Atomic Number and Atomic Mass

What is it that makes element different from another? Earlier, we said that elements in the same group on the periodic table usually have similar properties. Let's take a look at a specific group, group 11.  As you can see here, copper (Cu), silver (Ag), and gold (Au) all reside in the same group. These elements do, in fact, have similar properties (all metals, shiny, malleable, conduct electricity well), but we all know that they are very different elements. Take, for instance, their colors. Copper is seen in coins, wires, pipes, and a ton of other things and has a brownish hue to it. Silver has that nice grey color that's, well, silvery. Gold has that familiar rich yellowish color. What makes these 3 similar elements different from each other and other elements? This answer can be very complicated depending on the context of why it's asked, but for our purposes the answer is simple; they all have different numbers of protons in their atomic nucleus.


The number of protons in an atom;s nucleus is referred to as the atomic number. The atomic number for an element is unique to it; no 2 elements share the same atomic number. Also, all atoms of an elements always contain that number of protons e.g. hydrogen has 1 proton, helium has 2, etc That is the most basic thing that sets elements apart.

# of Protons = Atomic Number

The atomic mass is another characteristic that, while not exactly unique to an element, is an important piece of information about elements. As you may know, atoms don't consist of just protons; they can (and usually) contain also neutrons and electrons. Electrons are relatively mass-less compared to protons and neutrons, which have relatively identical masses. The atomic mass number is the value of the mass contained in the atom. Since electrons have almost no mass, and protons and neutrons have almost identical masses, the atomic mass number is equal to the number of protons and neutrons combined.

Atomic Mass Number = # of Protons + # of Neutrons

The atomic mass number is not always the same for every atom. Every element can have different numbers of neutrons in it's nucleus, and therefor different mass numbers. These different mass numbers for the same element are called isotopes. There can be many different isotopes for an element, and usually a small handful are stable, meaning they won't undergo decay.
For example: Carbon has 2 naturally occurring isotopes, Carbon 12 and Carbon 13.

Isotopes are represented with atomic mass number above the atomic number, all followed by the element's symbol. Examples below

Another way (and easier to do on a computer) to just have the atomic mass number in superscript before the atomic symbol and leave out the atomic number, we can do this because the atomic symbol and mass number mean the same thing. The only reason to keep the atomic number is to make it easier to see how many neutrons are in the atom.

     Ex.    ¹²C      ¹³C     ¹⁴N      ¹⁵N      ²³⁵U