SME
Grade 10 Science · Problem type
Fission of uranium-235
Use this for “U-235 takes in a neutron and splits into Ba-141, a second nucleus and 3 neutrons. What is the second nucleus?”
Diagram
Mass numbers (top) and atomic numbers (bottom) add to the same on both sides.
Neutrons in the first fragment: 85
This is an example. Type your own values to start a new problem.
235
92
U
+
1
0
n
→
X
+
Y
+
k
1
0
n
N₁
Neutrons in the first fragment
worked out (not typed)
Formulas
A₂
=
236
−
A₁
−
k
92 = 236 − 141 − 3
Z₂
=
92
−
Z₁
36 = 92 − 56
N₁
=
A₁
−
Z₁
85 = 141 − 56
Assumptions
•
A slow neutron splits the uranium-235 nucleus into two smaller ones and 2 or 3 neutrons.
•
Mass numbers and atomic numbers add to the same on both sides: 235 + 1 = 236 and 92.
Step-by-step
WE KNOW
A₁ = 141, Z₁ = 56, k = 3
FIND
mass number of the second fragment (A₂), atomic number of the second fragment (Z₂), neutrons in the first fragment (N₁)
Step 1 · Find mass number of the second fragment (A₂)
Use A₂ = 236 − A₁ − k
The left side’s mass numbers add to 235 + 1 = 236; take the first fragment’s and the neutrons’.
A₂
=
236
−
141
−
3
A₂
=
92
Step 2 · Find atomic number of the second fragment (Z₂)
Use Z₂ = 92 − Z₁
Uranium’s 92 protons are shared by the two fragments; neutrons carry none.
Z₂
=
92
−
56
Z₂
=
36
is
krypton:
Kr-92
Z₂
=
36
Step 3 · Find neutrons in the first fragment (N₁)
Use N₁ = A₁ − Z₁
Take the protons from the mass number.
N₁
=
141
−
56
N₁
=
85
Check
Put the numbers back in. Both sides should match.
92 = 236 − 141 − 3 ✓
36 = 92 − 56 ✓
85 = 141 − 56 ✓
Topic check
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