What factors limit electrovalency?

Answer 1

Electrovalency is a measurement of the net electric charge of an ion and is used when balancing chemical reactions. Electrovalency is related to the concepts of electronegativity and valence electrons, and indicates the number of electrons necessary for an ion to have a balanced electric charge. The electron configuration of elements determine what types of ions they form most easily. Most often, the ion formed will be the one which results in a closed-shell atom, or in other words, one that satisfies the octet rule. For instance, all the alkali metals form the +1 ion, all the alkaline earths form the +2 ion because the loss of one and two electrons, respectively, gives these atoms an especially stable noble gas electron configuration. Elements in these first two columns of the Periodic Table are very electropositive, which means that it does not require very much energy to remove one electron from the atom. In contrast, the halides are very electronegative. All the elements in this group of the periodic table are 1 electron short of a closed-shell configuration, and so these atoms all form -1 ions.
For the most part, the charge of an ion is determined by these simple rules. The transition metals however a bit more complicated and many transition metals form multiple types of ions. This is because the valence electrons for transition metals are the d-orbital electrons, and the occupancy of the d-orbitals is somewhat less important to the stability of the atom/ion.
Also note that sometimes the halides have positive oxidation numbers. For instance, chlorine can exist as at +7 ion! But if chlorine loses 7 electrons than it again reaches a closed-shell noble gas configuration (of neon). Oxidation states almost never go above 7 because there are only a total of 8 electrons in the s- and p-orbitals combined.
The charge of complex ions is simply determined by the oxidation numbers of the atoms they are made up of.
See the Web Links for more information.