Chemical Formulas: The Basics

Subscripts in Chemical Formulas numbers inside formulas numbers inside formulas lowered below line of type lowered below line of type show # of atoms of each element show # of atoms of each element

Rules for Subscripts in Formulas No subscript – understood to be 1 No subscript – understood to be 1 Subscripts refer to element immediately preceding number [Remember – every element symbol starts with capital letter] Subscripts refer to element immediately preceding number [Remember – every element symbol starts with capital letter] Subscripts after parentheses refer to everything inside ( ) Subscripts after parentheses refer to everything inside ( )

Chemical Formulas H 2 O has: 2 hydrogens and 1 oxygen H 2 O has: 2 hydrogens and 1 oxygen NH 3 has: 1 nitrogen and 3 hydrogens NH 3 has: 1 nitrogen and 3 hydrogens CH 4 has ? CH 4 has ? CO 2 has ? CO 2 has ? AgNO 3 has ? AgNO 3 has ? 1 carbon and 4 hydrogen 1 carbon and 2 oxygen 1 silver, 1 nitrogen, 3 oxygen

# atoms in formulas Al(NO 3 ) 3 Al(NO 3 ) 3 Al = Al = N = N = O = O = Total = Total = Subscript outside ( ) multiplied by subscript inside ( ) = # of atoms1 1 x 3 = 3 3 x 3 = 9 13 atoms

Coefficients Numbers in front of formulas Numbers in front of formulas Refers to everything following in formula Refers to everything following in formula No coefficient? It’s understood to be 1 No coefficient? It’s understood to be 1

Coefficients 2H 2 O means 2(H 2 O) 2H 2 O means 2(H 2 O) To count atoms - To count atoms - multiply: coefficient x subscript multiply: coefficient x subscript 2 x 2 = 4 H’s 2 x 2 = 4 H’s 2 x 1 = 2 O’s 2 x 1 = 2 O’s

Hydrates group of salts that have water molecules stuffed in their empty spaces group of salts that have water molecules stuffed in their empty spaces Ex: CuSO 4  5H 2 O Ex: CuSO 4  5H 2 O  means “is associated with” or “included”  means “is associated with” or “included” DOES NOT refer to multiplication DOES NOT refer to multiplication Not a true chemical bond: Not a true chemical bond: structure is definite so structure is definite so # empty spaces is definite & # empty spaces is definite & # water molecules is definite # water molecules is definite

CuSO 4  5H 2 O Count up the atoms! Count up the atoms! 1 x Cu 1 x S 4 x O 10 x H 5 x O total = 21 atoms

5[CuSO 4  5H 2 O] Count up the atoms! Count up the atoms! 5 x Cu 5 x S 20 x O 50 x H 25 x O total = 105 atoms Coefficients x Subscripts