Actinium - ₈₉Ac: radii of atoms and ions

One measure of size is the element-element distance within the element. It is not always easy to make sensible comparisons between the elements however as some bonds are quite short because of multiple bonding (for instance the O=O distance in O₂ is short because of the the double bond connecting the two atoms. The bond length in AcAc is: 375.6pm.

There are several other ways ways to define radius for atoms and ions. Follow the appropriate hyperlinks for literature references and definitions of each type of radius. All values of radii are given in picometres (pm). Conversion factors are:

• 1 pm = 1 × 10^‑12 metre (meter)
• 100 pm = 1 Ångstrom
• 1000 pm = 1 nanometre (nm, nanometer)

Neutral radii

The size of neutral atoms depends upon the way in which the measurement is made and the environment. Follow the appropriate hyperlinks for definitions of each radius type. The term "atomic radius" is not particularly helpful although its use is widespread. The problem is its meaning, which is clearly very different in different sources and books. Two values are given here, one is based upon calculations and the other upon observation - follow the appropriate link for further details.

Radius type	Radius value / pm	Periodicity link
Atomic radius (empirical)	195
Atomic radius (calculated)	-
Covalent radius (2008 values)	215
Molecular single bond covalent radius	186 (coordination number 3)
Molecular double bond covalent radius	153
Molecular triple bond covalent radius	140
Covalent radius (empirical)	-
van der Waals radius	283

Valence shell orbital radii

The following are calculated values of valence shell orbital radii, R_max

Table: valence shell orbital radii for actinium.

Orbital	Radius [/pm]	Radius [/AU]	Periodicity link
s orbital	235.4	4.44755
p orbital	-	-
d orbital	130.8	2.47083
f orbital	-	-

References

The R_max values for neutral gaseous element valence orbitals are abstracted from reference 1.

1. J.B. Mann, Atomic Structure Calculations II. Hartree-Fock wave functions and radial expectation values: hydrogen to lawrencium, LA-3691, Los Alamos Scientific Laboratory, USA, 1968.

Ionic radii

This table gives some ionic radii. In this table, geometry refers to the arrangment of the ion's nearest neighbours. Size does depend upon geometry and environment. For electronic configurations, where it matters, the values given for octahedral species are low spin unless stated to be high spin. The terms low spin and high spin refer to the electronic configurations of particular geomtries of certain d-block metal ions. Further information is available in inorganic chemistry textbooks, usually at Level 1 or First Year University level. For definitions of ionic radius and further information, follow the hypertext link.

Pauling ionic radii

This table shows Pauling radii for actinium

Ion	Pauling radius / pm	Periodicity link
Ac(I)	-