SME
Grade 10 Science · Chemistry
Atomic structure and isotopes
Use this for “An atom has 13 protons and 14 neutrons. What is its mass number, and how many electrons does it have?”
Diagram
Aluminum-27: 13 protons, so atomic number Z = 13.
Mass number A = 13 + 14 = 27.
13 electrons: charge 13 − 13 = 0, a neutral atom.
Shells: 2, 8, 3.
This is an example. Type your own values to start a new problem.
A
=
Z
+
N
q
=
Z
−
e
Formulas
A
=
Z
+
N
27 = 13 + 14
q
=
Z
−
e
0 = 13 − 13
Assumptions
•
The number of protons, the atomic number Z, names the element.
•
Isotopes of an element differ only in neutrons, so only the mass number changes.
•
Electrons have almost no mass, so they are left out of the mass number.
Step-by-step
WE KNOW
Z = 13, N = 14, e = 13
FIND
mass number (A), charge (q)
Step 1 · Find mass number (A)
Use A = Z + N
The mass number counts every particle in the nucleus.
A
=
13
+
14
A
=
27
Step 2 · Find charge (q)
Use q = Z − e
Each proton adds +1 and each electron −1.
q
=
13
−
13
q
=
0
Check
Put the numbers back in. Both sides should match.
27 = 13 + 14 ✓
0 = 13 − 13 ✓
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Ions: gaining and losing electrons
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