SME
Grade 10 Science · Problem type
Alpha decay
Use this for “Uranium-238 gives off an alpha particle. What nucleus is left?”
Diagram
Mass numbers (top) and atomic numbers (bottom) add to the same on both sides.
An alpha particle is a helium nucleus: 2 protons and 2 neutrons.
Neutrons before: 146
This is an example. Type your own values to start a new problem.
X
→
Y
+
4
2
He
N
Neutrons before
worked out (not typed)
Formulas
N
=
A
−
Z
146 = 238 − 92
A
=
A
′
+
4
238 = 234 + 4
Z
=
Z
′
+
2
92 = 90 + 2
Assumptions
•
The mass numbers (top) on the two sides add to the same total.
•
The atomic numbers (bottom) on the two sides add to the same total.
•
The alpha particle carries away 4 in mass number and 2 in atomic number.
•
The new atomic number names the new element.
Step-by-step
WE KNOW
A = 238, Z = 92
FIND
neutrons before (N), mass number after (A′), atomic number after (Z′)
Step 1 · Find neutrons before (N)
Use N = A − Z
The nucleons that are not protons are neutrons.
N
=
238
−
92
N
=
146
Step 2 · Find mass number after (A′)
Use A = A′ + 4
The alpha particle takes 4 of the mass number.
A
′
=
A
−
4
A
′
=
238
−
4
A
′
=
234
Step 3 · Find atomic number after (Z′)
Use Z = Z′ + 2
The alpha particle takes 2 of the atomic number.
Z
′
=
Z
−
2
Z
′
=
92
−
2
Z
′
=
90
is
thorium:
Th-234
Z
′
=
90
Check
Put the numbers back in. Both sides should match.
146 = 238 − 92 ✓
238 = 234 + 4 ✓
92 = 90 + 2 ✓
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Beta decay
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Nuclear chemistry: radioactive decay, half-life, fission and fusion
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