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? [Pg. No. 156]

Both atoms have 11 protons, so their atomic number is the same: 11. Since atomic number decides the element, both atoms are of the same element, sodium.

For the first atom:

Mass number = 11 + 12 = 23

For the second atom:

Mass number = 11 + 13 = 24

So, their atomic numbers are the same, but their mass numbers are different. They are isotopes of the same element because they have the same number of protons but different number of neutrons.