ATOMIC RADIUS

'Atomic radius', and more generally the 'size of an atom', is not a precisely defined physical quantity, nor is it constant in all circumstances.^[1] The value assigned to the radius of a particular atom will always depend on the definition chosen for "atomic radius", and different definitions are more appropriate for different situations.
The term "atomic radius" itself is problematic: it may be restricted to the size of free atoms, or it may be used as a general term for the different measures of the size of atoms, both bound in molecules and free. In the latter case, which is the approach adopted here, it should also include 'ionic radius', as the distinction between covalent and ionic bonding is itself somewhat arbitrary.^[2]
The atomic radius is determined entirely by the electrons: The size of the atomic nucleus is measured in femtometres, 100,000 times smaller than the cloud of electrons. However the electrons do not have definite positions—although they are more likely to be in certain regions than others—and the electron cloud does not have a sharp edge.
Despite (or maybe because of) these difficulties, many different attempts have been made to quantify the size of atoms (and ions), based both on experimental measurements and calculational methods. It is undeniable that atoms ''do'' behave as if they were spheres with a radius of 30–300 pm, that atomic size varies in a predictable and explicable manner across the periodic table and that this variation has important consequences for the chemistry of the elements.

Contents

Periodic trends in atomic radius

Lanthanide contraction

d-Block contraction

Empirically measured atomic radius

Calculated atomic radius

See also

References

External links

Periodic trends in atomic radius

Atomic radius tends to decrease on passing along a period of the periodic table from left to right, and to increase on descending a group. This is, in part, because the distribution of electrons is not completely random. The electrons in an atom are arranged in shells which are, on average, further and further from the nucleus, and which can only hold a certain number of electrons.^[3] Each new period of the periodic table corresponds to a new shell which starts to be filled up, and so the outermost electrons are further and further from the nucleus as a group is descended.
The second major effect which determines trends in atomic radius is the charge of the nucleus, which increases with the atomic number, ''Z''. The nucleus is positively charged, and tends to attract the negatively-charged electrons. Passing along a period from left to right, the nuclear charge increases while the electrons are still entering the same shell: the effect is that the physical size of the shell (and hence of the atom) decreases in response.
The increasing nuclear charge is partly counterbalanced by the increasing number of electrons in a phenomenon known as shielding, which is why the size of atoms usually increases as a group is descended. However, there are two occasions where shielding is less effective: in these cases, the atoms are smaller than would otherwise be expected.

Lanthanide contraction

Main articles: Lanthanide contraction

The electrons in the 4f-subshell, which is progressively filled from cerium (''Z'' = 58) to lutetium (''Z'' = 71), are not particularly effective at shielding the increasing nuclear charge from the sub-shells further out. The elements immediately following the lanthanides have atomic radii which are smaller than would be expected and which are almost identical to the atomic radii of the elements immediately above them.^[4] Hence hafnium has virtually the same atomic radius (and chemistry) as zirconium, tantalum as niobium etc. The effect of the lanthanide contraction is noticeable up to platinum (''Z'' = 78), after which it is masked by a relativistic effect known as the inert pair effect.

d-Block contraction

Main articles: d-Block contraction

The d-block contraction is less pronounced than the lanthanide contraction but arises from a similar cause. In this case, it is the poor shielding capacity of the 3d-electrons which affects the atomic radii and chemistries of the elements immediately following the first row of the transition metals, from gallium (''Z'' = 31) to bromine (''Z'' = 35).

Empirically measured atomic radius

Empirically measured atomic radius
in picometres (pm) to an accuracy of about 5 pm

'Group' (vertical)

'1'

'2'

'3'

'4'

'5'

'6'

'7'

'8'

'9'

'10'

'11'

'12'

'13'

'14'

'15'

'16'

'17'

'18'

'Period' (horizontal)

'1'

H 25

He

'2'

Li 145

Be 105

B 85

C 70

N 65

O 60

F 50

Ne

'3'

Na 180

Mg 150

Al 125

Si 110

P 100

S 100

Cl 100

Ar 71

'4'

K 220

Ca 180

Sc 160

Ti 140

V 135

Cr 140

Mn 140

Fe 140

Co 135

Ni 135

Cu 135

Zn 135

Ga 130

Ge 125

As 115

Se 115

Br 115

Kr

'5'

Rb 235

Sr 200

Y 180

Zr 155

Nb 145

Mo 145

Tc 135

Ru 130

Rh 135

Pd 140

Ag 160

Cd 155

In 155

Sn 145

Sb 145

Te 140

I 140

Xe

'6'

Cs 260

Ba 215

★

Hf 155

Ta 145

W 135

Re 135

Os 130

Ir 135

Pt 135

Au 135

Hg 150

Tl 190

Pb 180

Bi 160

Po 190

At

Rn

'7'

Fr

Ra 215

★ ★

Rf

Db

Sg

Bh

Hs

Mt

Ds

Rg

Uub

Uut

Uuq

Uup

Uuh

Uus

Uuo

Lanthanides

★

La 195

Ce 185

Pr 185

Nd 185

Pm 185

Sm 185

Eu 185

Gd 180

Tb 175

Dy 175

Ho 175

Er 175

Tm 175

Yb 175

Lu 175

Actinides

★ ★

Ac 195

Th 180

Pa 180

U 175

Np 175

Pu 175

Am 175

Cm

Bk

Cf

Es

Fm

Md

No

Lr

Periodic table of empirically measured atomic radius
in picometres (pm) to an accuracy of about 5 pm
See also Periodic table
'Reference:' J.C. Slater, ''J. Chem. Phys.'' 1964, '41', 3199.

Calculated atomic radius

Calculated atomic radius in picometres (pm)

'Group' (vertical)

'1'

'2'

'3'

'4'

'5'

'6'

'7'

'8'

'9'

'10'

'11'

'12'

'13'

'14'

'15'

'16'

'17'

'18'

'Period' (horizontal)

'1'

H 53

He 31

'2'

Li 167

Be 112

B 87

C 67

N 56

O 48

F 42

Ne 38

'3'

Na 190

Mg 145

Al 118

Si 111

P 98

S 88

Cl 79

Ar 71

'4'

K 243

Ca 194

Sc 184

Ti 176

V 171

Cr 166

Mn 161

Fe 156

Co 152

Ni 149

Cu 145

Zn 142

Ga 136

Ge 125

As 114

Se 103

Br 94

Kr 88

'5'

Rb 265

Sr 219

Y 212

Zr 206

Nb 198

Mo 190

Tc 183

Ru 178

Rh 173

Pd 169

Ag 165

Cd 161

In 156

Sn 145

Sb 133

Te 123

I 115

Xe 108

'6'

Cs 298

Ba 253

★

Hf 208

Ta 200

W 193

Re 188

Os 185

Ir 180

Pt 177

Au 174

Hg 171

Tl 156

Pb 154

Bi 143

Po 135

At

Rn 120

'7'

Fr

Ra

★ ★

Rf

Db

Sg

Bh

Hs

Mt

Ds

Rg

Uub

Uut

Uuq

Uup

Uuh

Uus

Uuo

Lanthanides

★

La

Ce

Pr 247

Nd 206

Pm 205

Sm 238

Eu 231

Gd 233

Tb 225

Dy 228

Ho

Er 226

Tm 222

Yb 222

Lu 217

Actinides

★ ★

Ac

Th

Pa

U

Np

Pu

Am

Cm

Bk

Cf

Es

Fm

Md

No

Lr

Periodic table of calculated atomic radius
in picometres (pm)
See also Periodic table
'Reference:' E. Clementi, D.L.Raimondi, and W.P. Reinhardt, ''J. Chem. Phys.'' 1963, '38', 2686.

See also

★ Atomic radii of the elements (data page)

★ Chemical bond

★ Bond length

★ Steric hindrance

References

Periodicity
1. Cotton, F. A.; Wilkinson, G. (1988). ''Advanced Inorganic Chemistry'' (5th Edn). New York: Wiley. ISBN 0-471-84997-9. p. 1385.
2. ''See also'' the definition of an 'atom' as "the smallest unit quantity of an element that is capable of existence whether alone or in chemical combination with other atoms of the same or other elements." IUPAC Commission on the Nomenclature of Inorganic Chemistry (1990). ''Nomenclature of Inorganic Chemistry.'' Oxford: Blackwell Scientific. ISBN 0-632-02494-1. p. 35.
3. The ''n''th electron shell can hold 2''n''² electrons.
4. Jolly, William L. (1991). ''Modern Inorganic Chemistry'' (2nd Edn.). New York: McGraw-Hill. ISBN 0-07-112651-1. p. 22.

External links

★ WebElements