Inorganic Chemistry with Help Guides

 Irrespective of which set of ionic radii are used, the following trends are observed:  1. In the main groups, radii increase on descending the group, e.g. u+ = 0.76A, Na+ = l.02A, K+- = l.38A, because extra shells of electrons are added. 2. The ionic radii decrease moving from left to right across any period in the periodic table, e.g. Na+ = 1.02A, Mg2+ = 0.720Aand Al3+ = 0.535 A. This is partly due to the increased number of charges on the nucleus, and also to the increasing charge on the ions.  3. The ionic radius decreases as more electrons are ionized off, that is as the valency incteases, e.g. Cr2+ = 0.80 A (high spin), Cr3+ = 0.615 A, Cr4+ = 0.55 A, CrH = 0.49 A and Cr6+ = 0.44 A.  4. The d and f orbitals do not shield the nuclear charge very effectively. Thus there is a significant reduction in the size of ions just after 10d or 14f electrons have been filled in. The latter is called the lanthanide contraction, and results in the sizes of the second and third...

 There are several problems in obtaining an accurate set of ionic radii.  1. Though it is possible to measure the internuclear distances in a crystal very accurately by X-ray diffraction, for example the distance between Na+ and p- in NaF, there is no universally accepted formula for apportioning this to the two ions. Historically several different sets of ionic radii have been estimated. The main ones are by Goldschmidt. Pauling and Ahrens. These are all calculated from observed internuclear distances, but differ in the method used to split the distance between the ions. The most recent values, which are probably the most accurate, are by Shannon (1976) .  2. Corrections to these radii are necessary if the charge on the ion is changed.  3. Corrections must also be made for the coordination number, and the geometry.  4. The assumption that ions are spherical is probably true for ions from the s- and p-blocks with a noble gas configuration, but is probably untrue...

  Metals usually form positive ions. These are formed by removing one or more electrons from the metal atom. Metal ions are smaller than the atoms from which they were formed for two. reasons:  1. The whole of the ·outer shell of electrons is usually ionized, i.e. removed. This is one reason why catfons· are much smaller than the original metal atom.  2. A second factor is the effective nuclear charge. In an atom, the number of positive charges on the nucleus is exactly the same as the number of orbital electrons. When a positive ion is formed; the number of positive charges on the nucleus exceeds the number of orbital electrons, and the effective nuclear charge (which is the ratio of the nun:iber of charges on the nucleus to the number of electrons) is increased. This results in the remaining electrons being more strongly attracted by the nucleus. Thus the electrons are pulled in - further reducing the size.