The elements in the periodic table are often divided into four categories: (1) main group elements, (2) transition metals, (3) lanthanides, and (4) actinides. The main group elements include the active metals in the two columns on the extreme left of the periodic table and the metals, semimetals, and nonmetals in the six columns on the far right. The transition metals are the metallic elements that serve as a bridge, or transition, between the two sides of the table. The lanthanides and the actinides at the bottom of the table are sometimes known as the inner transition metals because they have atomic numbers that fall between the first and second elements in the last two rows of the transition metals.
There is some controversy about the classification of the elements on the boundary between the main group and transition-metal elements on the right side of the table. The elements in question are zinc (Zn), cadmium (Cd), and mercury (Hg).
The disagreement about whether these elements should be classified as main group elements or transition metals suggests that the differences between these categories are not clear. Transition metals are like main group metals in many ways: They look like metals, they are malleable and ductile, they conduct heat and electricity, and they form positive ions. The fact the two best conductors of electricity are a transition metal (copper) and a main group metal (aluminum) shows the extent to which the physical properties of main group metals and transition metals overlap.
There are also differences between these metals. The transition metals are more electronegative than the main group metals, for example, and are therefore more likely to form covalent compounds.
Another difference between the main group metals and transition metals can be seen in the formulas of the compounds they form. The main group metals tend to form salts (such as NaCl, Mg3N2, and CaS) in which there are just enough negative ions to balance the charge on the positive ions. The transition metals form similar compounds [such as FeCl3, HgI2, or Cd(OH)2], but they are more likely than main group metals to form complexes, such as the FeCl4-, HgI42-, and Cd(OH)42- ions, that have an excess number of negative ions.
A third difference between main group and transition-metal ions is the ease with which they form stable compounds with neutral molecules, such as water or ammonia. Salts of main group metal ions dissolve in water to form aqueous solutions.
When we let the water evaporate, we get back the original starting material, NaCl(s). Salts of the transition-metal ions can display a very different behavior. Chromium(III) chloride, for example, is a violet compound, which dissolves in liquid ammonia to form a yellow compound with the formula CrCl3.6 NH3 that can be isolated when the ammonia is allowed to evaporate.
CrCl3(s) + 6 NH3(l) -> CrCl3.6 NH3(s)