The element hydrogen has three isotopes. The nucleus of a "normal" hydrogen atom has one proton (red) but no neutrons (blue). Hydrogen's other, far less abundant isotopes are deuterium (1 proton + 1 neutron) and tritium (1 proton + 2 neutrons). Click on image for full size (15 Kb) Original artwork by Windows to the Universe staff (Randy Russell).

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.

Hydrogen has three isotopes. The nucleus of a "normal" hydrogen atom has one proton (red) but no neutrons (blue). Hydrogen's other isotopes are deuterium (1 proton + 1 neutron) and tritium (1 proton + 2 neutrons). Click on image for full size (15 Kb) Original artwork by Windows to the Universe staff (Randy Russell).

Isotopes are different "versions" of an element. All atoms of an element have the same number of protons. For example, all hydrogen atoms have one proton, all carbon atoms have 6 protons, and all uranium atoms have 92 protons. However, atoms of an element can have different numbers of neutrons. Most carbon atoms have 6 neutrons, but about 1% of the carbon atoms on Earth have 7 neutrons. All carbon atoms have an atomic number (number of protons) of six, but these two different isotopes of carbon have an atomic mass (number of protons + neutrons) of 12 or 13.

Scientists use special "codes" to write the names of isotopes. One isotope of carbon has 8 neutrons. It has an atomic mass of 14 (6 protons + 8 neutrons). The "code" for this isotope is carbon-14 or ¹⁴C. Different isotopes of the same element behave almost exactly the same way in chemical reactions. For example, most oxygen is the isotope oxygen-16. Oxygen-18 is a rare isotope. However, adding two hydrogen atoms to one oxygen atom still makes water (H₂O), even if we use ¹⁸O instead of ¹⁶O. Some isotopes are radioactive, but others are not. Radioactive isotopes can "decay" by giving off radiation.

Where do different isotopes come from? Astronomers think the only elements created in the Big Bang were various isotopes of hydrogen, helium, and probably lithium, beryllium and boron. Supernova explosions created the rest of the elements, including most of their isotopes. Some isotopes form when high-energy cosmic rays crash into atoms in our atmosphere.

Isotopes help scientists learn about the history of some natural events. Carbon dating techniques help us figure out how old some objects are. Oxygen isotopes help us learn about past climates. Some of the water molecules in ice have the rare oxygen-18 isotope in them. The mix of ¹⁸O and ¹⁶O in ice core samples from Greenland and Antarctica give us clues about Earth's temperature in the past.

Hydrogen has three isotopes. The nucleus of a "normal" hydrogen atom has one proton (red) but no neutrons (blue). Hydrogen's other isotopes are deuterium (1 proton + 1 neutron) and tritium (1 proton + 2 neutrons). Click on image for full size (15 Kb) Original artwork by Windows to the Universe staff (Randy Russell).

Isotopes are different "versions" of an element. All atoms of an element have the same number of protons. All hydrogen atoms have one proton, all carbon atoms have 6 protons, and all uranium atoms have 92 protons. However, atoms of an element can have different numbers of neutrons. Most carbon atoms have 6 neutrons, but some have 7 neutrons.

Scientists use special "codes" to write the names of isotopes. One isotope of carbon has 8 neutrons. It has an atomic mass of 14 (6 protons + 8 neutrons). The "code" for this isotope is carbon-14 or ¹⁴C. Different isotopes of the same element behave almost exactly the same way in chemical reactions. For example, most oxygen is the isotope oxygen-16. Oxygen-18 is a rare isotope. However, adding two hydrogen atoms to one oxygen atom still makes water (H₂O), even if we use ¹⁸O instead of ¹⁶O. Some isotopes are radioactive, but others are not. Radioactive isotopes can "decay" by giving off radiation.

Where do different isotopes come from? Astronomers think the only elements created in the Big Bang were various isotopes of hydrogen, helium, and probably lithium, beryllium and boron. Supernova explosions created the rest of the elements, including most of their isotopes. Some isotopes form when high-energy cosmic rays crash into atoms in our atmosphere.