SAT Subject Physics Practice Question 147: Answer and Explanation

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Question: 147

10. What’s the missing particle in the following nuclear reaction?

A. Proton
B. Neutron
C. Electron
D. Positron
E. Gamma

Correct Answer: E

Explanation:

To balance the mass number (the superscripts), we must have 196 + 1 = 197 + A, so A = 0. To balance the charge (the subscripts), we need 78 + 0 = 78 + Z, so Z = 0. The only particle listed that has zero mass number and zero charge is a gamma-ray photon, .

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