4.3 COVALENT STRUCTURES
By: Merinda Sautel Alameda Int’l Jr/Sr High School Lakewood, CO [email protected]
ESSENTIAL IDEA
Lewis (electron dot) structures show the electron domains in the
valence shell and are used to predict molecular shape.
NATURE OF SCIENCE (1.10)
Scientists use models as representatives of the real world – the development of the
model of molecular shapes (VSEPR) to
explain observable properties.
THEORY OF KNOWLEDGE
Does the need for resonance
structures decrease the value or validity of Lewis (electron dot)
theory? What criteria do we use in assessing the validity of a
scientific theory?
UNDERSTANDING/KEY IDEA 4.3.A
Lewis (electron dot)
structures show all the valence electrons in a
covalently bonded species.
UNDERSTANDING/KEY IDEA 4.3.B
The “octet rule” refers to the tendency of atoms to gain a
valence shell with a total of 8
electrons.
APPLICATION/SKILLS
Be able to deduce the Lewis (electron dot) structures of molecules and ions showing
all valence electrons for up to four electron pairs on each
atom.
LEWIS STRUCTURE RULES
Add up the total number of valence electrons in the molecule.
Draw the skeletal structure.
Use a line between each element to symbolize an electron pair.
Distribute the remaining electrons around the elements in pairs to form octets. (Hydrogen can only ever have 2 electrons.)
If you do not have enough to form octets, make double or triple bonds.
Ions must have square brackets around them with the charge notated in the top right hand corner.
To be a correct Lewis structure, ALL electrons must be shown.
UNDERSTANDING/KEY IDEA 4.3.C
Some atoms, like Be and B,
might form stable compounds with incomplete octets of
electrons.
GUIDANCE
Coordinate covalent bonds
should be covered.
OCTET RULE EXCEPTIONS
Hydrogen will never have more than 2 electrons.
Be and B have less than 8 electrons.
Some elements like S and P can have
expanded octets which hold more than 8 electrons.
Coordinate covalent bonds are formed when both electrons originate from the same atom.
An arrow is used to denote the direction in a coordinate covalent bond showing the atom from which both electrons originated.
This is common in double and triple bonds.
UNDERSTANDING/KEY IDEA 4.3.D
Resonance structures occur
when there is more than one
possible position for a double
bond in a molecule.
RESONANCE
Resonance is a concept used to describe the structures when there are multiple ways to depict the same molecule.
If you can put a double bond in more than one position, you will be expected to draw the
resonance structures.
The electrons are actually delocalized in the areas of the double bonds and are spread out equally among all bonding positions.
Bond strength and length are in between that
of single and double bonds.
Resonance structures allow us to depict all the possible positions of the double bonds.
The true structure, however, is an intermediate form known as a resonance hybrid.
Double arrows are placed between all resonance structures.
Ref: myweb.astate.edu
APPLICATION/SKILLS
Be able to deduce resonance structures. Examples include but are not limited to C
6H
6,
CO
32-and O
3.
BENZENE
www.pixmule.com
CARBONATE
www.archives.evergreen.edu
OZONE
www.chemwiki.ucdavis.edu
UNDERSTANDING/KEY IDEA 4.3.E
Shapes of species are
determined by the repulsion
of electron pairs according to
the VSEPR theory.
APPLICATION/SKILLS
Be able to use the VSEPR
theory to predict the electron domain geometry and the
molecular geometry for
species with two, three and
four electron domains.
VSEPR THEORY
Valence Shell Electron Pair Repulsion theory.
States that in a small molecule, the pairs of valence electrons are arranged as far apart from each other as possible.
So far we have dealt with structural formulas
which only show the types of atoms, bonds
and lone pairs of electrons. They do not
show the shape of the molecule.
12 BASIC SHAPES
LINEAR – two atoms bonded to the central
atom, no lone pairs of electrons on the central atom.
BENT- two atoms bonded to the central atom
with one or two lone pairs of electrons on the
central atom.
TRIGONAL PLANAR(flat triangle) – three atoms bonded to the central atom, no lone pairs of electrons on the central atom.
TRIGONAL PYRAMIDAL – three atoms
bonded to the central atom, one lone pair of
electrons on the central atom.
TETRAHEDRAL – four atoms bonded to the central atom, no lone pairs of electrons on the central atom.
TRIGONAL BIPYRAMIDAL – five atoms
bonded to the central atom (octet rule
exception)
SEE SAW (also called unsymmetrical
tetrahedron) – derivative of the trigonal
bipyramidal with one lone pair of electrons.
Note that on the three trigonal bipyramidal shape derivatives, the shapes come from
pulling off atoms from the flat triangle, not
the top and bottom.
T-SHAPED – also a derivative of the
trigonal bipyramidal shape with 2 lone pairs of electrons on the central atom.
LINEAR – derivative of the trigonal
bipyramidal shape with three lone pairs of electrons on the central atom.
This linear is different from the other linear as it has 3 pairs of
lone electrons on the central atom.
OCTAHEDRAL – six atoms bonded to the central atom (another octet rule
exception).
SQUARE PYRAMIDAL – derivative of the octahedral with one lone pair of electrons on the top or bottom.
SQUARE PLANAR – derivative of the
octahedral shape with two lone pairs of
electrons – one on top and one on bottom.
APPLICATION/SKILLS
Be able to predict molecular
polarity from bond polarity
and molecular geometry.
MOLECULAR POLARITY
The polarity of a molecule depends upon:
The polar bonds it contains.
The shape of the molecule.
If the bonds are equally polar and arranged
symmetrically, then they cancel each other out and are non-polar.
If the molecule contains bonds of different polarities or the bonds are not arranged
symmetrically, then the molecule will be polar.
You can usually tell by the shape and lone pairs
of electrons if the molecule is polar or not.
APPLICATION/SKILLS
Be able to predict the bond angle from molecular
geometry and presence of non-bonding pairs of
electrons.
ELECTRON DOMAINS
Double and triple bonded electron pairs are orientated together and behave as a single unit known as an electron domain.
Lone pairs also count as electron domains so the 12 VSEPR shapes are narrowed down to 5 basic shapes.
For the electron domain shapes, you will need
to know bond angles and hybridization.
LINEAR
A linear molecule has two electron domains.
The angle is 180 degrees and it has “sp”
hybridization.
Non polar
TRIGONAL PLANAR
A trigonal planar molecule has 3 electron domains.
It has angles of 120 degrees and “sp
2” hybridization. Non polar
The bent molecule can also have 3 electron
domains. Polar
TETRAHEDRAL
A tetrahedral molecule has four electron domains.
It has angles of 109.5 degrees and “sp
3” hybridization. Non polar
Trigonal pyramidal and “bent” with 2 lone
pairs also have four electron domains. Polar
TRIGONAL BIPYRAMIDAL
A trigonal bipyramidal molecule has 5 electron domains.
It has angles of 90 and 120 degrees and
“dsp
3” hybridization.
Non polar (see saw and t-shaped are polar)
OCTAHEDRAL
An octahedral molecule has 6 electron domains.
It has angles of 90 degrees and “d
2sp
3”
hybridization. Non polar (square pyramidal –
polar)GUIDANCE
Allotropes of carbon
(diamond, graphite, graphene and C60
buckminsterfullerene) should
be covered.
ALLOTROPES OF CARBON
Some covalent structures are crystalline in nature like ionic lattices; however, they are linked together with covalent bonds.
The crystal is a single molecule with a regular repeating pattern of covalent bonds.
It is referred to as a giant molecular structure.
Allotropes are different forms of an element in the same physical state.
Different bonding within the structures gives rise to different physical properties.
Carbon has four allotropes.
GRAPHITE
Graphite
Each C atom is sp2 hybridized covalently bonded to 3 others forming hexagons in parallel layers with bond angles of 120 degrees.
The layers are held together by van der Waal’s forces so they can slide over each other.
Density is 2.26 g/cm3
Contains one non-bonded, delocalized electron per atom so graphite conducts electricity due to the movement of these electrons.
Not a good heat conductor
Very high melting point, most stable allotrope
Non lustrous – grey solid
Used as a lubricant and in pencils
DIAMOND
Diamond
Each C atom is sp
3hybridized covalently bonded to 4 others tetrahedrally in a regular repeating pattern with bond angles of 109.5 degrees.
It is the hardest known natural substance.
Density is 3.51 g/cm
3
All electrons are bonded so it does not conduct electricity.
Does conduct heat better than metals.
Very high melting point, brittle
Lustrous crystal
Used in jewelry and tools
FULLERENE, C 60
Fullerene
Each C atom is sp2 hybridized covalently bonded in a sphere of 60 carbon atoms consisting of 12 pentagons and 20 hexagons.
The structure is a closed spherical cage in which each carbon atom is bonded to 3 others.
Density is 1.726 g/cm3
It easily accepts electrons to form negative ions so it is a semiconductor at normal temp and pressure due to
some electron mobility.
Very low heat conductivity
Low melting point
Yellow crystalline solid – soluble in benzene
Related forms are used to make nanotubes for the electronics industry, catalysts and lubricants.
GRAPHENE
Graphene
Each C atom is sp2 hybridized covalently bonded to three other carbons forming hexagons with bond angles of 120.
The structure is a two dimensional single layer described as a honeycomb or chicken wire structure.
Density is 1.5 g/cm3
Contains one non-bonded, delocalized electron per atom so graphene conducts electricity due to the movement of these electrons.
Excellent heat conductor – better than diamonds
Very high melting point, thinnest material to exist, stronger than steel
Almost completely transparent
Used in touch screens, high performance electronics, etc
UNDERSTANDING/KEY IDEA 4.3.F
Carbon and silicon form giant covalent/network covalent
structures.
APPLICATION/SKILLS
Be able to explain the
properties of giant covalent
compounds in terms of their
structures.
Si and SiO 2
Silicon is a group 4 element with 4 valence electrons.
In its elemental state, each silicon atom is covalently bonded to four others in a tetrahedral arrangement.
This results in a giant lattice structure like a diamond.
SiO2 commonly known as silica or quartz, forms a giant covalent structure.
It is also a tetrahedral structure, but the bonds are between Si and O.
Each Si atom is covalently bonded to 4 oxygen atoms and each O to two Si atoms.
The formula SiO2 is the ratio of atoms within the giant molecule.
The structure is strong, insoluble in water, does not
conduct electricity or heat and has a high melting point – all properties of glass and sand which are different forms of silica.
Citations
International Baccalaureate Organization. Chemistry Guide, First assessment 2016. Updated 2015.
Brown, Catrin, and Mike Ford. Higher Level Chemistry.
2nd ed. N.p.: Pearson Baccalaureate, 2014. Print.
ISBN 978 1 447 95975 5 eBook 978 1 447 95976 2
Most of the information found in this power point comes directly from this textbook.
The power point has been made to directly complement the Higher Level Chemistry textbook by Brown and
Ford and is used for direct instructional purposes only.