Physical Science
Chapter 20: Chemical Bonds
Compounds
A compound is a combination of atoms
from 2 or more different elements, in a
definite ratio
 The properties of a compound may be
completely different from those of the
elements which make it up.


Example: water (H2O)
Chemical Formulas
• A short-hand way to write the
name of a chemical compound
is by using a chemical formula,
which is like a "recipe" for a
compound.
• It tells what elements are in the
compound, and how many
atoms of each element.
• The # of atoms of each element is shown
using a subscript.
• Example: Sulfuric acid, H2SO4
Hydrogen - 2 atoms
Sulfur - 1 atom (understood subscript)
Oxygen - 4 atoms
• Molecule - smallest sample of a
compound.
• Molecular mass - the mass of a single
molecule of a compound.
• Ex.
H2O
(2x1) + (1x16) = 18
• Why do you think table salt sinks
when you put it into water?
• C6H12O6 is the chemical formula for a
simple sugar. What is the molecular
mass of this compound?
Chemical stability
• Some elements do not join together with
other elements to form compounds.
These un-reactive elements are said to
already be chemically stable.
• All of the elements in G18 are this way.
• The way that an element reacts with other
elements is determined by it's outer shell electrons.
(Outer shell electrons refers to the electrons that are
farthest out from the nucleus in the electron cloud)
• Atoms which react with each other do so by either
gaining, losing, or sharing electrons with other atoms.
• Think of it like an egg carton. It can only
hold a certain number of eggs. And when
the carton is full, then it doesn’t need any
more.
• Likewise, atoms of different elements can only
have a certain # of electrons in their electron
clouds, and if they are full then they are stable,
and won’t react with other elements.
• Elements with 8 o.s.e.’s, or a full first level
(2 e-’s) are stable, and will not react.
• However, elements with only a partially-filled
outer shell may react with certain other
elements.
• An electron dot diagram can show how many
outer shell electrons an atom has, and
whether or not it can hold any more, and
therefore whether that atom will react or not.
Electron Dot Diagrams
• (Note: The only elements we will concern
ourselves with now are the G1 & G2 metals,
and the non-metals, since these are the most
common reactive elements)
• An element with 8 electrons in the outer shell is
considered full, and stable (un-reactive). This is
called the Octet Rule.
Ex. He-2 ose's, Ne-8, Ar-8, Kr-8
• A quick way to determine the # of outer
shell electrons (G1, G2, and the non-metals) is to
use the last # of the group #.
For example: Mg
N
Cl
Kr
G2
G15
G17
G18
2 o.s.e.’s
5 o.s.e.’s
7 o.s.e.’s
8 o.s.e.’s
• A dot diagram shows an elements
chemical symbol with 1 dot for each
outer shell electron up to 8.
So.....magnesium has 2 ose's, and would
therefore have 2 dots surrounding it.
Mg

Examples:
H
He
Al
C
N
O
Cl
Ne
• Noble gases have 8 o.s.e.’s, which is
why they are stable and un-reactive.
Chemical Bonds
• When 2 atoms do react with
each other to form a new
substance, this new substance
is held together with chemical
bonds. A chemical bond is a
force holding the atoms in a
compound together.
• Think of bonds like the
connecting sticks in a
Tinkertoy set.
Types of Bonds
• There are 2 main types of chemical bonds:
1. Ionic - bonds created by the attraction of
opposite charges (+, -) like a magnet
2. Covalent - bonds created by the sharing of
electrons
Ionic Bonds
• When 2 atoms react with each other,
they both want to have a stable
relationship (i.e. have 8 outer shell
electrons)
• Sometimes, one atom will "steal"
electrons from another atom in order to
become stable.
• For example, sodium (Na) has 1 outer shell
electron and chlorine (Cl) has 7.
If chlorine were to gain 1 electron, then it
would have 8, and be stable.
So it "steals" sodium's electron to become
stable.
Sodium, however, now drops down an
energy level and ends up with 8 outer shell
electrons also. So everybody's happy.
• When 1 atoms steals (gains) electrons, it
will take on a negative charge since that is
the charge of the electrons.
• Likewise, the atom which gave up
electrons will take on a positive charge,
since it lost negatively charged electrons.
• So………the 2 atoms now have opposite
charges which attract each other, just like
a magnet.
• When the atoms join together, the positive
and negative charges will cancel each
other out, forming a neutral molecule.
• Ions are individual atoms with a positive or
negative charge due to gained or lost
electrons
• Ionic bonds form when the oppositely
charged ions attract each other
Covalent Bonds
• A covalent bond is formed when atoms
combine together by sharing electrons.
• Covalent bonds usually occur between 2
non-metals
• Example: H has 1 o.s.e. and Cl has 7.
By sharing, H will have 2 (stable)
and Cl will have 8 (stable)
• A diatomic molecule is one with 2 atoms of
the same element covalently bonded.
• Ex.: Cl2 (chlorine gas)
O2 (oxygen gas)
Binary Compounds
• A binary compound is one composed of
atoms from exactly 2 different elements.
• In order to determine the ratio between 2
elements which will produce a compound
it is often useful to use oxidation numbers
(numbers which show how many electrons
are gained, lost, or shared when 1 atom
combines with another).
Using oxidation numbers to determine
chemical formulas.
Oxidation numbers show the number of
electrons gained, lost, or shared when an atom
combines.
Oxidation numbers are the same for elements in
the same groups.*
Ex.: G17 elements gain 1 electron when
bonding, and thus have an ox. # of -1.
+1 +2
+3 +4 -3 -2
-1
0
The criss-cross method
Oxidation numbers can be written as a
superscript for each of the elements in a binary
compound. Those numbers are then crisscrossed down and used as subscripts for the
other element.
• Example:
Mg+2
N-3
Cross the +2 and the -3 down to the subscript
of the other element, drop the signs, and you
end up with Mg3N2.
• Example #2:
Calcium has an
oxidation # of +2,
chlorine is -1,
what is the
formula for a
compound of
calcium and
chlorine?
Naming Binary Compounds
• To name a binary compound, you simply
say the name of the first element, and then
add the suffix -ide to the root of the
second element.
• Example: Na + Cl = sodium chloride
Al + N = aluminum nitride
Ca + P = calcium phosphide
Toxic and Corrosive Materials
• Many compounds are completely
harmless to humans, while some
are very harmful.
• Corrosive materials attack and/or
destroy metals, living tissue, and
other materials.
Acids are corrosive, although their
relative strength will vary greatly.
• Toxic materials are poisonous.
• If toxic materials are ingested,
vomiting is often induced to remove
the materials from the body.
This is usually not the case with
corrosive materials. Why?
Sample Problems
Write formulas, give names, and calculate
molecular masses for each of the following
compounds:
Al + Cl
Ca + O
Na + N
H+S
K+F
• The following covalently bonded
diatomic molecules are listed in order
from most stable to least stable. What
do you think is the main determining
factor of stability in these molecules?
•
•
•
•
He
N2
O2
Cl2
Reactants
Potassium &
Oxygen
Sodium &
Sulfur
Calcium &
Nitrogen
Magnesium
and
Oxygen
Carbon &
Sulfide
Hydrogen &
Phospho
rus
Dot structure
for each
reactant
Criss-Cross
Method
Molecular
Mass
Compound
Name
Compound
Formula
Reactants
Beryllium &
Iodine
Lithium &
Fluorine
Barium &
Chlorine
Cesium &
Nitrogen
Calcium &
Bromine
Magnesium
&
Nitrogen
Hydrogen &
Sulfur
Dot structure
for each
reactant
Criss-Cross
Method
Molecular
Mass
Compound
Name
Compound
Formula
• Identify 2 specific materials for each of
the following:
A.
B.
C.
D.
Corrosive
Toxic
Corrosive, but not toxic
Toxic, but not corrosive
(assume materials are at everyday common strengths)
+1 +2
+3 +4 -3 -2
-1
0
• Identify the following:
-P3 element with 6 outer shell electrons
-the lightest inert element
-the lightest binary compound (17 a.m.u.)
-the lightest diatomic molecule
• Provide the following information for a binary
compound composed of hydrogen and sulfur:
• Compound name
• Compound formula
• Molecular mass
Give 1 example for each of the following:
• A diatomic molecule which is very stable
• A compound formula which reduces down to
a 1:1 ratio
• An element which loses 2 electrons when
ionically bonding
• An element which does not follow the Octet
Rule
Physical Science
Ch. 21: Chemical Reactions
• A chemical reaction is where 1
or more substances are
chemically changed into new
substance(s)
• Example: Changing hydrogen
gas and oxygen gas into water

In order for a chemical reaction
to take place, you need both
reactants and products.

Reactants are the substances
used to produce the reaction.

Products are the new
substances formed as a result
of the reaction.
Chemical Equations
A short-hand way to show a chemical reaction by
using formulas and symbols is called a chemical
equation.
Example:
H2 + O2
g H20
In a chemical equation, the reactant(s) are
always on the left hand side, and the product(s)
are always on the right.
• Is it necessary to have the same # of
reactants as products?
Hmmm......
• No! You just saw that the formation of water
creates 1 product from 2 reactants.
H2 + O2 g H20
• So how come there are 2 atoms oxygen on the
left, but we only end up with 1 on the right?
We'll get to that later.
• There are several symbols used in
chemical equations which you will need to
be familiar with.
Those symbols are on pg. 635 in your
book.
Law of Conservation of Mass
• In a chemical reaction, mass can neither
be created or destroyed, it can only
change form.
Therefore, the mass of the reactants before the
reaction occurs must be exactly equal to the
mass of the products following the reaction.
• For example, if a log were placed in a sealed
metal box and set on fire. The products
(smoke, ash, gases, etc.) would have the
exact same mass as the log before it were
burnt.
• If the box were sitting on a scale as the log
burned, the weight would not change at all.
Molecular Mass
• Molecular mass is the mass of 1
molecule of a given compound.
Example: Calcium Chloride (CaCl2)
Calcium atomic mass = 40 a.m.u.
Chlorine atomic mass = 35 a.m.u. (x2)
40 + (35 x 2 atoms) = 110 a.m.u.
• Some chemicals when mixed produce
very mild reactions, some produce very
violent ones.
Certain chemicals, like strong acids and
bases, are just not made to be mixed
together (by sane people), unless under highly
controlled conditions.
• For example, some household
cleaners like Windex contain
ammonia (NH4), a strong base.
• Others, like Liquid Plumber,
contain sulfuric acid (H2SO4).
Mixing these can produce a
violent reaction.
• Or even worse, mixing
ammonia with bleach can
result in the production of
poisonous chlorine gas (Cl2).
Types of Reactions
• A synthesis reaction is one where 2 or more
reactants will combine to produce 1 product.
A + B
g
AB
A decomposition reaction is where 1
reactant will break down into 2 or more
products.
AB
g
A + B
Thermal Reactions
• An exothermic reaction is one where energy (heat,
light, or electricity) is given off during the reaction.
Example: burning, pocket hand warmers,
glowsticks, batteries, electric eels
• In an endothermic
reaction, heat is taken in
during the reaction. This
causes the surrounding
area to be cooled.
Example: chemical cold
pack
Catalysts
• A catalyst is a substance which will speed
up the rate of a chemical reaction, without
changing the product(s).
• Ex: blowing on a fire, moisture speeding
up the rate of corrosion, heating Luminol
Inhibitor
•An inhibitor is a substance
which slows down a chemical
reaction without changing the
product(s).
Ex: lemon juice on an apple,
food preservatives, CO2
on a fire, cooling Luminol
• Consider a chemical reaction where
sodium oxide and hydrogen chloride
combine to produce sodium chloride
and hydrogen oxide. Provide the
following:
• Formulas for all four compounds
• A balanced chemical equation for the
reaction
• The molecular mass of the reactants
• Bud decided to mix
sodium chloride (salt)
and carbon dioxide gas
together in a beaker.
Nothing happened. But
when he heated it up, a
reaction occurred
which produced carbon
chloride and sodium
oxide.
Write a balanced
chemical equation for
this reaction.