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's Atomic Structure and Symbolism.
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 IUPAC Gold Book's isotope entry.
Read the pattern like this:
isotopes⇒Zsame,n0different,Adifferent
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 n0 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.
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.
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
11H
1
1
0
Deuterium
12H
1
1
Tritium
13H
1
1
Deuterium is sometimes written as D, and tritium is sometimes written as T. Those shortcuts do not mean new elements. Deuterium and tritium are still hydrogen because their proton count is still 1.
Carbon always has 6 protons. In 612C, 613C, and 614C, the neutron count increases.
612C613C614C:n0=12−6=6:n0=13−6=7:n0=14−6=8
612C and 613C are stable. 614C is radioactive, so its amount in nature is extremely small. Because 614C 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.
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, C2− is a carbon ion because its electrons changed. In contrast, 614C is a carbon isotope because its neutron count is different from 612C.
If the atom is neutral, electrons still equal protons:
neutral atom⇒e−=p+
So neutral 614C has 6 protons, 6 electrons, and 8 neutrons.
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 DOE's Deuterium-Tritium Fusion Fuel.
614C 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 614C dating can determine the age of organic materials up to about 60000 years. The reference can be opened through the University of Chicago explainer.
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 614C are not materials for casual experiments, so their use requires safety rules.
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's Atomic Weights and Isotopic Compositions.
Carbon is an easy example:
Carbon isotope
Mass number
Natural abundance
612C
12
About 98.93%
613C
13
About 1.07%
614C
14
Extremely small
Because 612C is the most abundant carbon isotope, carbon's relative atomic mass on the periodic table is close to 12, but not exactly 12.
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 4092Zr:
n0=A−Z=92−40=52
So 4092Zr 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.
