Chemistry
Combining Elements,
Chemical Reactions
Why Do Elements Combine?
• Elements combine to become
more stable.
–Elements become stable by
filling their outermost level of
electrons.
What is a Compound?
A compound is what results when two or
more elements combine.
A binary compound is a compound
formed from two elements.
Example: CaF2 , Calcium fluoride.
How Do Elements Combine?
• Elements combine through
chemical bonding.
• There are 2 types of chemical
bonding:
– Ionic Bonding
– Covalent Bonding
Ionic Bonding
Ionic bonding happens when elements combine by
gaining or losing electrons.
Example: Na + Cl
Atoms become charged as electrons are
transferred from Na to Cl.
Na+ClOpposite charged atoms attract.
Attraction force forms an ionic bond between
elements and a new compound is formed. NaCl
This type compound is called an ionic
compound.
Note: the compound as a whole is neutral.
Elements Bond in
Predictable Ways
Metals will lose electrons and
become “+” charged.
Non-metals will gain electrons
and become “-“ charged.
Covalent bonding happens when elements
combine by sharing electrons.
For example: Cl + Cl
Each atom has the same number of protons and electrons, therefore
each have a neutral charge.
:: Cl :. .: Cl ::
Each atom is looking for one electron to fill outer energy level (8) so
each atom shares one electron with the other atom.
:: Cl ::: Cl ::
Since electrons are shared, no atom gains or loses electrons. The
result is a new molecule with a neutral charge.
Cl2
Note: the force of attraction between the “ –” electrons and both “+”
nucleus’ is what holds the molecule together.
What is a Molecule?
• A molecule is the neutral particle
formed as a result of atoms sharing
electrons.
•
Example: H. + .H  H:H
• Two hydrogen atoms each share one
electron to form a more stable hydrogen
molecule, or “H2”.
Polar and Non-polar Molecules
• Molecules can be classified as polar or
non-polar.
Polar molecules are molecules that have
two opposite charged ends, or poles.
Why Do Poles Form?
• The nucleus of the atom containing more
protons (here Oxygen with +8 nucleus) has
a greater force of attraction for the “-”
electrons than the less attractive Hydrogen’s
with +1 in each nucleus.
• As a result, the oxygen end of a water
molecule has a slight negative charge while
the hydrogen end has a slight positive
charge. The result is a polar molecule.
Non-polar Molecules
Non-polar molecules are molecules that
do not have charged ends.
Example: Cl2 ::Cl:::Cl::
The equal number of protons in both
nuclei results in an equal force of
attraction for the shared electrons,
therefore, no poles are created.
Non-polar Molecules
Writing Chemical Formulas
“Nomenclature” is the system of
shorthand used for writing
chemical formulas and
equations.
Nomenclature Terms - Subscript
A subscript is a number placed below and to the
right of an elements symbol. This number
indicates how many atoms of that element are
present. (the number 1 does not need to be
written since the symbol already indicates one
atom is present).
Examples:
O2 = 2 oxygen atoms
Cl2 = 2 chlorine atoms
H2O= 2 hydrogen atoms and 1 oxygen atom
C6H12O6 = 6 carbon atoms and 12 hydrogen atoms
and 6 oxygen atoms
Nomenclature Terms - Superscript
A superscript is a number placed above and
to the right of an elements symbol. This
number indicates the charge of an atom, or
the atom’s oxidation number.
Examples: O-2 means that the oxygen
gained 2 “-”electrons and now has a -2
charge.
K+1 means that the potassium atom has lost
one electron and has a +1 charge.
Nomenclature Terms - Oxidation
Number
An oxidation number is the number of
electrons that an atom gains, loses or
shares when bonding with another atom.
Example: when Lithium (Li) gives away an
electron it becomes Li+1. The +1 is the
oxidation number; it indicates that lithium
has lost one “-” electron and the result is a
+1 charge on the atom.
Nomenclature Terms - Coefficient
A coefficient is the number in front of each
“item” in a chemical equation. The
coefficient shows how many of each “item”
there is in the equation.
Example: 6 H2 + 6 O  6 H2O
How Do We Write Chemical
Formulas?
• Using a series of steps allows us to
correctly write chemical formulas.
Steps For Writing The Chemical
Formula For Binary Compounds
Write the symbol of the element with “+” oxidation number (all
metals and hydrogen). Example: Ca
Next write the symbol of the element with the “-” oxidation
number (all nonmetals). Example: Ca F
Write in the oxidation numbers for each element.
Example: Ca+2 F-1
Balance the formula. There must be an equal number of “+”
and “-“ charges in the completed formula to have a
neutral compound. Example: Ca+2 2F-1
Put in subscripts so the sum of the charges is equal to zero.
Example: Ca1F2 , drop the unnecessary “1” and get
CaF2.
Criss-Cross Method
A Short Cut is to use the criss-cross
method.
Change the oxidation number to a
subscript for the other element
Example: Ca+2 F-1 becomes Ca1F2
Drop the unnecessary “1” and get CaF2.
Naming Binary Compounds
•
•
•
Write the name of the first element.
Example: Calcium.
Write the root of the name of the second
element.
Ex: Calcium Flour.
Add the suffix “ide” to the root. Example:
Calcium Fluoride.
How Do Elements Combine?
Elements combine through chemical
bonding.
Chemical bonding involves chemical
reactions.
What Is Involved In Chemical
Reactions?
The theory of Conservation of
Matter states that matter can not
be created nor destroyed.
What Is Involved In Chemical
Reactions?
• Reactants are the elements or
molecules involved at the start of a
chemical reaction.
• Products are the results of the
chemical reaction. What we are left
with after the reaction.
How Do We Know What Happens
During Chemical Reactions?
Chemical equations are a way of showing
what is taking place during a chemical
reaction by using numbers and symbols.
Ex: Ag + H2S  Ag2S + H2
What is a Balanced Equation?
A balanced equation is when there are the
same number of atoms of each element on
both sides of the chemical equation.
What we start with = what we end with.
Ex: 2Ag + H2S  Ag2S + H2
Balancing Chemical Equations
1. Describe the chemical reaction in words.
Ex: silver nitrate plus sodium chloride yields silver
chloride plus sodium nitrate.
2. Write the chemical equation using formulas
and symbols.
Ex: AgNO3 + NaCl  AgCl + NaNO3
3. Check for balance. (make a table)
4. Determine the coefficients (if necessary).
Balance The Chemical Equation For
How The Human Body Makes Energy
Glucose plus oxygen produces carbon dioxide and water.
C6H12O6 + O2

CO2
Balanced = C6H12O6 + 6O2  6CO2 + 6H2O
+ H2O
Why Are Chemical Reactions
Important?
Life could not exist without chemical
reactions.
In chemical reactions atoms rearrange
themselves forming all of life’s
substances and compounds.
Types of Chemical Reactions
• There are four (4) types of chemical
reactions, all are based on the way
atoms rearrange themselves during the
reaction.
• These 4 are synthesis reactions,
decomposition reactions, single
replacement, double displacement.
Synthesis Reaction
• Synthesis reactions are when 2 or more
substances combine to form another substance.
• Most synthesis reactions give off energy in the
form of heat and light.
• General formula for synthesis reactions:
A + B  AB
• Example: Aluminum + oxygen produces aluminum
oxide
Al + O2  Al2O3
Balanced: 4 Al + 3O2  2 Al2O3
Decomposition Reaction
• Decomposition reactions are when one substance
breaks down into simpler substances.
• Most decomposition reactions require the addition
of energy (need energy)
• General formula for decomposition reactions:
AB  A + B
• Example: Carbonation (bubbles) in soda
Decomposition of carbonic acid yields water
and carbon dioxide.
H2CO3  H2O + CO2
Single Replacement Reaction
• Single replacement reactions are when one element
replaces another element in a compound.
• General formulas for single replacement reactions:
A + BC  AC + B
(the positive ion (B) is replaced (by A))
D + BC  BD + C
(the negative ion (C) is replaced (by D))
Example: Tarnish
Aluminum plus silver sulfide yields aluminum sulfide plus
silver
2Al + 3AgS  Al2S3 + 3Ag
Double Displacement Reactions
• Double displacement reactions are when the positive ion
of one compound replaces the positive ion of another
compound forming two new compounds.
• General formula for double displacement reactions:
AB + CD  AD + CB
• Most acid-base reactions are double displacement
reactions.
• Often a precipitate forms in these reactions.
Example: Antacid plus stomach acid yields precipitate plus
water
Magnesium hydroxide (antacid) plus hydrochloric acid
(stomach acid) yields magnesium chloride (precipitate) plus
water.
Mg(OH)2 + 2HCl  MgCl2 + 2H2O