Atoms and Bonding Interpreting and Writing Chemical Formulas

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 2Na 2 O 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 3Ba 3 (PO 4 ) 2 Ba = 3*3 = 9 P = 3* (2*1) = 3*2 = 6 O = 3* (4*2) = 3*8 = 24 Total # of atoms = = 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 1 st 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 H 2 O.

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. H2OH2O 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

+2 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