    In Al2O3, Al, in group 3, will be +3 while O, in group 6, will be -2. SUM = 2(+3) + 3(-2) = 0.
  3. Then write the structure according to the format below.

formula

CaCl2

MgO

Mg3N2

Lewis dot structure

Ca+2 + 2 Cl-1

Mg+2 + O-2.

3 Mg+2 + 2 N-3.

practice

 atoms: N, C, Al, Mg, F, As
ions: Cs+1 , F-1 , Mg+2 , O-2 , P-3 , Si-4 , B+3
ionic compounds: MgS, Ca3P2, NaCl, Na4Si, Fr2Se, Al2O3, Na3N

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Relationship of Molecular Shape to Electron Pair Geometry

The following table summarizes the interrelationships between the number of groups of electrons around a central atom, the molecular shape around the central atom, and the electron pair geometry around the central atom. Note that the number of unshared electron pairs that may be associated with the central atom are indicated.

A Summary of Molecular Shape and Electron Pair Geometry

No. of groups of electrons around a central atom Electron pair geometry Angles between groups of electrons No. of unshared electron pairs Molecular shape Diagram Example
2 linear 180o 0 linear Linear Geometry CO2
3 trigonal planar 120o 0 trigonal planar Trigonal Planar Geometry BCl3
3 trigonal planar 120o 1 bent Bent Geometry SO2
4 tetrahedral 109.5o 0 tetrahedral Tetrahedral Geometry CI4
4 tetrahedral 109.5o 1 pyramidal Pyramidal Geometry PCl3
4 tetrahedral 109.5o 2 bent Bent Geometry H2S
4 tetrahedral 109.5o 3 linear Linear Geometry HBr

E Chem- Molecular Models

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