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Atomic radius
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Atomic radius, is called the width of an atom, but it is true it is not a precisely defined physical quantity, nor is it constant in all circumstances. The value assigned to the radius of a particular atom always depends on the definition chosen for "atomic radius," and the appropriate definition depends on the context.
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.
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Atomic radius, is called the width of an atom, but it is true it is not a precisely defined physical quantity, nor is it constant in all circumstances. The value assigned to the radius of a particular atom always depends on the definition chosen for "atomic radius," and the appropriate definition depends on the context.
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.
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. Electrons, however, do not have definite positions—although they are more likely to be in certain regions than others—and the electron cloud does not have sharp edges.
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. Atomic radii decreases from Alkali Earth metals to the Noble Gases on the far right side of the periodic table. This is determined by the effective nuclear charge that increases with added electrons in a period. The atomic radii increases as you go down each column with added electrons in a period.
Periodic trends in atomic radius
Atomic radius tends to increase as one proceeds down any group of the periodic table. This trend is intuitively satisfying: atoms with more electrons have larger radii. As one proceeds across any row of the periodic table, a more complex rationale is required: atoms contract in size within a period from left to right. 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--this is also known as the nuclear charge of the atom.
| factor | principle | increase with... | tend to | effect |
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| electron shells | quantum mechanics | Principal Quantum Number, Azimuthal Quantum Number | atomic radius increase | increase when going down on group trend | | nuclear charge | attractive force acting on electrons by protons in nucleus | atomic number | atomic radius decrease | decrease when move from left to right on Periodic Table | | shielding | repulsive force acting on outermost shell electrons by inner electrons | number of electron shells | atomic radius increase | 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.
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. 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.
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
Reference: J.C. Slater, J. Chem. Phys. 1964, 41, 3199.
Calculated atomic radius
Calculated atomic radius in picometres (pm)
Reference: E. Clementi, D.L.Raimondi, and W.P. Reinhardt, J. Chem. Phys. 1967, 47, 1300.
See also
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