Chemical Formulas and Equations
NOTES
MOLECULE
A molecule is two or more atoms that have
chemically combined.
Examples:
H2O & O2
CHEMICAL FORMULA
A Chemical Formula is a description of the
number and kind of atoms found in a single
molecule of a substance, using the symbol for
each element and a subscript number to state
the number of atoms of an element.
Symbol for Hydrogen
H 2O
Subscript - Number of
hydrogen atoms
Symbol for Oxygen
Number of oxygen
atoms - no subscript
means one atom
COUNTING ELEMENTS
When asked how many different elements are
in a chemical formula, remember that each
element symbol starts with a capital letter. Be
careful not count an element twice if repeated
in the formula.
How many elements in the formula below?
HC2H3O2
Be careful… only count Hydrogen as an element once!
COUNTING ATOMS
To count the total number of atoms in a
formula, count each symbol without a
subscript number as one atom, and then add
together the rest of the subscript numbers.
How many atoms in the formula below?
HC2H3O2
Law of Conservation of Matter
In a chemical reaction, matter cannot be created
or destroyed. The atoms that were present
before the reaction took place simply rearranged
themselves into new substances.
In a baking soda/vinegar reaction the two
ingredients NaHCO3 and HC2H3O2, are rearranged
into three different substances with the same
number and kind of atoms:
NaC2H3O2, H2O and CO2
CHEMICAL EQUATIONS
A chemical equation is a description of a chemical
reaction, using the chemical formulas for each
reactant (ingredient) on the left side of the
equation and the chemical formulas of each
product (the end result) on the right side of the
equation. A right pointing arrow separates the
reactants from the products.
Al + CuCl2  AlCl2 + Cu
(Reactants)
Yield
(Products)
COEFFICIENTS
If there is more than one molecule of each
reactant or product, a large coefficient
number is placed in front of that substance’s
formula to show the number of molecules.
The coefficient number should be multiplied
by each subscript in the formula to find the
number of atoms (up to the + sign or  sign
only).
2Al + 3CuCl2  2AlCl3 + 3Cu
Coefficients
Recognizing Balanced Equations
Because of the Law of Conservation of Matter,
there must be exactly the same number and kind
of atoms on each side of a chemical equation. A
correctly stated chemical equation is said to be
balanced when the same number and kind of
atoms are on both sides.
Count the atoms of each element to see if this
equation is balanced.
K3PO4 + 3HCl 3KCl + H3PO4
(PARENTHESIS)
If part of the formula is enclosed in a
parenthesis, with an outside subscript number
to the right of the parenthesis pair, the
subscript number should be multiplied by the
subscript numbers for each symbol within.
For example in CO(NH2)2 there are a total of 8
atoms: one Carbon (C), One Oxygen (O), two
Nitrogen (N) and four Hydrogen (H).
UNBALANCED EQUATIONS
If an equation is not balanced, you must
balance it by changing coefficients ONLY!
You may never…ever change the subscripts!!!
Help me to balance the following equation:
K3PO4 + HCl  KCl + H3PO4
Warm-up 11/9/15
What do Lithium, Sodium, Potassium,
Rubidium, Cesium, and Francium have in
common?
How are they different? What do you think
more electrons might mean for reactivity?
https://www.youtube.com/watch?v=
m55kgyApYrY