When you look at the number of neutrons of elements in the periodic table, you will quickly notice that there are elements that have the same number of neutrons but different atomic numbers. These elements are called isotones. There are several factors that explain why isotones share the same neutron count despite being different elements:
I. Proton Count Variability
The number of neutrons in an atom is calculated by subtracting the atomic number (the number of protons) from the atomic mass number (the total number of protons and neutrons). Since the atomic mass of elements varies, different elements can have the same neutron count if their number of protons also differs.
II. The Relationship between Protons, Neutrons, and Stability
The stability of an atomic nucleus depends on the balance between protons and neutrons. For lighter elements, the ratio of neutrons to protons is close to 1:1, but as atomic number increases, more neutrons are required to stabilize the nucleus due to increasing proton-proton repulsion. Because of this natural tendency for nuclear stability, certain neutron counts are more common across different elements, leading to the formation of isotones.
III. Nuclear Forces and Magic Numbers
The strong nuclear force binds protons and neutrons together in the nucleus. This force favors specific numbers of neutrons and protons that lead to stable configurations. Magic numbers (such as 2, 8, 20, 28, 50, 82, and 126) correspond to especially stable numbers of protons or neutrons. Elements with neutron counts close to these numbers are more likely to appear as isotones.
IV. Nucleosynthesis and Element Formation
Elements are formed through nuclear reactions in stars, and the number of neutrons they acquire depends on processes like neutron capture, beta decay, and proton-neutron transformations. Since different elements can be formed under similar conditions, it is common for some to end up with the same neutron count but different proton numbers.
Chemical and Physical Differences
Despite having the same number of neutrons, isotones can have different chemical properties because the number of protons determines the element’s identity and its chemical behavior. The different proton count results in different electron configurations and different behavior in chemical reactions.
Examples of Isotones
Here are 20 examples of isotones—elements that have the same number of neutrons but different numbers of protons:
| Neutron Count | Element 1 | Element 2 |
| 6 | Boron-11 (₅B) | Carbon-12 (₆C) |
| 7 | Carbon-13 (₆C) | Nitrogen-14 (₇N) |
| 8 | Nitrogen-15 (₇N) | Oxygen-16 (₈O) |
| 9 | Oxygen-17 (₈O) | Fluorine-18 (₉F) |
| 10 | Fluorine-19 (₉F) | Neon-20 (₁₀Ne) |
| 11 | Neon-21 (₁₀Ne) | Sodium-22 (₁₁Na) |
| 12 | Sodium-23 (₁₁Na) | Magnesium-24 (₁₂Mg) |
| 13 | Magnesium-25 (₁₂Mg) | Aluminum-26 (₁₃Al) |
| 14 | Aluminum-27 (₁₃Al) | Silicon-28 (₁₄Si) |
| 15 | Silicon-29 (₁₄Si) | Phosphorus-30 (₁₅P) |
| 16 | Phosphorus-31 (₁₅P) | Sulfur-32 (₁₆S) |
| 17 | Sulfur-33 (₁₆S) | Chlorine-34 (₁₇Cl) |
| 18 | Chlorine-35 (₁₇Cl) | Argon-36 (₁₈Ar) |
| 19 | Argon-37 (₁₈Ar) | Potassium-38 (₁₉K) |
| 20 | Potassium-39 (₁₉K) | Calcium-40 (₂₀Ca) |
Each pair has the same number of neutrons but belongs to different elements due to their different proton counts.
