Changes

Atoms of the same element (having the same number of protons) that have different numbers of neutrons are known as [[isotope]]s. Some isotopes are more stable than others, and occur more often in nature, but there is no "standard" number of neutrons in a given element. The atomic weight of an element is a weighted average of the number of neutrons and protons (number of protons remains constant in a given element) in all naturally occurring isotopes (isotopes that are not radioactive). For example, [[chlorine]] has two naturally occuring occurring isotopes: chlorine-35 and chlorine-37, with relative abundances of 75.78% and 24.22%.<ref>https://www.webelements.com/chlorine/isotopes.html</ref>. The atomic weight of chlorine is the weighted average and so it is <math>(0.7578 \times 35 )+ (37 \times 0.2422)=35.48</math>. Note that the atomic weight of an element is not necessarily an integer due to the way it is calculated, as is true in this example. Many isotopes are [[radioactive]] and [[radioactive decay|decay]] over time.