Most chemistry textbooks have a periodic table printed on the inside cover.
If the number includes a decimal point, you’re probably looking at atomic mass instead.
You may see gaps after elements 56 (Barium) and 88 (Radium). There’s no actual gap; the elements with those atomic numbers are located in two rows below the rest of the chart. They are only separated like this to fit the periodic table in a more narrow shape.
To put it another way, every atom with eight protons is an oxygen atom. Two oxygen atoms might have different numbers of neutrons or (if one is an ion) different numbers of electrons, but they will always have eight protons each.
Some scientists prefer the term “relative atomic mass” instead of atomic weight.
This works because neutrons and protons are both very close to 1 AMU, and electrons are very close to 0 AMU. The atomic weight uses precise measurements to get the decimal value, but we’re only interested in the whole numbers that tell us the number of protons and neutrons. Remember, if you use the atomic weight, you’re getting an average for a typical sample. A bromine sample has an average mass number of 80, but as it turns out, a single bromine atom almost always has a mass number of 79 or 81. [5] X Research source
If an atom loses or gains electrons, it becomes an ion, which means that it is an electrically charged atom.
A single helium (He) atom has a mass number of 4 and an atomic number of 2. It must have 4 - 2 = 2 neutrons. A sample of silver (Ag) has an average mass number of 108 (based on the periodic table) and an atomic number of 47. On average, each silver atom in the sample has 108 - 47 = 61 neutrons.
Isotopes never change the atomic number. Every isotope of an element has the same number of protons.
