SME
Grade 8 Science · Problem type
Protons, neutrons and electrons from the card
Use this for “An atom has 8 protons and a mass number of 16. How many neutrons does it have?”
Diagram
Z = 8: Oxygen (O), group 16, period 2
This is an example. Type your own values to start a new problem.
n
=
A
−
Z
p
Protons
worked out (not typed)
e
Electrons
worked out (not typed)
Formulas
p
=
Z
(protons
=
atomic
number)
8 = 8
e
=
Z
(electrons
=
atomic
number)
8 = 8
n
=
A
−
Z
(neutrons
=
mass
number
−
atomic
number)
8 = 16 − 8
Assumptions
•
Protons and neutrons sit in the nucleus; electrons move round it.
•
The mass number is the protons plus the neutrons. Round the card’s mass to the nearest whole number for the common form of the element.
•
Atoms of one element can differ in neutrons (isotopes); they never differ in protons.
Step-by-step
WE KNOW
Z = 8 (atomic number), A = 16 (mass number)
FIND
protons (p), electrons (e), neutrons (n)
Step 1 · Find protons (p)
Use p = Z (protons = atomic number)
The atomic number is the number of protons.
p
=
8
Step 2 · Find electrons (e)
Use e = Z (electrons = atomic number)
A neutral atom has as many electrons as protons.
e
=
8
Step 3 · Find neutrons (n)
Use n = A − Z (neutrons = mass number − atomic number)
The mass number counts protons and neutrons together, so take the protons away.
n
=
16
−
8
n
=
8
Check
Put the numbers back in. Both sides should match.
8 = 8 ✓
8 = 8 ✓
8 = 16 − 8 ✓
Topic check
See what you know: 5 questions on this topic
New questions each time. Pass to mark it complete.