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MCAT Physics Question — Uranium

Uranium, U-235, undergoes beta decay giving off an electron. The resulting atom is:


a) Th-231

b) Pa-235

c) Np-235

d) U-234



In beta decay, an electron and a proton are created from a neutron. The electron is expelled leaving an atom with an increased atomic number and the same mass number. Remember: atomic number is the number of protons and mass number is the sum of protons and neutrons. Uranium, U-235, undergoes beta decay giving off an electron. The resulting atom has the same mass number, 235, and an atomic number increased by 1 to 93. The answer is Np-235.


a) Th-231, incorrect, This is the result of alpha decay losing a helium nucleus.

b) Pa-235, incorrect, Pa-235 results from positron emission when a proton becomes a neutron.

c) Np-235, correct.

d) U-234, incorrect, U-234 indicates loss of a neutron and decreased mass number. This does not occur because a proton is created leaving the mass number the same as before the decay.

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