# In the reaction above, X is…

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A. electron.

B. neutron.

C. neutrino.

D. proton.

B

## DETAILS…

The sum of the mass numbers on the right-hand side = that on the left-hand side

Also, the sum of the atomic numbers on the right-hand side = that on the left-hand side.

Sum of mass numbers on the left-hand side = 235 + 1 = 236

Hence, 236 = 144 + 90 + 2X

So, the mass number of 2X = 236 – 144 – 90 = 2

Therefore, mass number of X = 2/2 = 1

Also, sum of atomic numbers on the left-hand side = 92 + 0 = 92

Hence, 92 = 56 + 36 + 2X

2X = 92 – 56 – 36

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2X = 0

X = 0

This means X = 1 mass number, and 0 atomic number

X = neutron.

### Now for the right answer to the above question:

1. Option A is incorrect. electron would have 0 mass number and -1 atomic number.
2. Option B is correct. neutron have these features.
3. C is incorrect. neutrinos are special kind of sub-atomic particles with almost zero mass, they are seldom represented in nuclear equations.
4. D is not the correct answer. A proton would have 1 mass number and 1 atomic number.