Atomic and ionic radii
The size of an atom can be visualized from its atomic radius. The term atomic or ionic radius is generally defined as the distance between the centers of the nucleus and the outermost shell of electrons in an atom or ion. For example, the atomic radius of hydrogen atom is equal to 74/2 pm = 37 (bond distance in hydrogen molecule (H2) is 74pm).
Atomic and ionic radii both decrease from left to right across a period in the periodic table when we consider only normal elements, e.g. in the elements of 2nd period the covalent radii decrease as we move from Li to F as shown below:
Elements of 2nd period :
Li Be B C N O F
Covalent radii Values decreasing from Li to F
Thus in any period the alkali metals (that are present at the extreme left of the periodic table) have the largest size while the halogens (that are present at the extreme right, excluding the zero group elements) have the smallest size.
We know that as we proceed from left to right in a period, the electrons are added to the orbitals of the same main energy level. Addition of different electrons to the same main energy level puts the electrons, on the average, no farther from the nucleus and hence the size can not be increased. But with the addition of each electron, the nuclear charge (i.e. atomic number) increases by one. The increased nuclear charge attracts the electrons more strongly close to the nucleus and thus decreases the size of the atoms.
Atomic Radii / pm Across the periods
Atom Atomic Atom Atomic
Li 152 Na 186
Be 111 Mg 160
B 88 Al 143
C 77 Si 117
N 70 P 110
O 74 S 104
F 72 Cl 99
(b) In a group
On moving down a group of regular elements both atomic and ionic radii increase with increasing atomic number, e.g. in the elements of IA Group both covalent and ionic radii of M+ ions increase when we pass from Li to Cs
Elements of IA Group : Li Na K Rb Cs
Covalent radii/Ionic radii Values increasing from Li to Cs
On proceeding downwards in a group the electrons are added to higher main energy levels, which are, on the average, farther from the nucleus. This effect decreases the electrostatic attraction between the nucleus and the valence-shell electrons and this decreased electrostatic attraction increases the atomic and ionic radii.
Table : Atomic Radii / pm Down the Group Across a Family
Atom Atomic Atom Atomic
Li 152 F 72
Na 186 Cl 99
K 231 Br 114
Rb 244 I 133
Cs 262 At 140
When we find some atoms and ions, which contain the same number of electrons, we call them isoelectronic. For example, O2-, F-, Na+ and Mg2+ have the same number of electrons (10). Their radii would be different because of their different nuclear charges. The cation with the greater positive charge will have a smaller radius because of the greater attraction of the electrons to the nucleus. Anions with the greater negative charge will have the larger radius. In this case, the net repulsion of the electrons will outweigh the nuclear charge and the ion will expand in size.
Which of the following species will have the largest and the smallest size Mg, Mg2+, Al, Al3+?.
Atomic radii decrease across a period. Cations are smaller than their parent atoms. Among isoelectronic ions, the one with the large positive nuclear charge will have a smaller radius.
Hence the largest species is Mg; the smallest one is Al3+
The size of an anion greater while that of the cation is smaller than that of its parent atom, e.g. F- (=1.36 Å)>F(=0.72 Å); Cl -(=1.81 Å)>Cl(=0.99Å); Na +(=0.95Å)<Na(=1.90Å); Ca 2+(=0.99 Å)<Ca(=1.97Å).
Let us consider the radii of Na, Na+, Cl and Cl-. The reason of the fact that Na+ ion is smaller than Na atom is that Na+ ion has 10 electrons (Na+->1s2,2s2p6) while Na atom has 11electrons (Na ->1s2,2s2p6,3s1). The nuclear charge (charge on the nucleus) in each case is the same, i.e. equal to +11 (atomic number of Na). This nuclear charge of +11 can pull 10 electrons of Na+ ion inward more effectively than it can pull a greater number of 11 electrons of Na atom. Thus Na+ ion is smaller than Na atom.
The reason why Cl- ion is bigger than Cl atom can also be explained on a similar basis. The Cl- ion has 18 electrons (Cl-->1s2,2s2p6,3s2p6) while Cl atom has only 17 electrons (Cl->1s2,2s2p6,3s2p5). The nuclear charge in each case is +17, which cannot pull 18 electrons of Cl- ion as effectively as it can pull 17 electrons of Cl atom inward. Thus Cl- ion is bigger than Cl atom.