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

OSIRIS-REx Returns This Sunday!

On Sunday, September 24th, NASA’s OSIRIS-REx mission will deliver samples from the asteroid Benny.

11 hours ago

Machine Learning Algorithms Can Find Anomalous Needles in Cosmic Haystacks

The face of astronomy is changing. Though narrow-field point-and-shoot astronomy still matters (JWST anyone?), large…

20 hours ago

The JWST is Forcing Astronomers to Rethink Early Galaxies

The JWST has surprised astronomers again. Contrary to our existing understanding, the JWST showed us…

2 days ago

The JWST Just Found Carbon on Europa, Boosting the Moon’s Potential Habitability

Most planets and moons in the Solar System are clearly dead and totally unsuitable for…

2 days ago

If You Could See Gravitational Waves, the Universe Would Look Like This

Our biology limits our vision. Our eyes can only perceive specific wavelengths of light. But…

2 days ago

Solar Sails Could Reach Mars in Just 26 Days

A recent study submitted to Acta Astronautica explores the potential for using aerographite solar sails…

2 days ago