Name(s):___Haley Bockelman_____ CHM 1020 Lab
Week 4 Assignment a, b, and c
Atomic Structure, Chemical Bonding, Lewis Structure, and 3D Molecular Shape
Objectives
In this lab, you will apply valence bond theory to draw appropriate Lewis structures, use
electronegativity differences to classify bonds as ionic, polar covalent, or nonpolar covalent, and apply
valence shell electron pair repulsion theory (VSEPR) to predict molecular geometry. We will also use
molecular kits to create ball and stick models of molecules of interest to you.
Theory
Atoms of certain species tend to bond together. An atom is more stable if its valence shell
(electrons in its outermost energy level) is similar to that of a Noble Gas (typically eight electrons), and
this number is most often achieved when atoms combine. Notice that atoms of elements in the same
group on the periodic table tend to have the same number of valence electrons; for example, halogens,
Group 7A elements (Figure 1), have seven valence electrons.
Figure 1: Dot Diagrams
For most elements, a full outer energy level has eight electrons, an octet. The elements in group
8A have a full outer energy level. Helium (He), in period 1, is an exception, requiring only two electrons.
Because group 8A elements’ atoms already have a full outer energy level, those elements tend to be
nonreactive—they rarely combine with other atoms to form compounds. Atoms can fill their outer
energy level by transferring or sharing electrons to form either ionic or covalent compounds.
Figure 2: Two Bonding Types
Page 1 of 18
,Name(s):___Haley Bockelman_____ CHM 1020 Lab
Whether two given atoms tend to bond ionically or covalently is determined by the difference in
their electronegativity. Electronegativity is a dimensionless number that is a measure of an atom’s
attraction for bonding valence electrons. Electronegativities show periodic trends on a periodic table.
Figure 3: Table of Electronegativities
Excluding the noble gases, the most electronegative element is fluorine, which is assigned a
value of 4.0. The other elements’ values are calculated on the basis of that of fluorine. Across each
period, electronegativities tend to increase. The nonmetal families of nitrogen, oxygen, and fluorine
have the highest values. Due to their atoms’ small radii, the positive nuclei exert a greater attraction for
bonding electrons. The alkali metals and alkaline earth metals (the groups on the left side of the
periodic table) have the lowest electronegativities because their atoms have the largest radii. Cesium
(and francium – not shown in Figure 3), with the largest radii, have the lowest electronegativity, at 0.7.
Lewis Dot Structures
A Lewis dot structure is one way to represent the arrangement of valence electrons in a
molecule (Figure 1). In Lewis structures, an element symbol is surrounded by a specific number of dots
representing valence electrons. Most atoms obey the octet rule; they need eight valence electrons to fill
their outer shell. (Exceptions include hydrogen and helium, which need only two. Also, boron and
beryllium may form compounds with fewer than eight, and elements in periods 3–6 may use more than
eight.)
To fill their outer shells, elements can form covalent bonds by sharing electrons. To show those
bonds, Lewis structures are often used. Covalent compounds may have single, double, or triple bonds
between atoms. These bonds are represented in a Lewis structure with dashes between the chemical
symbols of the bonded elements. The number of dashes corresponds to the number of bonds. Lewis
structures also indicate the lone pairs of electrons on different atoms. Use the guide below if it helps in
drawing Lewis Structures.
Page 2 of 18
, Name(s):___Haley Bockelman_____ CHM 1020 Lab
Page 3 of 18
Week 4 Assignment a, b, and c
Atomic Structure, Chemical Bonding, Lewis Structure, and 3D Molecular Shape
Objectives
In this lab, you will apply valence bond theory to draw appropriate Lewis structures, use
electronegativity differences to classify bonds as ionic, polar covalent, or nonpolar covalent, and apply
valence shell electron pair repulsion theory (VSEPR) to predict molecular geometry. We will also use
molecular kits to create ball and stick models of molecules of interest to you.
Theory
Atoms of certain species tend to bond together. An atom is more stable if its valence shell
(electrons in its outermost energy level) is similar to that of a Noble Gas (typically eight electrons), and
this number is most often achieved when atoms combine. Notice that atoms of elements in the same
group on the periodic table tend to have the same number of valence electrons; for example, halogens,
Group 7A elements (Figure 1), have seven valence electrons.
Figure 1: Dot Diagrams
For most elements, a full outer energy level has eight electrons, an octet. The elements in group
8A have a full outer energy level. Helium (He), in period 1, is an exception, requiring only two electrons.
Because group 8A elements’ atoms already have a full outer energy level, those elements tend to be
nonreactive—they rarely combine with other atoms to form compounds. Atoms can fill their outer
energy level by transferring or sharing electrons to form either ionic or covalent compounds.
Figure 2: Two Bonding Types
Page 1 of 18
,Name(s):___Haley Bockelman_____ CHM 1020 Lab
Whether two given atoms tend to bond ionically or covalently is determined by the difference in
their electronegativity. Electronegativity is a dimensionless number that is a measure of an atom’s
attraction for bonding valence electrons. Electronegativities show periodic trends on a periodic table.
Figure 3: Table of Electronegativities
Excluding the noble gases, the most electronegative element is fluorine, which is assigned a
value of 4.0. The other elements’ values are calculated on the basis of that of fluorine. Across each
period, electronegativities tend to increase. The nonmetal families of nitrogen, oxygen, and fluorine
have the highest values. Due to their atoms’ small radii, the positive nuclei exert a greater attraction for
bonding electrons. The alkali metals and alkaline earth metals (the groups on the left side of the
periodic table) have the lowest electronegativities because their atoms have the largest radii. Cesium
(and francium – not shown in Figure 3), with the largest radii, have the lowest electronegativity, at 0.7.
Lewis Dot Structures
A Lewis dot structure is one way to represent the arrangement of valence electrons in a
molecule (Figure 1). In Lewis structures, an element symbol is surrounded by a specific number of dots
representing valence electrons. Most atoms obey the octet rule; they need eight valence electrons to fill
their outer shell. (Exceptions include hydrogen and helium, which need only two. Also, boron and
beryllium may form compounds with fewer than eight, and elements in periods 3–6 may use more than
eight.)
To fill their outer shells, elements can form covalent bonds by sharing electrons. To show those
bonds, Lewis structures are often used. Covalent compounds may have single, double, or triple bonds
between atoms. These bonds are represented in a Lewis structure with dashes between the chemical
symbols of the bonded elements. The number of dashes corresponds to the number of bonds. Lewis
structures also indicate the lone pairs of electrons on different atoms. Use the guide below if it helps in
drawing Lewis Structures.
Page 2 of 18
, Name(s):___Haley Bockelman_____ CHM 1020 Lab
Page 3 of 18
