Changes

==Isotopes==

==Isotopes==

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 occurring isotopes: chlorine-35 and chlorine-37, with relative abundances of 75.78% and 24.22%.<ref>{{cite web|url=https://www.webelements.com/chlorine/isotopes.html|title=Chlorine: isotope data|accessdate=2019-01-24}}</ref> The atomic weight of chlorine is the weighted average and so it is (0.7578×35)+(37×0.2422)=35.48. 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.

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 atomic weights of all naturally occurring isotopes (isotopes that are not radioactive). The atomic weight of an isotope is approximately equal to the number of neutrons and protons (number of protons remains constant in a given element), For example, [[chlorine]] has two naturally occurring isotopes: chlorine-35 and chlorine-37, with relative abundances of 75.78% and 24.22%.<ref>{{cite web|url=https://www.webelements.com/chlorine/isotopes.html|title=Chlorine: isotope data|accessdate=2019-01-24}}</ref> The atomic weights of those isotopes are 34.96885268 and 36.96590259, respectively.  The atomic weight of chlorine is the weighted average, so it is (0.7578×34.96885268)+(0.2422×36.96590259)=35.45293758. So the atomic weight of an element is not necessarily an integer for two reasons: the element as it occurs in nature may be (as in this case) a mixture of two or more isotopes, and the individual isotopes may have non-integral atomic weights due to the mass defect.  Mass defects are calculated relative to the [[carbon]]-12 isotope, which is defined to have an atomic weight of exactly 12.  Other isotopes generally have non-integral atomic weights because the nuclear binding energy ties up some of the mass according to the formula E=mc². See [[Quantitative Analysis of Alpha Decay]]. Many isotopes are [[radioactive]] and [[radioactive decay|decay]] over time.

==History of the Atomic Model==

==History of the Atomic Model==