How Isotopes Change an Element's Mass

The nucleus of an atom contains protons and neutrons. The number of protons is the atomic number, and it determines the element. An atom with six protons is carbon, while an atom with seven protons is nitrogen. Changing the proton count creates a different element.

Neutrons add mass but carry no electrical charge. Atoms of one element can contain different numbers of neutrons without changing their chemical identity. Carbon-12 has six protons and six neutrons, while carbon-14 has six protons and eight neutrons.

The number after the element name is the mass number, equal to protons plus neutrons. Electrons have very little mass compared with nuclear particles, so they are not included. Chemistry has kindly placed the important arithmetic in the nucleus, where nobody can casually misplace it.

Neutral isotopes of an element have the same number of electrons and generally the same electron arrangement. Because chemical reactions mainly involve electrons, isotopes usually behave in very similar chemical ways.

Their masses differ, which can create small differences in reaction speed or movement. These isotope effects are especially noticeable for light elements such as hydrogen. Scientists can use mass differences to trace substances through chemical and biological systems. This makes isotopes useful as tracers because instruments can distinguish their masses even when their reactions look almost identical.

Some isotope nuclei are stable, while others are unstable and radioactive. An unstable nucleus can change into another nucleus while releasing particles or electromagnetic radiation. Radioactivity is a nuclear process, not a sign that the atom is chemically eager or glowing green for theatrical convenience.

The atomic mass shown on a periodic table is often a decimal because a natural sample contains a mixture of isotopes. The value is a weighted average, so common isotopes influence it more strongly than rare ones.

For example, most natural chlorine atoms are chlorine-35, while a smaller share are chlorine-37. The listed atomic mass lies between 35 and 37 but closer to 35. It does not mean each chlorine atom contains a fractional collection of neutrons.

Keep these quantities separate:

• •Atomic number equals the number of protons.
• •Mass number equals protons plus neutrons.
• •Isotopes have the same proton number.
• •Isotopes have different neutron numbers.
• •Relative atomic mass reflects isotope abundance.

Radiocarbon dating shows isotopes at work. Carbon-14 forms continually in the upper atmosphere when radiation from space strikes nitrogen atoms. It becomes part of carbon dioxide, which plants absorb, and animals take it in by eating plants or other animals. While an organism is alive, the share of carbon-14 in its body stays roughly in step with the atmosphere. When it dies, it stops taking in new carbon, and the carbon-14 already present decays steadily back into nitrogen while the stable carbon-12 remains.

Carbon-14 has a half-life of about 5,730 years, meaning half of any sample decays in that time. A piece of wood with half the expected carbon-14 is therefore about 5,730 years old, and one with a quarter is about twice that age. After roughly ten half-lives too little remains to measure reliably, so the method works for organic material up to about fifty thousand years old. Scientists also calibrate results, for example using tree rings, because the amount of carbon-14 in the atmosphere has not been perfectly constant over time.

Most oxygen atoms are oxygen-16, but a small share are the heavier oxygen-18. Water molecules containing the lighter isotope evaporate slightly more easily, while those containing the heavier one condense and fall as rain slightly more easily. As moist air travels from warm oceans towards the poles, it keeps losing heavy water in rain and snow along the way. The colder the climate, the further this process goes, so snow falling on polar ice sheets in cold periods contains an even smaller share of oxygen-18.

That snow is compressed into ice layer by layer, keeping a record of conditions when it fell. By drilling deep ice cores in Antarctica and Greenland and measuring the isotope ratio at each depth, scientists can reconstruct temperature changes over hundreds of thousands of years, including the cycle of ice ages. Ocean sediments provide a matching record. Tiny shelled organisms build their shells from seawater, and when large ice sheets lock up lots of light water, the oceans become richer in oxygen-18. The two archives can be checked against each other.

The first clear sign that a stable element could have atoms of different masses came in 1913. J. J. Thomson was sending streams of charged neon atoms through electric and magnetic fields, which bent their paths onto a photographic plate. Instead of one trace he found two, matching masses of about 20 and 22. His assistant Francis Aston went on to build the mass spectrograph, an instrument that separates charged atoms by mass because lighter ions are deflected more by a magnetic field. Aston found isotopes in many elements and received the Nobel Prize in Chemistry in 1922.

Modern mass spectrometers work on the same principle and also measure how common each isotope is, which supplies the numbers behind relative atomic mass. For chlorine, roughly three quarters of atoms are chlorine-35 and one quarter are chlorine-37. Imagine four typical atoms. Three with mass 35 and one with mass 37 give a total of 142, and dividing by four gives 35.5, the value shown in many school periodic tables. More precise measurements of each isotope's exact mass and abundance give 35.45, but the method is the same: multiply each mass by its share and add the results.

Hydrogen shows the biggest isotope effects because adding one neutron doubles the mass of its nucleus. Deuterium, or hydrogen-2, has one proton and one neutron, and water made with deuterium instead of ordinary hydrogen is called heavy water. It looks just like normal water, but it is about 11 per cent denser, so an ice cube made from heavy water sinks in an ordinary drink. It melts at about 3.8 degrees Celsius and boils at about 101.4 degrees. The chemistry is essentially the same, but the extra mass slightly changes how the molecules move and hold together.

Heavy water became strategically important because of nuclear reactors. In a reactor, fast neutrons must be slowed down before they can split uranium nuclei efficiently. Ordinary hydrogen slows them well but also absorbs many of them, while deuterium absorbs far fewer, so reactors using heavy water can run on natural, unenriched uranium. During the Second World War the Norsk Hydro plant at Vemork in occupied Norway was a major source of heavy water, which Germany wanted for nuclear research. In February 1943, Norwegian commandos trained in Britain blew up the plant's heavy water production cells in a celebrated act of sabotage.

Isotopes are atoms of the same element with different neutron numbers and masses. Their shared electron structures give them similar chemistry, while their nuclear stability and mass can differ. Distinguish atomic number, mass number and average atomic mass, and isotope questions lose most of their unnecessary fog.