Isotopes are different "versions" of a chemical element. All atoms of an element have the same number of protons. For example, all hydrogen atoms have one proton, all carbon atoms have six protons, and all uranium atoms have 92 protons. However, the number of neutrons in atoms of an element can vary. For instance, while most carbon atoms have six neutrons, about 1% of the carbon atoms found on Earth have seven neutrons and an even smaller fraction of carbon atoms have eight neutrons. All carbon atoms have an atomic number (number of protons) of six, but the different isotopes of carbon can have an atomic mass (number of protons + neutrons) of 12, 13 or 14.
Two types of notation are commonly used to indicate isotopes. As an example, carbon's 8-neutron isotope is written as either carbon-14 or 14C. For the most part, all of the isotopes of a particular element behave the same way in chemical reactions. For instance, an atom of the rare oxygen-18 isotope will combine just as readily with two hydrogen atoms to form water (H2O) as will a more common oxygen-16 atom.
Some isotopes are radioactive, while others are not. Atoms of radioactive isotopes can "decay" by emitting energy in the form of some type of radiation. Atoms that decay typically lose one or more proton(s) and/or neutron(s), converting the atom to a different isotope or even a different element. Isotopes that are not radioactive are called "stable" isotopes. Of the 94 elements that occur naturally on Earth, 80 have at least one stable isotope. There are 26 elements that have only one stable isotope. Tin has the most, with ten stable isotopes. Of the 80 elements with at least one stable isotope, the average number of stable isotopes is 3.2 per element.
Tables that list the atomic masses of elements factor in the relative abundance of each isotope. For example, the atomic mass of chlorine is reported as 35.5 since 76% of chlorine found on Earth is the lighter 35Cl while 24% is the heavier 37Cl. Where do these different isotopes come from, and why are there differing quantities of each in nature? Astronomers believe that the only elements created in the Big Bang were various isotopes of hydrogen, helium, and probably lithium, beryllium and boron. Since then, enormous supernova explosions have created the remaining elements, including most of their isotopes. Some isotopes form when high-energy cosmic rays collide with various atoms. Isotopes are also created when other radioactive isotopes decay.
Isotopes often give scientists clues about the history of natural processes. Carbon dating techniques help us determine the age of objects based on the relative abundance of stable 12C as compared to radioactive 14C (which decays and "disappears" over time). The ratio of deuterium to normal hydrogen atoms in water molecules in ice core samples helps us estimate global temperatures in the past; the slightly heavier water molecules that contain a deuterium atom evaporate from the oceans (and later snow out over the poles) at a different rate as compared to their normal hydrogen cousins, hinting at average temperatures.
Isotopu ni "matoleo" tofauti-tofauti ya elementi ya kemikali. Atomu zote za elementi moja zina idadi ileile ya protoni. Kwa mfano, atomu zote za hidrojeni zina protoni moja, atomu zote za kaboni zina protoni sita, na atomu zote za urani zina protoni 92. Hata hivyo, idadi ya neutroni katika atomu za elementi inaweza kutofautiana. Kwa mfano, wakati atomu nyingi za kaboni zina neutroni sita, karibu 1% ya atomu za kaboni zilizopatikana duniani zina neutroni saba na sehemu ndogo zaidi ya atomu za kaboni zina neutroni nane. Atomu zote za kaboni zina nambari ya atomu (idadi ya protoni) ya sita, lakini isotopu tofauti za kaboni zinaweza kuwa na wingi wa atomu (idadi ya protoni + neutroni) ya 12, 13 au 14. Aina mbili za alama hutumiwa kwa kawaida kuashiria isotopu. Isotopu ya kaboni yenye neutroni nane huitwa kaboni 14 au 14C. Kwa sehemu kubwa, isotopu zote za elementi fulani hujiendesha kwa njia ileile katika athari za kemikali. Kwa mfano, atomu ya isotopu ya oksijeni-18 isiyo ya kawaida itaungana kwa urahisi na atomu mbili za hidrojeni kuunda maji (H2O) kama vile atomu ya oksijeni-16 ya kawaida. Isotopu fulani ni zenye mnururisho, ilhali nyingine si hivyo. Atomu za isotopu zenye mnururisho zinaweza "kuharibika" kwa kutoa nishati katika namna fulani ya mnururisho. Atomu zinazoharibika kwa kawaida hupoteza protoni moja au zaidi na au neutroni, na kugeuza atomu kuwa isotopu tofauti au hata kipengele tofauti. Isotopu zisizo zenye mnururisho huitwa isotopu "zenye uthabiti". Kati ya vitu 94 vya asili duniani, 80 vina isotopu moja. Kuna elementi 26 ambazo zina isotopu moja tu ya kudumu. Chuma kina isotopu kumi thabiti. Kati ya elementi 80 zenye isotopu moja ya kudumu, kiwango cha wastani cha isotopu za kudumu ni 3.2. Jedwali linaloorodhesha uzito wa atomu wa elementi hutia ndani wingi wa kila isotopu. Kwa mfano, uzito wa atomu ya klorini ni 35.5 kwa kuwa asilimia 76 ya klorini inayopatikana duniani ni 35Cl, wakati asilimia 24 ni 37Cl. Isotopu hizo tofauti-tofauti zinatoka wapi, na kwa nini kuna kiasi tofauti-tofauti cha kila moja katika asili? Wataalamu wa nyota wanaamini kwamba elementi pekee zilizotengenezwa katika Big Bang zilikuwa isotopu mbalimbali za hidrojeni, heliamu, na labda lithiamu, berylliamu na boroni. Tangu wakati huo, milipuko mikubwa ya supernova imeunda elementi zilizobaki, kutia ndani isotopu zao nyingi. Isotopu fulani hufanyizwa wakati miale ya angani yenye nishati nyingi inapogongana na atomu mbalimbali. Isotopu hutokezwa pia wakati isotopu nyingine zenye mnururisho zinapoharibika. Mara nyingi isotopu huwapa wanasayansi habari kuhusu historia ya mambo ya asili. Utafiti wa kaboni unatuwezesha kuamua umri wa vitu kulingana na wingi wa 12C thabiti ikilinganishwa na 14C ya radioactive (ambayo inaharibika na kutoweka kwa muda). Kiwango cha deuteriamu na atomu za kawaida za hidrojeni katika molekuli za maji katika sampuli za barafu hutusaidia kukadiria joto la dunia katika siku za nyuma; molekuli za maji nzito kidogo ambazo zina atomu ya deuteriamu huyeyuka kutoka baharini (na baadaye kutolewa kwenye barafu juu ya nguzo) kwa kiwango tofauti ikilinganishwa na binamu zao wa kawaida wa hidrojeni, ikionyesha joto la wastani.