Reading Chemical Formulas:
Element- made up of only one kind of atom. The simplest form of matter.
Molecule- when two or more atoms combine (Example: O2)
Compound- when two or more different atoms combine (Example: CO2)
Every element on the periodic table is represented by a symbol. Molecules and compounds are represented by a
combination of the element’s symbol and numbers representing the number of atoms or molecules in the
compound.
For example, H2 is the chemical formula for a molecule of Hydrogen. The H is the chemical
symbol for hydrogen and the subscript 2 indicates that there are 2 atoms of hydrogen in that
molecule.
H2O is the compound for water representing two atoms of Hydrogen and one of Oxygen!
Coefficient- The number in front of the element or compound. It is equal to the number of
molecules!
Subscript- The number after and below the element symbol. It is equal to the number of atoms of
that element present in the compound!
**If there is NO coefficient or subscript it is understood to be 1**
A chemical equation is a way of describing a chemical reaction using chemical formulas. Ex:
Hydrogen and Oxygen atoms react chemically to form water. The chemical equation that shows
this is
H2 + O 

***We call the molecules on the left side of the equation are always called the
REACTANTS. They “react” to form the product. The molecules on the right side of the
equation are always called the PRODUCTS***
Law of conservation of mass- states that matter cannot be created or destroyed. This
means that the number and kind of atoms in the reactants must equal the number and
kinds of atoms in the product! If that is true then we say the equation is balanced!
PRACTICE:
C6H12O6
Has 3 elements: Carbon (C), Hydrogen (H), and Oxygen (O)
Has 6 atoms of carbon, 12 atoms of Hydrogen, and 6 atoms of oxygen
2H2O
Has 2 molecules of water. So to find the number of atoms of each element we have the distribute the
coefficient. That means multiply the coefficient by the subscript.
2 x 2 = 4 so there are 4 atoms of Hydrogen
2 x 1 = 2 remember if there is no subscript we assume it is one so there are 2 atoms of oxygen
Chemical Reactions
In a chemical reaction elements are rearranged to create
new substances with
different properties :) Don't forget that these equations must be balanced (have the same
number of atoms of each element on both sides) because the Law of Conservation of Mass says that
matter cannot be created or destroyed.
Evidence that a CHEMICAL CHANGE has occurred: That
means if you see any of the things below a chemical
change NOT a physical change has occurred:
Please- Precipitate (a solid that forms from a solution)
Excuse- Endothermic (heat is absorbs, it feels cold)/ Exothermic (heat is released it feels hot)
Coughs-Color change (unexpected not from something like food coloring)
Sneezes-Smell (A new smell is produced)
Burps-Bubbles (a gas is being produced)
Or-Oxidation (rust)
Chunks-Combustion (fire)