What is Atomic Weight?
Atomic weight, more properly relative atomic mass, is the weight of atoms of an element. The science behind this measurement is extremely complex, and it has changed quite a bit since the early 1800s, when the concept of atomic weight was first proposed. It is also important to distinguish between atomic weight and atomic mass: atomic weight or relative atomic mass reflects the average weight of a single atom of an element, while atomic mass refers to the weight of a specific atom. Simple textbooks and science courses often describe atomic weight as the number of protons and neutrons in an element added together. An element with two protons and two neutrons, for example, would be considered to have an atomic weight of four under this system. Unfortunately, neutrons actually weigh more than protons, which makes this rough guideline a bit inaccurate. Individual atoms are extremely small, and rather challenging to weigh on their own. Early researchers arrived at a system which involved de
The nuclei of atoms also contain neutrons, which help hold the nucleus together. A neutron has no electrical charge and is slightly more massive than a proton. Because a neutron can decay into a proton plus an electron (the essence of beta decay), it is sometimes helpful to think of a neutron as an electron and a proton blended together, although this is at best an oversimplification. Because a neutron has no charge, a neutron has no effect on the number of electrons orbiting the nucleus. However, because it is even more massive than a proton, a neutron can add significantly to the weight of an atom. The total weight of an atom is called the atomic weight. It is approximately equal to the number of protons and neutrons, with a little extra added by the electrons. The stability of the nucleus, and hence the atom’s radioactivity, is heavily dependent upon the number of neutrons it contains.
