SME
Grade 7 Science · Problem type
Counting atoms in water molecules
Use this for “What atoms make up a molecule of water?” and “How many atoms are in 6 water molecules?”
Diagram
3 H₂O molecules (water), 3 atoms in each
Hydrogen atoms = 2 × 3 = 6
Oxygen atoms = 1 × 3 = 3
This is an example. Type your own values to start a new problem.
n
Water molecules
example
h
Hydrogen atoms
worked out (not typed)
o
Oxygen atoms
worked out (not typed)
A
Atoms in all
worked out (not typed)
Formulas
h
=
2
×
n
(hydrogen
atoms
=
2
×
water
molecules)
6 = 2 × 3
o
=
n
(oxygen
atoms
=
water
molecules)
3 = 3
A
=
h
+
o
(atoms
in
all
=
hydrogen
atoms
+
oxygen
atoms)
9 = 6 + 3
Assumptions
•
H₂O means 2 hydrogen atoms and 1 oxygen atom in each molecule; a small number counts the atom before it.
•
Every water molecule is the same, so the atoms grow in step with the molecules.
•
A count of atoms is a whole number.
Step-by-step
WE KNOW
n = 3 (water molecules)
FIND
hydrogen atoms (h), oxygen atoms (o), atoms in all (A)
Step 1 · Find hydrogen atoms (h)
Use h = 2 × n (hydrogen atoms = 2 × water molecules)
Each water molecule has 2 hydrogen atoms: multiply by the molecules.
h
=
2
×
3
h
=
6
Step 2 · Find oxygen atoms (o)
Use o = n (oxygen atoms = water molecules)
Each water molecule has 1 oxygen atom.
o
=
3
Step 3 · Find atoms in all (A)
Use A = h + o (atoms in all = hydrogen atoms + oxygen atoms)
Add the hydrogen atoms and the oxygen atoms.
A
=
6
+
3
A
=
9
Check
Put the numbers back in. Both sides should match.
6 = 2 × 3 ✓
3 = 3 ✓
9 = 6 + 3 ✓
Topic check
See what you know: 5 questions on this topic
New questions each time. Pass to mark it complete.