Atomic radius

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Atomic radius:
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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 calculation 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

[edit] Periodic trends in atomic radius

Atomic radius tends to increase as one proceeds down any group of the periodic table. This satisfies simple intuition: atoms with more electrons have larger radii. However as one proceeds across any row of the periodic table, a deeper intuition is required: atoms with more numerous electrons exhibit decreasing radius. This contraction results from the increasing number of protons in the nucleus. Protons make little contribution to the size of the atom, but they increase the positive charge of the nucleus, which draws the electrons into tighter orbitals.

factor principle increase with... tend to effect
electron shells quantum mechanics Principal Quantum Number, Azimuthal Quantum Number atomic radius↑ increase on descending a group
nuclear charge attractive force acting on electrons by protons in nucleus atomic number atomic radius↓ decrease on passing along a period
shielding repulsive force acting on outermost shell electrons by inner electrons number of electron shells atomic radius↑ reduce the effect of the 2nd factor

The increasing nuclear charge is partly counterbalanced by the increasing number of electrons in a phenomenon that is 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.

[edit] Lanthanide contraction

Main article: 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.[3] Hence hafnium has virtually the same atomic radius (and chemistry) as zirconium, and tantalum has an atomic radius similar to niobium, and so forth. 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.

[edit] d-Block contraction

Main article: 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).[3]

[edit] 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: Bob Jones, J. Chem. Phys. 1964, 41, 3199.

[edit] 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. 1967, 47, 1300.

[edit] See also

[edit] 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. ^ a b Jolly, William L. (1991). Modern Inorganic Chemistry (2nd Edn.). New York: McGraw-Hill. ISBN 0-07-112651-1. p. 22.