Identify the missing information for each atom or ion.

Question

Note that the atoms and ions are not necessarily neutral. A Se ion has a mass number of 77 and a charge of −2 . Determine the number of neutrons, protons, and electrons in this ion.

Answer 1

33 neutrons, 34 protons, and 36 electrons.

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Protons will always be equal to the atom's atomic number on the periodic table. Selenium (Se) has an atomic number of 34. Also note that electrons don't significantly contribute to an atom's mass, while protons and neutrons each contribute 1 amu to mass number (amu is the units for mass number).

That being said, we know that with an atomic number of 34, the Se^2- ion has 34 protons.

Given a mass number of 77, 77 - 34 amu (from 34 protons) = 33 neutrons.

-2 charge indicates there are 2 more electrons than protons. 34 protons + 2 = 36 electrons.