Atomic radius

Atomic radius
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Atomic radius:
Ionic radius
Covalent radius
Metallic radius
van der Waals radius
edit

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 [hide]
1 Periodic trends in atomic radius
1.1 Lanthanide contraction
1.2 d-Block contraction
2 Empirically measured atomic radius
3 Calculated atomic radius
4 See also
5 References
6 External links



[edit] 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.


[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.[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.


[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).[4]


[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: J.C. Slater, 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. 1963, 38, 2686.


[edit] See also
Atomic radii of the elements (data page)
Chemical bond
Bond length
Steric hindrance

[edit] References
^ Cotton, F. A.; Wilkinson, G. (1988). Advanced Inorganic Chemistry (5th Edn). New York: Wiley. ISBN 0-471-84997-9. p. 1385.
^ 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.
^ The nth electron shell can hold 2n2 electrons.
^ a b Jolly, William L. (1991). Modern Inorganic Chemistry (2nd Edn.). New York: McGraw-Hill. ISBN 0-07-112651-1. p. 22.

[edit] External links
WebElements
Retrieved from "http://en.wikipedia.org/wiki/Atomic_radius"
Categories: Chemical properties | Chemical bonding