Chemistry
4/30/2018
1
N'vida E. Houndonougbo
Part 2: Covalent bonds and molecules
Learning Objectives
Explain the difference between a covalent bond and an ionic
bond. (4.6)
Use electronegativity of elements to determine bond
polarity. (4.7)
Classify a covalent bond as polar or nonpolar. (4.7)
Predict the number of bonds formed by common nonmetals
. (4.8)
Name and write formulas for covalent compounds. (4.6)
Write Lewis formulas for simple molecules (4.10)
Explain how shape and composition change the properties
of molecules
Chapter 4 : Chemical bonds_Part 2
N'vida E. Houndonougbo
Chapter 4 : Chemical bonds
Outline Chapter 4_part 2
I. Molecule and covalent bond
II. Polar Covalent Bonds and Electronegativity
III. Lewis Structure
IV. Molecular shapes and Properties
4/30/2018
2
N'vida E. Houndonougbo
Covalent bonds and Molecules
Many nonmetal elements react by sharing electrons rather than by gaining or losing electrons.
When two atoms share a pair of electrons, a covalent bond is formed.
Example: Formation of the molecule of hydrogen (H2)
The two electrons travel about both nuclei
N'vida E. Houndonougbo
Covalent Bonding and Molecules
Single covalent bond : sharing
of a pair of electrons.
Double covalent bond: sharing
of two pairs of electrons.
Triple covalent bond: sharing of
three pairs of electrons.
4/30/2018
3
N'vida E. Houndonougbo
Polar Covalent Bonds and Electronegativity
• Electrons in a covalent bond occupy the region between the
bonded atoms.
• If the atoms are identical, as in H2 and Cl2, electrons are
attracted equally to both atoms and are shared equally.
• If the atoms are not identical, however, as in HCl, the
bonding electrons may be attracted more strongly by one
atom than by the other and thus shared unequally. Such
bonds are known as polar covalent bonds.
N'vida E. Houndonougbo
In a polar covalent bonds partial charges are represented
by a d- on the more negative atom and d+ on the more
positive atom.
.
Polar Covalent Bonds and Electronegativity
4/30/2018
4
N'vida E. Houndonougbo
• The ability of an atom to attract electrons is called the
atom’s electronegativity.
• Fluorine, the most electronegative element, assigned a
value of 4, and less electronegative atoms assigned lower
values.
N'vida E. Houndonougbo
• The difference in electronegativity between two bonded atoms can be used to determine the type of bond.
• As a rule of thumb, electronegativity differences
Example: in class
4/30/2018
5
N'vida E. Houndonougbo
Drawing a Lewis Structure
A Lewis structure shows the bonds between atoms and helps us visualize the arrangement of atoms in a molecule.
Some Lewis structures can be assembled like a puzzle. Each piece an atom, fitting together by connecting bonding sites. Others require more hints.
1. Determine the number of valence electrons in the molecule
• For a cation, subtract one electron for each positive charge on it
• For an anion, add one electron for each negative charge on it.
9
N'vida E. Houndonougbo
Drawing a Lewis Structure
2. Determine the connectivity of atoms.
3. Connect the atoms by single bonds.
4. Show bonding electrons as a single line; show nonbonding
electrons as a pair of Lewis dots.
5. In a single bond, atoms share one pair of electrons; in a
double bond, they share two pairs, and in a triple bond they
share three pairs.
10
4/30/2018
6
N'vida E. Houndonougbo
Drawing a Lewis Structure: practice
in class
11
N'vida E. Houndonougbo
Exceptions to the Octet Rule
• The octet “rule” is a useful guideline, but it has numerous exceptions.
• Boron has only 3 valence electrons it can share and thus forms compounds in which it has only 3 covalent bonds.
• Elements in the third row and below have vacant d orbitals that can be used to make more than 4 bonds.
12
4/30/2018
7
N'vida E. Houndonougbo
Naming Binary Molecular Compounds
• Binary molecular compounds are
composed of only two elements
• The formulas of binary molecular
compounds are written with the less
electronegative element first.
1. Name the first element in the formula,
using a prefix to indicate the
number of atoms.
• The prefix mono- is omitted for the
first element
N'vida E. Houndonougbo
Naming Binary Molecular Compounds
2. Name the second element in the formula, using the
ending -ide along with the prefix needed.
• 3. If the prefix ends with a or o and the name of the element begins with a vowel, the two successive vowels are often elided into one.
Example in class
4/30/2018
8
N'vida E. Houndonougbo
Some molecules are known by their common name or generic name
H2O ( water / dihydrogen monoxide)
H2S ( hydrogen sulfide / dihydrogen monosulfide
CH4 (methane / carbon tetrahydride)
NH3 (ammonia / nittrogen trihydride)
.
N'vida E. Houndonougbo
Molecular Shapes: The VSEPR Theory
The Valence Shell Electron Pair Repulsion (VSEPR) theory
predicts the shape of molecules and polyatomic ions based on
repulsions of electron pairs on central atoms.
The molecular shape indicates the arrangement of atoms
around the central atom as a result of electron repulsion.
Practice in class: Use the table 4.6: bonding and the shapes
of molecules to predict the shape of a molecule
4/30/2018
9
N'vida E. Houndonougbo
Molecular Shapes: The VSEPR Theory
N'vida E. Houndonougbo
Shapes and Properties: Polar and Nonpolar
Molecules
In order for a molecule to be polar, two conditions must
be met:
1. It must have polar bonds.
2. The bonds must be arranged such that a
separation of charge exists.
4/30/2018
10
N'vida E. Houndonougbo
Shapes and Properties: Polar and Non
polar Molecules
Why water is polar molecule and carbon dioxide, CO2, is not?
In water, H2O, the central O atom is surrounded by two nonbonding pairs and two bonding pairs.
Thus, the molecular shape is bent
N'vida E. Houndonougbo
Shapes and Properties: Polar and Non
polar Molecules
• In carbon dioxide, CO2, the central C atom is surrounded by
four bonding pairs.
• According to VSEPR theory, the electron pairs will repel each
other and they will be at opposite sides of the C atom.
• Thus, the molecular shape are both linear.
4/30/2018
11
N'vida E. Houndonougbo
Molecular Polarity
• Polarity has a dramatic effect on the physical properties of molecules, particularly on melting points, boiling points, and solubility.
• Example :
The polarity of the bond in water molecule profoundly affects the property of water including what dissolves in it.
N'vida E. Houndonougbo
Like Dissolves Like
We can predict whether a solid will dissolve in a liquid by applying the like dissolves like rule
Ionic compounds such as sodium chloride, NaCl and Polar compounds, such as table sugar (C12H22O11), are soluble in polar solvents such as water, H2O.
Polar solvents such as water have large dipole moments (“partial charges”); they contain bonds between atoms with very different electronegativities, such as oxygen and hydrogen.
4/30/2018
12
N'vida E. Houndonougbo
Hydrophilic molecules
Polar molecules such as Methanol, CH3OH that
dissolve easily in water are called Hydrophilic
molecules
Methanol is soluble in water because its molecular
structure (-OH groups form hydrogen bonds).
N'vida E. Houndonougbo
Hydrophilic molecules
Hydrogen bonding is a weak attraction
between Molecules.
It is an interaction between a hydrogen atom and an
electronegative atom such as oxygen, nitrogen and
fluorine.
4/30/2018
13
N'vida E. Houndonougbo
The hydrophobic effect
Nonpolar molecules such as
(methane ) do not dissolve in water
because they cannot form hydrogen
bonds.
Methane is nonpolar and when
forced into water, the polar water
molecules form a cage-like
structure.
N'vida E. Houndonougbo
The hydrophobic effect
• Nonpolar molecules such as
methane, CH4 that do not mix with water
are called Hydrophobic molecules.
• When hydrophobic molecules are placed
in water, the nonpolar molecules group
together to minimize their interaction
with the solvent. This is called
the hydrophobic effect.
Nonpolar molecules CH4 are soluble in
nonpolar solvents such as hexane C6H14