Atoms of a given element have a defined number of protons, neutrons, and electrons, but atoms can gain or lose electrons to form ions. How many neutrons and electrons are present in a ⁸⁰Se²⁻ ion formed from a neutral selenium-80 atom?

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11ecba2d_1238_802c_a3bf_dbece3650534_MD0008_00 Isotopes are specified either by using a chemical symbol with the mass number given as a superscript to the upper left (eg, ⁸⁰Se) or by stating the element name hyphenated with its mass number (eg, selenium-80). Because the atomic number (the number of protons) determines the identity of an element and is the same for all atoms of that element, the atomic number is not always explicitly stated in isotope notation; however, when stated, it is placed as a subscript to the lower left of the chemical symbol (below the mass number). An atom of any isotope in its elemental state will be neutral (have no net charge), and the number of negatively charged electrons will be equal to the number of positively charged protons. However, elements may react to form ions by gaining or losing electrons. A neutral atom will acquire a net charge of +1 for each electron lost and a net charge of −1 for each electron gained. Selenium-80 has a mass number (protons plus neutrons) of 80. Because all atoms of selenium have 34 protons, the number of neutrons is found by subtracting the atomic number (protons) from the mass number (80 − 34 = 46). A neutral (uncharged) atom of selenium-80 in its elemental state has 34 protons and 34 electrons. However, the −2 charge in the ⁸⁰Se²⁻ ion indicates that the ⁸⁰Se atom has gained two additional electrons during ion formation. Therefore, a ⁸⁰Se²⁻ ion has 46 neutrons and 36 electrons. (Choice A) The mass number (protons plus neutrons) is 80. (Choices C and D) ⁸⁰Se has 34 protons, 46 neutrons, and 34 electrons, but ⁸⁰Se²⁻ has 34 protons, 46 neutrons, and 36 electrons. Educational objective: Isotopes are specified by their mass number (the number of protons and neutrons in the nucleus). The number of neutrons in an isotope can be determined by subtracting the element's atomic number (number of protons) from the mass number. An atom in its elemental state is neutral (equal number of protons and electrons) but acquires a net charge of +1 for each electron lost and −1 for each electron gained during ionization.

Atoms of a Given Element Have a Defined Number of Protons