Notes Lewis Dot Structures
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Lewis dot structures (a.k.a. Lewis electron dot formulas) use dots arranged around the chemical
symbol for an element to represent the valence electron configuration of the atoms in the
element.
For example, a hydrogen atom has one valence electron. The Lewis dot structure for hydrogen
would be written as
H·
A hydrogen molecule is formed when two hydrogen atoms mutually share valence electrons. The
Lewis dot structure would be written as
H:H
The pair of dots between the two hydrogen symbols represents the pair of shared electrons.
The electron dot formula is simply a two-dimensional representation of the positions of the
valence electrons in an atom, ion, or molecule.
Lewis Dot Structures for Neutral Atoms
When writing Lewis structure for single atoms, the s sublevel valence electrons usually are
placed at the top of the element symbol. The remaining three sides are used for the three
orbitals in the p sublevel. The p sublevel electrons are placed following the rule of “one electron
per orbital until each orbital contains one electron; then pair up the remaining p electrons.”
For example, beryllium has s2 p1 valence electron configuration. The Lewis structure would be
written as
..
Be ·
The Lewis dot structure clearly shows the pair of s electrons and the one unpaired p electron.
The Lewis dot structure for nitrogen (s2p3) would be written as
..
·N·
·
The Lewis dot structure clearly shows the one pair of s electrons and the 3 unpaired p electrons.
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The Lewis dot structure for oxygen (s2p4) would be written as
..
:O·
·
The Lewis dot structure clearly shows the one pair of s electrons, the one pair of p electrons and
the two unpaired p electrons.
Lewis Dot Structures for Ions of Single Atoms
For positive ions, the Lewis dot structures are extremely easy: these ions are formed when a
metallic atom LOSES it valence electrons! Therefore, the Lewis dot structure for a positive ion
has no dots!
The Lewis dot structure for a positive ion is simply the chemical symbol for the element and the
charge on the ion written at the upper right of the symbol:
H+1
Ca+2
Al+3
Lewis dot structures for negative ions require that all of the valence electrons be represented.
Brackets are then drawn around the chemical symbol and dots, and the ion charge is written at
the upper right OUTSIDE the brackets:
..
[ : F : ]-1
..
[ : P : ]-3
..
..
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Lewis Dot Structures for Ion Pairs (Ionic Compounds)
Write the chemical formula for the ionic compound. Be sure to include any subscripts after
element symbols if they are given. Then write the appropriate Lewis dot structures for the
respective ions. If there is more than one of a particular ion in the chemical formula, you must
write more than one Lewis dot structure for that ion.
For example, the Lewis dot structure for sodium chloride, NaCl, would be written as
..
Na+1 [: Cl : ]-1
..
The Lewis dot structure for calcium chloride, CaCl2, would be written as
..
Ca+2 [: Cl : ]-1
..
..
[: Cl : ]-1
..
Lewis Dot Structures for Molecules
Lewis dot structures for molecules show the sharing of valence electrons between atoms.
The Lewis dot structure for a molecule of hydrogen composed of two atoms of hydrogen sharing
each other’s single valence electron to form a bond pair of electrons is H : H.
In order to draw Lewis dot structures for molecules, you must first be able to draw a skeleton
structure using lines to represent chemical bonds.
The molecule of hydrogen may be represented as H — H in which the single line drawn
between the two symbols represents a pair of shared electrons. This is an example of a single
covalent bond: one pair of electrons is shared by two atoms.
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The skeleton structure for water is
The Lewis dot structure would be
H―O
|
H
..
H‫׃‬O:
..
H
The Lewis dot structure shows the pair of shared electrons between each hydrogen atom and the
oxygen atom. The unshared pairs of valence electrons in the oxygen atom are also shown.
Those unshared electron pairs are potential bonding sites for any atom that has 2 unfilled valence
orbitals.
Rules for Writing the Skeleton Structure of a Molecule
1. Many small molecules or polyatomic ions consist of a central atom around which are
bonded atoms having greater electronegativity, such as F, Cl, and O. In some cases, H
atoms surround a less electronegative atom because H atoms can form only one bond.
For example, CCl4 and CH4:
Cl

Cl  C  Cl

Cl
H

H CH

H
2. Molecules or polyatomic ions that have symmetrical formulas usually have symmetrical
structures. For example, disulfur dichloride:
Cl  S  S  Cl
3. Oxoacids are substances in which O atoms (and possibly other electronegative atoms) are
bonded to a central atom with one or more H atoms usuallybonded to O atoms. For
example, perchloric acid, HClO3:
O

O  Cl  O  H

O
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4. Of several possible structures, the one in which the atoms have their usual number of
covalent bonds is usually preferred. For example, oxygen difluoride, OF2:
FO

F
Steps in Writing Lewis Dot Structures
1. Calculate the total number of valence electrons for the molecule by adding up the
number of valence electrons for each atom. If we are writing the Lewis formula for a
negatively charged polyatomic ion, we ADD the number of negative charges to this total
(because there are more electrons than are provided by a neutral molecule). If we are
writing the Lewis formula for a positively chaarged polyatomic ion, we SUBRACT the
number of positive charges from the total (because there are fewer electrons than are
provided by a neutral molecule). (Please remember that plain numbers do not have
a + or - sign! “Numer of...” means just that: the NUMBER!!))
2. Write the skeleton structure of the molecule or ion, connecting every bonded pair of
atoms by a pair of dots (or a dash).
3. Distribute electrons to the atoms surrounding the central atom (or atoms) to satisfy
the octet rule (having 8 valence electrons) for these atoms (Remember that hydrogen can
only make one single covalent bond.)
4. Distribute the remaining electrons as pairs to the central atom (or atoms), after
subtracting the number of electrons already distributed from the total of valence electrons
calculated in Step 1.
If there are fewer than 8 electrons on the central atom, this suggests a multiple bond
is present (double or triple).
Two electrons fewer than 8 suggests a double bond; four fewer suggests a triple bond or
two double bonds.
To write a multiple bond, move one or two electron pairs (depending on whether the
bond is a double or triple bond) from a surrounding atom to the bond connecting the
atom. Atoms that often form multiple bonds are C, N, O, P, and S.
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