Isotope
Theatomsof achemical elementcan exist in different types that have very similar behavior, but weigh different amounts. These are calledisotopesof the element. Atoms of the same element have the same number ofprotons,but different isotopes have different numbers ofneutrons.Different isotopes of the same element have different masses because they have a different number of neutrons.Massis the word for how muchsubstance(ormatter) something has. Things with different masses also have differentweights.
Some isotopes are not stable so they change to another isotope or element byradioactive decay.These are calledradioactive isotopes.Others are not radioactive. These are called stable isotopes.
Atoms of the sameelementhave the same number ofprotons,which is called theatomic number.Different isotopes of an element have different numbers ofneutrons.As a result, they also have differentmass numbers,which is the number of protons plus the number of neutrons. An isotope is usually named by giving the element and the mass number. For example, the symbol for carbon is C and all carbon atoms have 6 protons. The most common isotope of carbon also has 6 neutrons, giving a mass number of 12, and it is writtencarbon-12or12C. The isotope of carbon that has 8 neutrons is writtencarbon-14or14C.
The word "isotope", meaning "at the same place", comes from the fact that isotopes of the same element are at the same place on theperiodic table.
Chemical properties[change|change source]
In anyneutralatom,the number ofelectronsis the same as the number ofprotons.As a result, isotopes of the same element also have the same number of electrons and the sameelectronic structure.The way an atom acts is decided by its electronic structure, so isotopes of the same element have almost the same chemical behavior, such as the molecules it can form. It is very difficult to separate the isotopes of an element from a mixture of different isotopes because of how similar this behavior is.
Heavier isotopes react chemically more slowly than lighter isotopes of the same element. This "mass effect" is large forprotium(1H) anddeuterium(2H), because deuterium has twice the mass of protium. For heavier elements, the relativeatomic weightratio between isotopes is much less, so the mass effect is usually small.
Stability[change|change source]
Some isotopes are not stable so they change to a different isotope or element byradioactive decay.These are called unstable isotopes, orradioactive isotopes.The average time it takes an isotope to decay is called thehalf-life.Other isotopes do not decay so they are not radioactive. They are called stable isotopes.
Every atom has anatomic nucleus,which is made from protons and neutrons that are held together by thenuclear force.Because protons have positiveelectrical charge,they repel each other. Neutrons are neutral and they stabilize the nucleus. The neutrons hold the protons slightly apart. This reduces theelectrostatic repulsionbetween the protons, so the nuclear force can hold the protons and neutrons together. One or more neutrons are necessary for two or more protons to bind into a nucleus. As the number of protons increases, so does the number of neutrons needed to have a stable nucleus.
Some elements only have a single isotope that is stable. For example,fluorine-19 (19F) is the only stable isotope of fluorine. The other isotopes of fluorine decay rapidly so they are not found in nature. Other elements have many stable isotopes. For example,xenonhas seven stable isotopes. It also has two isotopes that decay very slowly that are found in nature. The largest number of stable isotopes for any element is ten, for the elementtin.Some elements do not have any stable isotopes, such ascurium.These only exist on Earth because they are created innuclear reactors,nuclear explosions,orparticle accelerators.
Some unstable isotopes exist naturally onEarthbecause they have a very long half-life. For example,uranium-238 has a half-life of 4468 million years. The half life ofradium-226 is only 1600 years, and it is found in nature because it is constantly forming from the decay of uranium-238.
Hydrogen has three common isotopes. The most common isotope of hydrogen is called protium (1H), which has one proton and no neutrons. A hydrogen atom that has a proton and a neutron (atomic mass of 2) is called deuterium (2H). Hydrogen with one proton and two neutrons (atomic mass of 3) is calledtritium(3H). Protium and deuterium are stable isotopes, while tritium is a radioactive isotope.
The heaviest elements in theperiodic tableare all radioactive. All of the isotopes ofradon,thorium,anduraniumare radioactive, since they are very heavy. This is because the nuclear forces inside thenucleusof theatomcannot hold together all the protons and neutrons.