AP Chemistry Question?

Bromine is a halogen that tends to gain 1 electron to have a full valence shell. But notice there are two bromide ions meaning the "X" has a 2+ charge, so it had to have lost two electrons to the two bromines.

So that means when the ion X is a neutral atom, it actually has 88 electrons. And remember that the atomic number = number of protons = number of electrons in an atom and it's the protons that give the element it's identity. Therefore, X = Radium.

Now when you look at the periodic table, you'll notice that it says that Ra's atomic mass is 226 amu but keep in mind that most elements have isotops. So in this particular case, Ra has an additional 4 neutrons.

But the thing to keep in mind above all else:

-It's the # of protons that give an element its identity, not it's atomic mass nor the # of electrons.

-In an atom (not an ion), the # protons = # electrons.

because the element has 86 electrons, it will have the same number of protons, and that leads us to Radon

230 - 86 = 134 neutrons