Categories: Astronomy

[/caption]
If you can imagine an atom, of an element, as a sphere, then the radius of that sphere would be the atomic radius of that element. However, as atoms are things far better described using quantum mechanics than classical physics, the definition – even the concept – of atomic radius is tricky (and, in fact, there are actually several different definitions!).

Start with the Bohr atomic model, and an atom of hydrogen. In this model, the atomic radius (Bohr radius) is related to the lowest energy level of the electron, and has an exact value which involves Planck’s constant, the fine structure constant, c (speed of light), and the mass of the electron ( h/(2πcαme) – approx 53 pm … that’s picometers, trillionths of a meter, in case you were wondering). Although the Bohr model of the atom is no longer used, except in teaching, the Bohr atom radius for hydrogen is a key physical constant.

If you have a crystal, of a salt, you can study it with x-rays, and work out how far apart the entities in the crystal lattice are; those entities are ions (not atoms), so the atomic radii estimated this way are called ionic radii. No surprise that the ionic radii of a particular element depend on the ionization state of the ions!

In a metal, one or more outer electrons become part of the sea of electrons throughout the metal, which give the metal its high electrical conductivity. The atomic radii of metal atoms in this environment are called metallic radii.

By now you should be able to guess what the covalent radius is (in molecules with covalent bonds, the atomic radii are estimated from the bond lengths), and what the Van der Waals radius is (if two atoms are not bound in a molecule, the minimum distance between them is determined by the Van der Waals force, and radii estimated this way are …).

Chemguide, a UK site, has a nice intro, Atomic and Ionic Radius; this Frostburg State University page is a bit more advanced ; and here is a list of the elements, sorted by atomic radius.

Astronomy Cast episodes relevant to this topic include Quantum Mechanics, Wave Particle Duality, and Inside the Atom.

Jean Tate

Hi! When I was only six (or so), I went out one clear but windy night with my uncle and peered through the eyepiece of his home-made 6" Newtonian reflector. The dazzling, shimmering, perfect globe-and-ring of Saturn entranced me, and I was hooked on astronomy, for life. Today I'm a freelance writer, and began writing for Universe Today in late 2009. Like Tammy, I do like my coffee, European strength please. Contact me: JeanTate.UT@gmail.com

Share
Jean Tate

## China’s Chang’e-6 Probe Drops Off Samples From Moon’s Far Side

Three weeks after it lifted off from the far side of the moon, China's Chang'e-6…

5 hours ago

## Simulating the Last Moments Before Neutron Stars Merge

When stars reach the end of their life cycle, they shed their outer layers in…

11 hours ago

## Growing Black Holes Have Much in Common With Baby Stars

First looks would tell most observers that supermassive black holes (SMBHs) and very young stars…

16 hours ago

## NASA Doesn't Know When Starliner Will Return From Orbit

After helium leaks and thruster problems with Boeing’s Starliner capsule, NASA has been pushing back…

18 hours ago

## Advanced Optics Could Help Us Find Earth 2.0

NASA has long been interested in building bigger and better space telescopes. Its Institute for…

20 hours ago

## Satellites are Going to Track Garbage Drifting Across the Oceans

We are all too aware of the pollution on planet Earth. There are increased amounts…

21 hours ago