Atomic Nomenclature

Introduction:

Throughout this web site we will use different terminology to describe atomic processes. It is important that you know this terminology, or "nomenclature", so that you can understand what we are talking about! In this page we are going to highlight some of the vocabulary you must understand.

Atomic Number:

The atomic number of an atom is essentially the number of protons in its nucleus. For example, carbon has 6 protons. Thus, its atomic number is 6. Earlier, we talked about isotopes. Remember that each isotope has the same number of protons? Well, every isotope of an element has the same atomic number.

Atomic Mass:

The atomic mass of an element is the sum of the number of protons AND neutrons. For example, carbon has six protons. One isotope of carbon also has 6 neutrons. This means that its atomic mass is 12, or 6 protons + 6 neutrons. Another isotope of carbon has 8 neutrons. This means that its atomic mass would be 14, or 6 protons + 8 neutrons.

What exactly is an Isotope?

An isotope is a type of an atom. One element can have many different isotopes. This is because an atom is defined by its number of protons, not it's number of neutrons. Isotopes are simply atoms of an element with different numbers of neutrons, but the same number of protons.

Each isotope has one proton but a different number of neutrons. Note: Picture Not To Scale

Referring to Isotopes:

Elements are often referred to by their symbol, and not their name. For instance, we might call Beryllium Be. (Scientists are lazy people. Why write the full name when they can abbreviate it?) So, how do we distinguish between different isotopes? They are the same element, after all. Well, it turns out that there is a notation that says what type of element we're talking about AND tells the isotope. This is done by giving the element symbol and the isotope mass in smaller text to the left of the symbol. This is written like this: 244Pu .
Pu is the symbol for plutonium, an element used in nuclear fission. The number 244 is the atomic mass. This tells the isotope because we know that plutonium has 94 protons. This means that we are referring to the isotope of plutonium with 150 neutrons. 244Pu is pronounced "plutonium 244" or "Pu 244".

Periodic Table of Elements:

The Periodic Table of Elements is a listing of all the known elements (types of atoms) that exist. They are listed in order of atomic number (number of protons). In our table, we have each element's atomic number at the top of each box, its atomic symbol in the center, and its atomic mass at the bottom. The atomic mass of each element is not a whole number. At first, this appears to make no sense. How could the sum of an atom's protons and neutrons be 1.01 (the atomic mass of hydrogen)? It is impossible, after all, to have .1 neutrons or .1 protons. To understand this, first recall that the same type of atoms can have different numbers of neutrons, and thus different atomic masses. The reason that the atomic masses are not whole numbers is because the atomic masses of the isotopes are averaged to get the overall atomic mass for the element. We will explain more about this later. For now, remember that the mass of the isotope of an element that occurs most often in nature is the atomic mass rounded to the nearest whole number. For example, hydrogen's (H) mass is 1.01. So, th e mass of its most common isotope is 1. Click here to view our Periodic Table of Elements.