Answer: By looking at the periodic table, you will see that Fluorine has 9 protons. Since the number of electrons equal the number of protons, Fluorine has 9 electrons as well. Meanwhile, it's mass number of 19, minus 10 neutrons, gives you 9 protons or electrons. Hence, the atom would be Fluorine.
There are now two oxygen atoms on the left and four on the right (in one N2O and three H2O's), so we balance the oxygen atoms by placing a 2 in front of the O2.
