Counting Atoms in Chemical Formulas Problems Notes/Examples
Name:________________________Date:___________Section:_______
Part A, #2
SiO2
1 atom silicon (Si), 2 atoms oxygen (O)
*The subscript following the element’s symbol tells you how many atoms
of that element there are in the formula. If there is no subscript
following an element’s symbol, it is understood to be a one.
Part A, #6
CH4CH2CH2CH3
4 atoms carbon (C), 11 atoms hydrogen (H)
*If the same element is repeated throughout the formula, add the
subscripts together.
Part A, #10
(C2H5)2O
4 atoms carbon (C), 10 atoms hydrogen (H), 1 atom oxygen (O)
*If there is a subscript outside the parentheses than multiply that
subscript by the subscripts inside the parentheses.
Part A, #14
Fe4[Fe(CN)6]3
7 atoms iron (Fe), 18 atoms carbon (C), 18 atoms nitrogen (N)
*If there are brackets and parentheses, determine the parentheses first,
than multiply the number outside the brackets by everything inside the
brackets.
Part B, #2
Al2O3  SiO3  9H2O
Al – 2
Si – 1
H – 18
O–3
O–3
O-9
2 atoms aluminum (Al), 1 atom silicon (Si),
18 atoms hydrogen (H), 15 atoms oxygen (O)
*The  between formulas acts as an addition sign. Determine the number
of atoms of each element in each formula first (separately) and then add
like atoms together. If there is a number in front of a formula (it is
called a coefficient), distribute (multiply) the number to everything in the
formula.
BrassbridgeChemistry/UnitTwo/NotesCountAtoms