Identify the missing information for each neutral isotope. A Se atom has a mass number of 77 . Determine the number of neutrons, protons, and electrons in this neutral isotope.

Answer 1

Hi Jessica! To answer this question, you need to know a few things.

1. The atomic number of any element is, by definition, the number of protons that element contains (you can find the atomic number in the top left hand corner of each element on the periodic table).
2. Protons and neutrons both weigh 1 atomic mass unit (amu), while electrons are so tiny they don't weigh anything (they do have weight, it is just so tiny we ignore it).
3. In an atom that has neutral charge (no net positive or negative), the number of protons and electrons are the same.

Since Se is element 34, it contains 34 protons. Because the atom is neutral and has no net positive or negative charge, we know there are also 34 electrons. This last part I am a little confused- looking at the periodic table, Se actually has a weight of 78.97 amu (you may have typed the wrong number in your question, or your teacher gave you an incorrect number). Since the weight of an atom is (total weight) = (number of protons * 1 amu) + (number of neutrons * 1 amu), we can calculate that 78 = 34 - x (number of neutrons). Subtraction shows that the number of neutrons is 45. So, there are 45 neutrons, 34 protons, and 34 electrons in a neutral Se atom. I hope this helped you!