Imagine an element that has 15 protons, 16 neutrons, and 18 electrons. Which of the following would NOT be considered an isotope of this element? * 15 protons, 16 neutrons, and 15 electrons
15 protons, 17 neutrons, and 15 electrons
15 protons, 15 neutrons, and 18 electrons

A: 15 protons, 16 neutrons, and 15 electrons

Explanation: An isotope of an element is when neutrons are taken away from the nucleus, or added to the nucleus.  

If you change the amount of protons in the nucleus, not only will the charge become positive due to their being one more proton than an electron, but the atom will change to a new element, Sulfur. If you change the electron count, that will just affect how prominent the negative charge is on the atom, whether it be more negative by adding an electron, or more postitive by removing an electron. Therefore, the amount of neutrons that you add to the nucleus of an atom, will change the isotope of the atom, and that's why the answer is A because every other answer changes the neutron count other than A.