Atoms and Bonding

Interpreting and Writing ChemicalFormulas

Chemical Formulas represent compounds.

Interpreting Chemical Formulas• Coefficient

• Number that is placed in front of a chemical formula or a symbol in a chemical equation that indicates the number of moles or molecules of this substance involved in the reaction

• Subscript• Number placed to the lower right of a

chemical symbol to indicate the number of atoms of the element in the compound.

Interpreting Chemical Formulas

• 2Na2O• 2 = coefficient= 2 moles of this

element• Number of atoms for each element

coefficient * subscript• If you do not see a subscript, it is

understood to be 1• Na = 2*2 = 4• O = 2*1 = 2• Total # of atoms = 4+2 = 6

Interpreting Chemical Formulas• 3Ba3(PO4)2

• Ba = 3*3 = 9• P = 3* (2*1) = 3*2 = 6• O = 3* (4*2) = 3*8 = 24• Total # of atoms = 9+6+24 = 39

• When you have parenthesis, you must multiply the subscript on the outside of the parenthesis by every subscript on the inside of the parenthesis before you multiply by the coefficient

Writing Chemical Formulas

• You need to know the chemical symbol for the element

• You need to know the element’s oxidation number• Oxidation number is the number of valence

electrons an element gains, loses, or shares during bonding

• For metals, the oxidation number is positive• For nonmetals, the oxidation number is negative• Column 14 is considered an exception; its

oxidation number is +/-4 depending on what it is bonding with

Writing Chemical Formulas

• Metals• Oxidation number = # of valence electrons

it gives away when bonding (in short = number of valence electrons

• Nonmetals• Oxidation number = # of valence electrons

– 8• Family 14

• Oxidation number is +/-4• If 1st element in compound will be +4; if second

element in compound will be -4

• Family 18• Oxidation number is 0

Understanding Chemical Formulas • Chemical

formulas are composed of a positive half and a negative half.

• Water is a compound you know to have a formula of H2O.

Understanding Chemical Formulas• The element with the positive

oxidation number is always written first.

• The element with the negative oxidation number is always written second.

• H O+1 -2

Add subscripts after a chemical symbol, when needed, to make the oxidation numbers total zero.

H2O

Understanding Chemical Formulas

• The total of the oxidation numbers in a compound must equal zero.

The easiest way to think of writing chemical formulas is to use the oxidation number (without the + or -) of one element as the subscript of the other element.

+2 -1

+2 -1

Writing and Understanding Chemical Formulas

Cross over the oxidation numbers without the charges!!!

2

Remember, DO NOT write a subscript of 1.

Writing and Understanding Chemical Formulas

Examples of Writing Chemical Formulas

Examples of Writing Chemical Formulas