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.