Question

Two different atoms have 11 protons each, but one has 12 neutrons, and the other has 13 neutrons. How do their atomic numbers and mass numbers compare? Are they the same element or different elements?

Answer 1

Both atoms have atomic number 11 because each has 11 protons. Their mass numbers are 23 (11 + 12) and 24 (11 + 13). They are the same element, sodium, but are different isotopes.