Periodic Trends — Atomic and Ionic Radii

1A 2A 3A 4A 5A 6A 7A 8A
(1) (2) (13) (14) (15) (16) (17) (18)
3B 4B 5B 6B 7B 8B 1B 2B
(3) (4) (5) (6) (7) (8) (9) (10) (11) (12)
1

H

78

He

32

2

Li

152

Be

113

B

83

C

77

N

71

O

73

F

71

Ne

70

3

Na

186

Mg

160

Al

143

Si

117

P

115

S

104

Cl

99

Ar

98

4

K

227

Ca

197

Sc

161

Ti

145

V

132

Cr

125

Mn

124

Fe

124

Co

125

Ni

125

Cu

128

Zn

133

Ga

122

Ge

123

As

125

Se

117

Br

114

Kr

112

5

Rb

248

Sr

215

Y

181

Zr

160

Nb

143

Mo

136

Tc

136

Ru

134

Rh

134

Pd

138

Ag

144

Cd

149

In

163

Sn

141

Sb

141

Te

143

I

133

Xe

130

6

Cs

265

Ba

217

La

188

 

Hf

156

Ta

143

W

137

Re

137

Os

135

Ir

136

Pt

138

Au

144

Hg

160

Tl

170

Pb

175

Bi

155

Po

167

At

n.a.

Rn

145

7

Fr

270

Ra

223

Ac

188

 

Rf

150

Db

139

Sg

132

Bh

128

Hs

126

Mt

n.a.

Ds

n.a.

Rg

n.a.

Uub

n.a.

Uuq

n.a.

6

 

Ce

182

Pr

183

Nd

182

Pm

181

Sm

180

Eu

185

Gd

180

Tb

178

Dy

177

Ho

177

Er

176

Tm

175

Yb

170

Lu

173

7

 

Th

180

Pa

161

U

154

Np

150

Pu

175

Am

173

Cm

174

Bk

170

Cf

169

Es

203

Fm

n.a.

Md

n.a.

No

n.a.

Lr

n.a.

Atomic radii reported in units of picometers (pm).

Data taken from John Emsley, The Elements, 3rd edition.  Oxford:  Clarendon Press, 1998.

 

The atomic radius is the distance from the nucleus of an atom to the outermost electrons.  Since the orbitals around an atom are defined in terms of a probability distribution in quantum mechanics, and do not have fixed boundaries, determining where an atom "stops" is not very straightforward.  By comparing the bond lengths of a number of representative compounds of an element, an average size for most atoms can be determined.

The atomic radius can also be defined in other ways.  The van der Waals radius (also known as the nonbonding atomic radius) is the radius of an atom which is not bonded to other atoms; this is determined by measuring the distance between atomic nuclei which are in direct but nonbonding contact with each other in a crystal lattice.  The covalent atomic radius (also known as the bonding atomic radius) is determined for metals by taking one-half of the distance between two adjacent atoms in a metallic crystal, or one-half the distance between like bonded atoms for nonmetals.

Unfortunately, it is not possible to determine the radius for every element on the periodic table in the same way, and consequently, it is sometimes difficult to make comparisons between different sets of data.  In the table above, most of the atomic radii listed are average atomic radii, while for the halogens (Group 7A) and the noble gases (Group 8A) the covalent radius is used.

Atomic radii vary in a predictable way across the periodic table.  As can be seen in the figures below, the atomic radius increases from top to bottom in a group, and decreases from left to right across a period.  Thus, helium is the smallest element, and francium is the largest.

  • From top to bottom in a group, orbitals corresponding to higher values of the principal quantum number (n) are being added, which are on average further away from the nucleus, thus causing the size of the atom to increase.
  • From left to right across a period, more protons are being added to the nucleus, but the electrons which are being added are being added to the valence shell, not to the lower energy levels.  As more protons are added to the nucleus, the electrons in the valence shell feel a higher effective nuclear charge — the sum of the charges on the protons in the nucleus and the charges on the inner, core electrons.  (See figure below.)  The valence electrons are therefore held more tightly, and the size of the atom contracts across a period.

 

The following charts illustrate the general trends in the radii of atoms:

 

 

 

The sizes of cations and anions follow similar trends to those of neutral atoms.  In general, anions are larger than the corresponding neutral atom, since adding electrons increases the number of electron-electron repulsion interactions that take place.  Cations are smaller than the corresponding neutral atoms, since the valence electrons, which are furthest away from the nucleus, are lost.  Taking more electrons away from the cation further reduces the radius of the ion.

The table below illustrates these trends for the main group elements.  For elements which form more than one cation, the cation charges and sizes are listed in two separate columns.  The transition metals and inner transition metals have been omitted, since almost all of those elements can form two or more possible cations.

 

Sizes of Common Cations and Anions of the Main Group Elements

Atomic

Number

Name

Neutral

 Atom

(ppm)

Cation1

Charge

Cation1

Radius

(ppm)

Cation2

Charge

Cation2

Radius

(ppm)

Anion

Charge

Anion

Radius

(ppm)

1 Hydrogen 78 1+ 0.00066     1- 154
2 Helium 32            
3 Lithium 152 1+ 78        
4 Beryllium 113 2+ 34        
5 Boron 83 3+ 23        
6 Carbon 77         4- 260
7 Nitrogen 71         3- 171
8 Oxygen 73         2- 132
9 Fluorine 71         1- 133
10 Neon 70            
11 Sodium 186 1+ 98        
12 Magnesium 160 2+ 79        
13 Aluminum 143 3+ 57        
14 Silicon 117 4+ 26     4- 271
15 Phosphorus 115         3- 212
16 Sulfur 104         2- 184
17 Chlorine 99         1- 181
18 Argon 98            
19 Potassium 227 1+ 133        
20 Calcium 197 2+ 106        
31 Gallium 122 3+ 62 1+ 113    
32 Germanium 123 2+ 90     4- 272
33 Arsenic 125 5+ 46 3+ 69 3- 222
34 Selenium 117 4+ 69     2- 191
35 Bromine 114         1- 195
36 Krypton 112            
37 Rubidium 248 1+ 149        
38 Strontium 215 2+ 127        
49 Indium 163 3+ 92 1+ 132    
50 Tin 141 4+ 74 2+ 93 4- 294
51 Antimony 141 5+ 62 3+ 89 3- 245
52 Tellurium 143 6+ 56 4+ 97 2- 211
53 Iodine 133         1- 196
54 Xenon 130            
55 Cesium 265 1+ 165        
56 Barium 217 2+ 143        
81 Thallium 170 3+ 105 1+ 149    
82 Lead 175 4+ 84 2+ 132    
83 Bismuth 155 5+ 74 3+ 96    
84 Polonium 167 4+ 65     2- 230
85 Astatine   5+ 57     1- 227
86 Radon 145            
87 Francium 270 1+ 180        
88 Radium 223 2+ 152        

Data taken from John Emsley, The Elements, 3rd edition.  Oxford:  Clarendon Press, 1998.