During beta decay, a radioactive atomic nucleus emits either an electron (β⁻ decay) or a positron (β⁺ decay). If a cobalt-59 atom is formed from the β⁻ decay of a radionuclide, what must the original nucleus have been?

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11ecba2d_123a_ca32_a3bf_2fed069ea29b_MD0008_00 In radioactive nuclei, beta decay can occur in three forms: β⁻ decay (electron emission), β⁺ decay (positron emission), and electron capture. In β⁻-decay, a neutron converts to a nuclear proton and emits an electron. In β⁺ decay, a nuclear proton converts to a neutron (the opposite of β⁻ decay) and ejects a positron (an electron with a positive charge). In electron capture, a proton captures an electron near the nucleus and converts to a neutron without a positron or electron emission. In all three forms of beta decay, the mass number remains unchanged whereas the atomic number either increases (β⁻ decay) or decreases (β⁺ decay and electron capture) by 1. As the atomic number changes, the identity of the element changes accordingly. Consequently, if an atom of ⁵⁹Co were formed as the result of a β⁻ decay, the original radionuclide would have the same mass number (59) but its atomic number would be one unit less, as shown in the following nuclear reaction: 11ecba2d_123a_ca33_a3bf_91b8622c7d9a_MD0008_00 Therefore, ⁵⁹Co would be formed by the β⁻ decay of ⁵⁹Fe. (Choice B) During β⁻ decay, the mass number remains unchanged and the atomic number increases by one unit. ⁶⁰Co has a higher mass number and an unchanged atomic number, which is inconsistent with a β⁻ decay precursor. (Choice C) Forming a ⁵⁹Co nucleus from ⁵⁹Ni would require ⁵⁹Ni to undergo β⁺ decay (positron emission) or electron capture instead of the β⁻ decay (electron emission) specified by the question. (Choice D) A ⁵⁹Co nucleus could form from ⁶³Cu only if ⁶³Cu were to undergo alpha decay, but the question states that ⁵⁹Co was formed by β⁻ decay. Educational objective: In all forms of beta decay, the mass number remains unchanged whereas the atomic number increases (β⁻ decay) or decreases (β⁺ decay and electron capture) by 1. β⁻ decay converts a neutron into a proton and emits an electron. β⁺ decay and electron capture convert a proton into a neutron (the opposite of a β⁻ decay).

During Beta Decay, a Radioactive Atomic Nucleus Emits Either an Electron