Isotopes and atomic mass

Same element, different mass

All atoms of an element have the same number of protons, but they don't all have the same number of neutrons. Atoms of the same element with different numbers of neutrons are called isotopes. Isotopes of an element have:

• the same atomic number (same protons, so the same element)
• the same number of electrons in a neutral atom, so they react in the same way chemically
• different mass numbers, so they have slightly different masses
• sometimes different stability: some isotopes are radioactive

Isotopes are named by their mass number: carbon-12, carbon-13 and carbon-14 all have 6 protons, but 6, 7 and 8 neutrons. Carbon-12 and carbon-13 are stable. Carbon-14 is radioactive, which is what makes radiocarbon dating possible (1F).

Hydrogen is the only element whose isotopes have their own names. "Heavy water", made with deuterium instead of ordinary hydrogen, is about 10% denser than ordinary water, so an ice cube made from it sinks.

Why the periodic table shows decimals

Most elements are a mixture of isotopes, and the mixture is almost the same wherever the element is found on Earth. The relative atomic mass on the periodic table is the weighted average mass of an element's atoms, taking into account how common each isotope is (its abundance).

Worked example: chlorine. Chlorine is 75.8% chlorine-35 and 24.2% chlorine-37. Imagine 100 chlorine atoms:

• 75.8 atoms × 35 = 2653
• 24.2 atoms × 37 = 895.4
• total mass = 3548.4, so the average = 3548.4 ÷ 100 = 35.48

The periodic table gives 35.45. The small difference is because the real masses of isotopes are not exactly whole numbers (chlorine-35 is actually 34.97 amu). Notice that the average is closer to 35 than to 37, because chlorine-35 is much more common. A weighted average always lies closer to the more abundant isotope.

Scientists measure isotope abundances with a mass spectrometer. It turns atoms into ions, speeds them up and sends them through a magnetic field. Lighter ions are bent more than heavier ones, so each isotope lands in a different place, and the height of each peak shows how common it is.

Isotopes as fingerprints

Because the ratio of isotopes can vary slightly with where and how something formed, isotopes act as natural fingerprints:

• Rain that falls in hot places contains slightly more of the heavy oxygen isotope oxygen-18 than rain in cold places. Ice cores from Antarctica record these ratios layer by layer, giving a record of past temperatures going back 800 000 years. Australian scientists drilling at Law Dome and elsewhere in Antarctica have contributed to this record.
• Food scientists can tell where honey, wine or beef came from by measuring hydrogen, oxygen and strontium isotopes, which helps catch food fraud.
• Police and customs use isotope ratios to trace the origin of illegal drugs and explosives.