Isotopes

Same Element, Different Mass

Hydrogen in the Sun, carbon in our bodies, and oxygen in water do not always appear as only one mass version. One element can have several atoms with the same number of protons but different numbers of neutrons. Those atoms are called isotopes.

The Atomic Structure and Symbolism section from OpenStax explains that isotopes are atoms of the same element with the same atomic number but different mass numbers. The source can be opened through openstax.org.

The IUPAC Gold Book emphasizes the same idea: isotopes have the same atomic number but different mass numbers. The term reference can be opened through goldbook.iupac.org.

Read the pattern like this:

\text{isotopes} \Rightarrow Z\ \text{same},\ n^0\ \text{different},\ A\ \text{different}

The symbol $Z$ is the atomic number, which equals the number of protons. The symbol $A$ is the mass number, which equals protons plus neutrons. If $Z$ is the same, the element is the same. If $n^0$ is different, the mass number also changes.

So isotopes are mass versions of the same element, not new elements. The key stays the proton count. Neutrons only change the mass number.

Isotope Mini Lab

Choose an isotope below. First notice what does not change: the number of protons. Then compare the neutron count.

Isotope Reader

Compare hydrogen and carbon isotopes. The element stays the same when the atomic number stays the same, but the mass number changes when neutrons change.

{}^{1}_{1}\mathrm{H}

{}^{1}_{1}\mathrm{H}

Atomic number: $Z = 1$
Mass number: $A = 1$
Neutrons: $n^0 = 1 - 1 = 0$
Natural abundance: About $99.9885\%$

Protons: $1$
Neutral electrons: $1$
Neutrons: Has no neutrons.

Hydrogen Has Three Important Examples

All hydrogen isotopes have $1$ proton. Because their proton count is $1$ , they are still hydrogen. The difference is their neutron count.

Hydrogen isotope	Symbol	Protons	Neutral electrons	Neutrons
Light hydrogen	${}^{1}_{1}\mathrm{H}$	$1$	$1$	$0$
Deuterium	${}^{2}_{1}\mathrm{H}$	$1$	$1$	$1$
Tritium	${}^{3}_{1}\mathrm{H}$	$1$	$1$	$2$

Deuterium is sometimes written as $\mathrm{D}$ , and tritium is sometimes written as $\mathrm{T}$ . Those shortcuts do not mean new elements. Deuterium and tritium are still hydrogen because their proton count is still $1$ .

Carbon Shows the Role of Neutrons

Carbon always has $6$ protons. In ${}^{12}_{6}\mathrm{C}$ , ${}^{13}_{6}\mathrm{C}$ , and ${}^{14}_{6}\mathrm{C}$ , the neutron count increases.

\begin{aligned} {}^{12}_{6}\mathrm{C} &: n^0 = 12 - 6 = 6 \\ {}^{13}_{6}\mathrm{C} &: n^0 = 13 - 6 = 7 \\ {}^{14}_{6}\mathrm{C} &: n^0 = 14 - 6 = 8 \end{aligned}

${}^{12}_{6}\mathrm{C}$ and ${}^{13}_{6}\mathrm{C}$ are stable. ${}^{14}_{6}\mathrm{C}$ is radioactive, so its amount in nature is extremely small. Because ${}^{14}_{6}\mathrm{C}$ decays over time, scientists can use it to estimate the age of objects that once came from living things, such as old wood or bone.

Do Not Mix It Up with Ions

Ions and isotopes both change numbers in an atom, but the changing part is different.

Concept	What changes	What locks the element
Ion	Electrons	Protons
Isotope	Neutrons	Protons

For example, $\mathrm{C}^{2-}$ is a carbon ion because its electrons changed. In contrast, ${}^{14}_{6}\mathrm{C}$ is a carbon isotope because its neutron count is different from ${}^{12}_{6}\mathrm{C}$ .

If the atom is neutral, electrons still equal protons:

\text{neutral atom} \Rightarrow e^- = p^+

So neutral ${}^{14}_{6}\mathrm{C}$ has $6$ protons, $6$ electrons, and $8$ neutrons.

Uses of Isotopes

Isotopes do not only differ in tables. Different neutron counts can make some isotopes useful for research, energy, or age estimation.

Deuterium and tritium, two hydrogen isotopes, are widely discussed in nuclear fusion research. The Department of Energy explains deuterium-tritium fuel as an important isotope pair in fusion energy development. The reference can be opened through energy.gov.

${}^{14}_{6}\mathrm{C}$ is used in radiocarbon dating to estimate the age of organic materials, meaning materials that once came from living things. The University of Chicago explains that ${}^{14}_{6}\mathrm{C}$ dating can determine the age of organic materials up to about $60\,000$ years. The reference can be opened through news.uchicago.edu.

The important note is that isotope uses must be read together with stability, meaning whether the nucleus tends to decay. Radioactive isotopes such as tritium and ${}^{14}_{6}\mathrm{C}$ are not materials for casual experiments, so their use requires safety rules.

From Isotopes to Average Atomic Mass

On the periodic table, relative atomic mass is often a decimal. That number is not the mass number of one isotope. It is a weighted average of the isotopes found in nature.

NIST explains that an element's relative atomic mass is calculated from isotope masses and isotopic composition. The column notes can be opened through nist.gov.

Carbon is an easy example:

Carbon isotope	Mass number	Natural abundance
${}^{12}_{6}\mathrm{C}$	$12$	About $98.93\%$
${}^{13}_{6}\mathrm{C}$	$13$	About $1.07\%$
${}^{14}_{6}\mathrm{C}$	$14$	Extremely small

Because ${}^{12}_{6}\mathrm{C}$ is the most abundant carbon isotope, carbon's relative atomic mass on the periodic table is close to $12$ , but not exactly $12$ .

Distinguishing Isotopes by Atomic and Mass Numbers

Use these three questions whenever you read an isotope:

How many protons are there?
What is the mass number?
How many neutrons come from $A - Z$ ?

For ${}^{92}_{40}\mathrm{Zr}$ :

\begin{aligned} n^0 &= A - Z \\ &= 92 - 40 \\ &= 52 \end{aligned}

So ${}^{92}_{40}\mathrm{Zr}$ has $52$ neutrons. This method is safer than memorizing tables because you can calculate any isotope when the atomic number and mass number are known.