6-1
CHEMISTRY UNIT 6
(Chemical Bonds)
Valence electrons are the electrons in an incomplete
highest energy level
A chemical bond is the linkage between atoms produced
by the transfer or sharing of electrons
6-2
Electron transfer results in ionic bonding
Ex: Na and Cl
6-3
Electron sharing produces covalent bonding
Ex: water H2O
6-4
Finding Energy changes from Ionic bonding
Ex: The formation of Na and Cl ions from Na and Cl
atoms. What is the total energy change? Is the
reaction Endothermic or exothermic.
Na + Cl 
Na+
+ Cl- (what happened)
1st: Na loses an electron [endothermic]
Na + ionization energy  + eNa+
2nd: Cl gains an electron [exothermic]
Cl + e-  Cl- + electron affinity
3rd:
and Cl- bond to form Cl- which we are
told gives off 189 kcal
Na+
Na+
So: 1 mole Na + 119 kcal  1 mole Na++ e1 mole Cl + e 1 mole Cl + 83 kcal
____1 mole + 1 mole Cl- 1 mole Na+Cl +189 kcal
1 mole Na + 1 mole Cl 1 mole Na+Cl +153 kcal
Na+
By canceling above, the total reaction is exothermic.
6-5
A Molecular formula indicates the kinds of atoms in a
compound formed and the ratio of these atoms
Ex: H6O3 Na22Cl22
An Empirical formula indicates (1) the kinds of atoms in
the compound formed (2) the simplest whole
number ratio of the atoms in a compound
Ex: Na17Cl17 is a Molecular formula
NaCl is an Empirical formula
The Oxidation state of an element is represented by a
signed number called the oxidation number
Oxidation number rules:
1. The oxidation number of a free element is zero.
2. The oxidation number of a mono-atomic ion is
equal to its charge.
3. The algebraic sum of the oxidation numbers of all
the atoms in a compound is zero.
A chemical reaction in which an element attains a more
positive oxidation state (loss of electrons) is called
oxidation
6-6
A chemical reaction in which an element attains a more
negative oxidation state (gain of electrons) is called
reduction
The substance that is reduced is called the oxidizing
agent
The substance that is oxidized is called the reducing
agent
Ex: In the reaction between sodium and sulfur
(Na0
+
S0  Na2+1S-2)
Describes the following:
Sulfur reduced by sodium
Sodium oxidized by sulfur
Sulfur is the oxidizing agent
Sodium is the reducing agent
Positive ions are called cations
Ex: Na+
Negative ions are called anions
Ex: Cl-
6-7
A covalent electron pair is two shared electrons that
forms the bond between two atoms
Ex: water
A molecule is the smallest chemical unit of a substance
that is capable of stable independent existence
Ex: one water molecule
Diatomic molecules consist of two atoms
Ex. H2 F2 Cl2 Br2 I2
O
2
N2
6-8
Bond energy is the energy required to break chemical
bonds
Ex: Breaking up a H - H bond
A single covalent bond consist of 1 pair of electrons
being shared
Ex: Diatomic Flourine F2
A double covalent bond consists of 2 pairs of electrons
being shared
Ex: Calcium Oxide
A triple covalent bond consists of 3 pairs (6) electrons
being shared
Ex: Ethyne C2H2
In a molecule containing three atoms either the atoms are
in a straight line or they form a bent molecule
Ex. Straight: Be Cl2 Bent: H2O
6-9
Ex: If Na is in with F and Cl. Which one will get to
bond with Na?
Ex: In a water molecule, where do the electrons stay
most of the time?
Percentage of Ionic character of a bond A-B, X is
electronegativity
XA - XB
% I.C. = ---------------------- * 100%
XA
Ex: Na-Cl What is % ionic character?
When is a bond covalent or ionic; polar or non-polar
Bonds with more than 50% ionic character are ionic
Bonds with between 5% and 50% ionic character are
polar covalent
Bonds with less than 5% ionic character are
considered to be non-polar covalent
6-10
Hybridization is the combining of two or more orbitals
of nearly the same energy into new orbitals of equal
energy
Ex: CH4
Electronegativity is the degree of attraction an atom has
for the shared electron in a covalent bond
In a non-polar covalent bond there is an equal attraction
for the shared electron, which results in a balanced
distribution of charge
Ex: H-H
In a polar covalent bond there is an unequal attraction
for the shared electrons, which result in an unbalanced
distribution of charge
Ex: H-F
6-11
A non-polar molecule is composed of non-polar bonds
or uniformly spaced, like polar bonds
Ex: H-H
Polar molecules have an unbalanced distribution of
electrons, thus have two regions of different electric
charges
Ex: Water
Resonance is a bonding situation that cannon be
represented by a single formula
Ex: sulfur dioxide (SO2)
A polyatomic ion is a charged group of covalently
bonded atoms
Ex: NH4+ ;
NO3- ;
PO4---