Learning Objective

Ionic radius Ions are particles in the same way as atoms only they carry an electrical charge. This is due to the number of electrons in the outer shells. If a particle has one fewer electrons then it carries a positive charge (there is an excess of protons in the nucleus). Revised effective ionic radii and systematic studies of interatomic distances in halides and chalcogenides. Search for more papers by this author.

• Identify the general trends of the ionic radius size for the periodic table.

Ionic Radii Definition

Key Points

• The ionic radius is the distance between the nucleus and the electron in the outermost shell of an ion.
• When an atom loses an electron to form a cation, the lost electron no longer contributes to shielding the other electrons from the charge of the nucleus; consequently, the other electrons are more strongly attracted to the nucleus, and the radius of the atom gets smaller.
• When an electron is added to an atom, forming an anion, the added electron repels other electrons, resulting in an increase in the size of the atom.
• The trend observed in size of ionic radii is due to shielding of the outermost electrons by the inner-shell electrons so that the outer shell electrons do not “feel” the entire positive charge of the nucleus.

Terms

• cationA positively charged ion, as opposed to an anion.
• ionAn atom or group of atoms bearing an electrical charge, such as the sodium and chlorine atoms in a salt solution.
• anionA negatively charged ion, as opposed to a cation.

In chemistry, periodic trends are the tendencies of certain elemental characteristics to increase or decrease along a period (row) or group (column) of the periodic table of elements. Ionic radius (r_ion) is the radius of an ion, regardless of whether it is an anion or a cation. Although neither atoms nor ions have sharp boundaries, it is useful to treat them as if they are hard spheres with radii. In this way, the sum of ionic radii of a cation and an anion can give us the distance between the ions in a crystal lattice. Ionic radii are typically given in units of either picometers (pm) or Angstroms (Å), with 1 Å = 100 pm. Typical values range from 30 pm (0.3 Å) to over 200 pm (2 Å).

Trends in Ionic Radii

Ions may be larger or smaller than the neutral atom, depending on the ion’s charge. When an atom loses an electron to form a cation, the lost electron no longer contributes to shielding the other electrons from the charge of the nucleus; consequently, the other electrons are more strongly attracted to the nucleus, and the radius of the atom gets smaller. Similarly, when an electron is added to an atom, forming an anion, the added electron repels other electrons, resulting in an increase in the size of the atom.

The ionic radius is not a fixed property of a given ion; rather, it varies with coordination number, spin state, and other parameters. For our purposes, we are considering the ions to be as close to their ground state as possible. Nevertheless, ionic radius values are sufficiently transferable to allow periodic trends to be recognized.

As with other types of atomic radii, ionic radii increase upon descending a group and decrease going across a period. Note that this only applies if the elements are the same type of ion, either cations or anions. For example, while neutral lithium is larger than neutral fluorine, the lithium cation is much smaller than the fluorine anion, due to the lithium cation having a different highest energy shell.

Ionic Radii Of Transition Metals

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Ionic Radii Shannon

http://en.wiktionary.org/wiki/anion
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http://en.wiktionary.org/wiki/cation
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http://en.wiktionary.org/wiki/ion
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http://en.wikipedia.org/wiki/Ionic_radius
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http://en.wikipedia.org/wiki/Periodic_trends
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Sorted by Ionic Radius

Ionic Radii Of Transition Metals

Name	Sym	#
0.012 Å	Hydrogen	H	1
0.13 Å	Nitrogen	N	7
0.23 Å	Boron	B	5
0.35 Å	Beryllium	Be	4
0.37 Å	Sulfur	S	16
0.38 Å	Phosphorus	P	15
0.4 Å	Silicon	Si	14
0.46 Å	Manganese	Mn	25
0.5 Å	Selenium	Se	34
0.52 Å	Uranium	U	92
0.52 Å	Chromium	Cr	24
0.53 Å	Germanium	Ge	32
0.535 Å	Aluminum	Al	13
0.56 Å	Technetium	Tc	43
0.56 Å	Rhenium	Re	75
0.58 Å	Arsenic	As	33
0.59 Å	Vanadium	V	23
0.605 Å	Titanium	Ti	22
0.62 Å	Gallium	Ga	31
0.62 Å	Tungsten	W	74
0.625 Å	Platinum	Pt	78
0.625 Å	Iridium	Ir	77
0.63 Å	Osmium	Os	76
0.64 Å	Tantalum	Ta	73
0.645 Å	Iron	Fe	26
0.65 Å	Molybdenum	Mo	42
0.68 Å	Ruthenium	Ru	44
0.68 Å	Rhodium	Rh	45
0.69 Å	Tin	Sn	50
0.69 Å	Nickel	Ni	28
0.69 Å	Niobium	Nb	41
0.71 Å	Hafnium	Hf	72
0.72 Å	Zirconium	Zr	40
0.72 Å	Magnesium	Mg	12
0.73 Å	Copper	Cu	29
0.74 Å	Zinc	Zn	30
0.745 Å	Cobalt	Co	27
0.745 Å	Scandium	Sc	21
0.75 Å	Neptunium	Np	93
0.76 Å	Antimony	Sb	51
0.76 Å	Lithium	Li	3
0.78 Å	Protactinium	Pa	91
0.8 Å	Indium	In	49
0.848 Å	Lutetium	Lu	71
0.85 Å	Gold	Au	79
0.858 Å	Ytterbium	Yb	70
0.86 Å	Palladium	Pd	46
0.869 Å	Thulium	Tm	69
0.881 Å	Erbium	Er	68
0.887 Å	Plutonium	Pu	94
0.9 Å	Yttrium	Y	39
0.901 Å	Holmium	Ho	67
0.912 Å	Dysprosium	Dy	66
0.923 Å	Terbium	Tb	65
0.925 Å	Einsteinium	Es	99
0.934 Å	Californium	Cf	98
0.938 Å	Gadolinium	Gd	64
0.947 Å	Europium	Eu	63
0.949 Å	Berkelium	Bk	97
0.964 Å	Samarium	Sm	62
0.97 Å	Tellurium	Te	52
0.97 Å	Cadmium	Cd	48
0.97 Å	Curium	Cm	96
0.972 Å	Thorium	Th	90
0.979 Å	Promethium	Pm	61
0.982 Å	Americium	Am	95
0.99 Å	Calcium	Ca	20
0.995 Å	Neodymium	Nd	60
1.013 Å	Praseodymium	Pr	59
1.02 Å	Sodium	Na	11
1.02 Å	Mercury	Hg	80
1.03 Å	Bismuth	Bi	83
1.034 Å	Cerium	Ce	58
1.061 Å	Lanthanum	La	57
1.1 Å	Nobelium	No	102
1.119 Å	Actinium	Ac	89
1.12 Å	Strontium	Sr	38
1.19 Å	Lead	Pb	82
1.26 Å	Silver	Ag	47
1.33 Å	Fluorine	F	9
1.35 Å	Barium	Ba	56
1.38 Å	Potassium	K	19
1.4 Å	Oxygen	O	8
1.43 Å	Radium	Ra	88
1.5 Å	Thallium	Tl	81
1.52 Å	Rubidium	Rb	37
1.67 Å	Cesium	Cs	55
1.8 Å	Francium	Fr	87
1.81 Å	Chlorine	Cl	17
1.96 Å	Bromine	Br	35
2.2 Å	Iodine	I	53
2.3 Å	Polonium	Po	84

[Last Updated: 2/22/2007]

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