Name _________________________________________ Date ______________________ Hour _______
Lewis Structures for Molecules
Ionic Bonds:
Atoms with a sizeable electronegativity difference (> 1.70) form ionic bonds. Electrons from the least
electronegative element are attracted to and transfer electrons to the element with the higher electronegative value.
This transfer of electrons creates two charged stable ions, a cation (positive ion) and an anion (negative element).
The oppositely charged ions have an electrostatic attraction to each other called an ionic bond. Below is an
example of an ionic bond between magnesium and chlorine to form magnesium chloride. The octet rule states that
atoms tend to lose, gain or share electrons in order to acquire a full set of eight valence electrons. This diagram
illustrates the ionic bond formed between magnesium and chlorine to form a magnesium chloride formula unit.
The electronegativity difference between magnesium and chlorine is shown below.
3.16
-1.31
1.85
chlorine’s electronegative value
magnesium electronegative value
electronegativity difference
Use the electronegativity chart (page 169) and the example above, to determine which compounds below are ionic
or covalent. Using your molecular and ionic notes sheet, write the name of each compound below.
CH4
NaF
SrF2
BH3
Covalent Bonds:
Generally, ionic bonds form when the electronegativity difference is greater than 1.70. When the electronegativity
difference between elements is less than 1.70, the elements tend to share electrons forming a covalent bond. The
sharing of electrons completes the valence shell (2 or 8) for both of the elements. The resulting bond results in the
formation of a molecule.
Lewis Dot Diagrams use electron–dot diagrams in order to show how the valence electrons are arranged in
molecules. For example, hydrogen atoms have only one energy shell. This small inner shell can hold a maximum of
two electrons. The Lewis structure illustrating two hydrogen atoms bonding is shown below. Each hydrogen atom
possesses one electron. By bonding together, they share the two electrons thus filling both of their energy shells.
Atoms are more stable when their valence shell is “full”. Below, draw the two electrons hydrogen atoms share.
Two shared electrons forms a single bond
H
H
There are seven nonmetal atoms that occur in nature as diatomic molecules, hydrogen (H2), fluorine (F2), bromine
(Br2), oxygen (O2), chlorine (Cl2), iodine (I2) and nitrogen (N2). These seven elements are found as diatomic
molecules unless they are bonded to another element. The sharing of two electrons forms a single bond.
Fluorine, chlorine, bromine and iodine all share one electron between them to give them eight electrons in their
valence shell. Oxygen atoms possess six valence electrons. The octet rule says both atoms require eight valence
electrons to be stable. Oxygen atoms share two electrons forming a double bond. Below, draw the four electrons
oxygen atoms share as well as the other electrons that surround the molecule O2.
Four shared electrons forms a double bond
O
O
Nitrogen atoms each have five electrons in their valence shells. Complete the Lewis structure showing what kind of
bond will form between two nitrogen atoms.
_______ shared electrons forms a ____________ bond
N
N
The pairs of electrons that are not shared are called unshared pairs or lone pairs. Triple bonds are the maximum
amount of bonds that exist between two atoms. It is customary to draw a line (dash) for two shared electrons.
Polar Covalent Bonds:
In covalent bonds, the sharing of electrons is not always equal. The atom with a larger electronegative
value produces a greater pull on the electrons. This causes the electrons to spend more time around the
atom with a higher electronegative value. This bond is called a polar covalent bond. The resulting
molecule has a partially negative (δ-) end and a partially positive end (δ+). The resulting polar covalent
bond is referred to as a dipole (two poles) one slightly negative pole and one slightly positive pole.
Polar or Nonpolar Molecule?
H2O and CCl4 are examples of covalent bonds. Hydrogen has an electronegative value of 2.20; oxygen’s
value is 3.44. The electrons will spend more time around the oxygen atom causing it to be slightly
negative. The hydrogen atoms will be slightly positive since the electrons will spend less time around
them. Chlorine has an electronegative value of 3.16; carbon’s value is 2.55. In the compound CCl4,
which atom will be slightly negative and which will be slightly positive? The shape of this molecule is
tetrahedral. Although the chlorine atoms have a greater electronegative value (3.16) than the carbon atom
(2.55), the chlorine atoms are symmetrically arranged around the carbon atom. The electrical charge is the
same when measured at the same distance from the carbon atom on all sides of the molecule. This is a
nonpolar covalent molecule; all of the atoms share the electrons equally.
Lewis Structure (electron dot diagram) Practice: Complete Practice Problems #1-5 on page 244. Refer to
example Problem 9-1 to help you complete this exercise. Using the electronegativity values on page 169 and your
diagrams, predict which of the molecules will be polar covalent and which will be nonpolar covalent molecules and
then circle your choice.
1. polar covalent or nonpolar covalent
2. polar covalent or nonpolar covalent
3. polar covalent or nonpolar covalent
4. polar covalent or nonpolar covalent
5. polar covalent or nonpolar covalent