The atomic number is the number of protons. Remember that in a neutral atom, the number of protons and electrons is equal, but the electrons come and go, so the proton number is a reliable indicator of the chemical properties of that element.

Mass number is the sum of protons and neutrons. Electrons are so light that they do not contribute to the mass number and are ignored.

The atomic mass (or atomic weight) is the average mass of all naturally occurring combinations of protons and neutrons in the nucleus. That is, the atomic mass depends on what isotopes of the element are found in nature.

If we use an atomic butterfly net (not yet invented, but we can imagine it) and capture 100 atoms of hydrogen, we find that one out of 100 H atoms in nature is hydrogen 2, deuterium (²H). This is how we calculate the atomic mass (or atomic weight) that is shown in the periodic table. If 99 atoms are ¹H, with a mass of 1 Da each, and 1 atom is ²H, with a mass of 2 Da, then the total mass of the 100 hydrogen atoms is 101 (99+2) and the average mass is 1.01 Da. If the math freaks you out, notice there are 100 hydrogen atoms in the figure above. Count the number of large particles (red protons and purple neutrons) and divide by 100 to get the average mass.

Carbon exists as ¹²C and ¹³C. There are about 99 ¹²Cs for every one ¹³C, so its atomic mass is 12.01. Nitrogen exists as ¹⁴N or ¹⁵N, and there are about 99 ¹⁴Ns for every ¹⁵N. That gives an atomic mass of 14.01.

The most common isotopes of iron are ⁵⁴Fe, ⁵⁶Fe, ⁵⁷Fe and ⁵⁸Fe. What do the numbers 54, 56, 57, and 58 represent? How does that compare to 26, the atomic number for iron? How many neutrons are present in each case? (Hint: 54 massive particles are found in the nucleus of ⁵⁴Fe, and the number of protons is always 26, so the number of neutrons is 54 – 26 = 28.)

Wikipedia says that the proportions of iron isotopes are 5.8% of ⁵⁴Fe, 91.7% of ⁵⁶Fe, 2.1% of ⁵⁷Fe and 0.3% of ⁵⁸Fe. Given this information, you can check and see if it agrees with the block of the periodic table. Since the most common isotope by far is ⁵⁶Fe, we expect to get a number close to 56, and we do.

By rounding the atomic mass, we can easily determine the most common isotope of the element, with a couple of exceptions that you will not be tested on. The atomic mass of hydrogen is 1.01, and the most common isotope of hydrogen is ¹H, as we’ve already explained.

The atomic mass of carbon is 12.01, and the most common isotope of carbon is ¹²C. The atomic mass of nitrogen is 14.01, and the most common isotope of nitrogen is ¹⁴N. The atomic mass of iron is 55.85, and the most common isotope of iron is ⁵⁶Fe.