If it has a third energy level then you know to look on the third row (period) of the periodic table. Since it has only 1 electron in that level you know to look in the first column (group) of the periodic table. Row plus column meets at sodium.
To check our answer, sodium has an atomic number of 11 (11 protons) and a mass of 22.99 (23 for normal sodium) which indeed gives a neutron number of 12.
To check our answer, sodium has an atomic number of 11 (11 protons) and a mass of 22.99 (23 for normal sodium) which indeed gives a neutron number of 12.
