Atomic Radius of Different Elements in Picometers (pm)
A complete guide to understanding atomic radii measured in picometers, including a reference list of common elements like Hydrogen, Carbon, and Oxygen.
Staring at an atomic radius on a periodic table or inside a textbook and trying to visualize its actual size is a nightmare. Human brains do not naturally process something that is 10⁻¹² metres wide. When calculating bond lengths or lattice structures, the decimals pile up fast, and panic sets in.
But let's look at the numbers. Scientists rely almost exclusively on the picometer (pm) and the ångström (Å) to measure atoms because using anything larger, like millimetres, results in microscopic decimals that guarantee calculation errors. The picometer provides a clean, whole-number scale so you can focus on the chemistry, not the zeros.
Here is the reality: you do not have to do the math in your head. Use the calculator below to instantly convert any atomic radius into nanometers or ångströms.
What is Atomic Radius?
The atomic radius is a measure of the size of an atom. Simply put, it is the typical distance from the center of the nucleus to the boundary of the surrounding electron shells.
Because electrons do not have a hard edge, the exact number can change depending on if it is a covalent radius, Van der Waals radius, or metallic radius. The one constant? It is almost always reported in picometers (pm).
The Size of Common Elements
Here is a quick reference table showing the empirically measured (covalent) atomic radii of several essential elements. Keep this handy when double-checking your math.
| Element | Symbol | Atomic Radius (pm) | In Ångströms (Å) | In Nanometers (nm) |
|---|---|---|---|---|
| Hydrogen | H | 53 pm | 0.53 Å | 0.053 nm |
| Helium | He | 31 pm | 0.31 Å | 0.031 nm |
| Carbon | C | 77 pm | 0.77 Å | 0.077 nm |
| Nitrogen | N | 75 pm | 0.75 Å | 0.075 nm |
| Oxygen | O | 73 pm | 0.73 Å | 0.073 nm |
| Sodium | Na | 190 pm | 1.90 Å | 0.190 nm |
| Silicon | Si | 111 pm | 1.11 Å | 0.111 nm |
| Gold | Au | 144 pm | 1.44 Å | 0.144 nm |
The Smallest and Largest Atoms
- The Smallest: Helium (He) holds the record at roughly 31 pm. Even though Hydrogen has fewer protons, Helium's extra proton pulls its electron shell tighter.
- The Largest: Cesium (Cs) is massive, clocking in at an atomic radius of approximately 343 pm.
Why Does Atomic Size Matter?
The atomic radius is not just a trivia fact; it dictates exactly how an element behaves in a reaction:
- Bond Lengths: The distance between bonded nuclei is just the sum of their radii. A Carbon-Carbon single bond is about 154 pm (77 pm + 77 pm).
- Material Density: In solids, atom size dictates how tightly they pack. Tighter packing means higher density.
- Reactivity: Large atoms have electrons far from the nucleus. This makes it easier for them to lose those electrons and react violently.
How to Convert Atomic Radii
If you are stuck staring at an older paper using ångströms, the math is straightforward.
Picometers to Ångströms: Divide by 100. Picometers to Nanometers: Divide by 1,000.
For example, a Carbon radius of 0.77 Å becomes 77 pm.
If your brain feels fried and you just need the answer, head over to our main Picometer Converter or read What Is a Picometer? to brush up on the fundamentals.
Ready to run the numbers?
Get your result instantly — private, in your browser.