Atoms of completely different elements can share the same number of neutrons. When they do, they’re called isotones. For example, sulfur-36 (16 protons, 20 neutrons) and chlorine-37 (17 protons, 20 neutrons) are isotones because both nuclei contain exactly 20 neutrons, even though they are different elements with different chemical properties.
What Isotones Are
An isotone is any atom that has the same neutron count as another atom of a different element. The term comes from the same Greek root as “isotope,” but where isotopes share the same number of protons, isotones share the same number of neutrons. A simple mnemonic: the “n” in isotone stands for neutrons.
To find the neutron number of any atom, subtract its atomic number (the number of protons, Z) from its mass number (the total count of protons plus neutrons, A). The formula is straightforward: N = A − Z. If two atoms from different elements yield the same N, they are isotones.
Isotones vs. Isotopes vs. Isobars
These three terms describe different ways atoms can resemble each other at the nuclear level. Keeping them straight is easier when you focus on what stays the same in each case.
- Isotopes: Same element, same number of protons, different number of neutrons. Hydrogen-1, hydrogen-2, and hydrogen-3 are all hydrogen, but they contain 0, 1, and 2 neutrons respectively.
- Isotones: Different elements, different number of protons, same number of neutrons. Sulfur-36 and chlorine-37 both have 20 neutrons.
- Isobars: Different elements, same mass number (total protons + neutrons), but different proportions of each. Argon-40 and calcium-40 both have a mass number of 40, yet argon has 18 protons and 22 neutrons while calcium has 20 of each.
The key distinction is which number matches. Isotopes match on proton count, isotones match on neutron count, and isobars match on the total mass number.
How to Find Isotone Pairs
You can identify isotones yourself with a periodic table and a list of common isotopes. Pick any atom, calculate its neutron number (mass number minus atomic number), then look for atoms of other elements with the same result. A few well-known isotone groups:
- N = 2 (2 neutrons): Helium-4 (2 protons) and lithium-5 (3 protons)
- N = 8 (8 neutrons): Carbon-14 (6 protons), nitrogen-15 (7 protons), oxygen-16 (8 protons)
- N = 20 (20 neutrons): Sulfur-36 (16 protons), chlorine-37 (17 protons), argon-38 (18 protons), potassium-39 (19 protons), calcium-40 (20 protons)
The N = 20 group is particularly large because 20 is one of the so-called “magic numbers” in nuclear physics, which brings us to why neutron count matters in the first place.
Why Neutron Number Matters for Stability
Neutrons play a central role in holding a nucleus together. They add to the strong nuclear force that binds protons and neutrons, without adding the electrical repulsion that extra protons would. The ratio of neutrons to protons largely determines whether a nucleus is stable or radioactive.
Certain neutron counts produce unusually stable nuclei. Nuclear physicists call these “magic numbers”: 2, 8, 20, 28, 50, 82, and 126. At these numbers, neutrons fill complete energy shells inside the nucleus, similar to how electrons fill shells around the atom. When a shell is completely filled, the nucleus resists breaking apart. That’s why you’ll find more stable isotones clustered around these magic numbers than at other neutron counts.
Isotones don’t share chemical properties the way isotopes do. Chemistry is governed by electrons, which are determined by proton count. Two isotones like sulfur and chlorine behave completely differently in chemical reactions. What they share is a nuclear structural similarity: the same neutron framework, which can lead to comparable nuclear stability and similar patterns of radioactive decay.
Isotones on the Chart of Nuclides
Scientists visualize all known nuclei on a grid called the chart of nuclides (sometimes called the Segrè chart). The horizontal axis plots neutron number and the vertical axis plots proton number. On this chart, isotopes (same element, different neutron count) appear as horizontal rows, isobars (same mass number) fall along diagonal lines, and isotones (same neutron count) line up in vertical columns. Each vertical column is a family of isotones sharing one neutron number, making it easy to compare nuclear properties across different elements at a glance.
This visual arrangement is especially useful for spotting stability patterns. The vertical columns at magic numbers tend to contain more stable nuclei than neighboring columns, confirming that certain neutron counts genuinely confer extra nuclear resilience regardless of which element the atom belongs to.