Download Counting Atoms Worksheet

Chemical Reactions: Counting Atoms
Text-Dependent Questions
1.
What do the element symbol and subscript in a
compound represent?
2.
If the element does not have a subscript (i.e.
CaCl2), how many atoms are present?
3.
Explain how subscripts work with polyatomic
ions present in a compound. Provide an example.
4.
Application: How many atoms of each element is
present in Mg3(PO4)2?
Directions: Read the passage to learn about balancing chemical reactions.
The formula for a compound indicates the elements that make up the compound
and the number of atoms of each element present in the compound.
These numbers of atoms are indicated by the use of small numbers
called subscripts.
Sometimes groups of atoms act as a single atom. Such a group of atoms is
called a polyatomic ion. If a polyatomic ion is used in a formula more than
once, it is put in parentheses and the subscript appears outside the parentheses.
When a subscript appears outside the parentheses, it indicates that all the
elements inside the parentheses should be multiplied by that subscript. For
example, the formula Fe(OH)3 indicates the combination of 1 atom of Fe,
3 atoms of O, and 3 atoms of H.
In the formula of a chemical reaction, numbers will be present before
each compound, these are called coefficients.
H2 O
Fe(OH)3
2 H2O
# Atoms of H: 2(2) = 4
# Atoms of O: 2(1) = 2
2H2O2  2H2O + H2
A coefficient is a number written in front of a chemical formula and
indicates the number of molecules of that compound. Coefficients apply
to the entire compound. A coefficient multiplies the number of atoms of
each element in the compound. Below are 2 examples:
Mg atoms= ____________
P atoms= ____________
O atoms= ____________
5.
3 CuSO4
___3_____ molecules of CuSO4
What information do the coefficients provide?
4 Pb(NO3)2
__4___ molecules of Pb(NO3)2
6. Complete the table below:
formula
Atoms in formula
___3(1) =3_____ Cu atoms (copper)
_4(1) = 4 Pb (Lead)
C9H8O4
(aspirin)
_3(1) = 3_______ S atoms (sulphur)
4(2) = 2_ N (nitrogen)
5NaCl3
_3(4) = 12_______ O atoms(oxygen)
4(3x2) = 24 O (oxygen)
3(NH2)3O2
Counting Atoms Practice
Name _______________________________
Directions for each problem:
1) write down the different elements in each compound.
2) write down how many of that particular atom there are
3) how many atoms are there total in the compound.
Examples:
LiOH
2 NaCl3
5 Si(SO4 )
3 (NH2)3O2
Li -
Na –
Si –
N-
O -
Cl –
S–
H-
H–
Total:
1) NaOH
2) MgCl2
3) Li2SO4
4) NaC2H3O2
OTotal:
Total:
6) 4 HNO3
7) 4 Li2O
8) 3 H2O
9) 3 Al2O3
OTotal:
11) 4 Mg(OH)2
12) 2 NaOH
13) 4 Al2(SO3)3
14) 2 (NH4)3PO4