Re: How come transition elements display a variety of valencies?

This question is not easily answered in simple terms without going into quantum mechanics of electron orbitals and suborbitals. I would encourage you to spend some time with a college-level basic chemistry book where the subject of periodic law or energy levels and the periodic table are discussed. In simple terms, valance electrons are the outer most, highest energy level electrons, but it is the outer most sublevel, which holds the valance electrons, and the number of these electrons changes with the transition elements. I will attempt to explain below without going into too much detail. Periodic law, or the manner in which the periodic table is laid out, defines each new row as beginning to add electrons to a different energy level, beginning at the K shell, then going to the L shell and so on. Frequently these levels are numerically indicated as 1, 2, 3, etc. Level 1 only allows at most 2 electrons; level 2 allows 8 electrons, distributed into 2 sub-levels, one containing 2 electrons in a s-sublevel and the other containing 6 electrons in a p-sublevel. In level 3 beginning with the element Na, there are 18 electrons allowed, but one of the sublevels of this level does not begin getting filled until after the element Ca, which has two electrons in its 4th level. This 3d sublevel can hold 10 electrons, and these elements begin the transition series of elements. The additional valance electrons come from the fact that the 3-d sublevel is actually at a slightly higher energy level than the 4-s sublevel. Therefore with the transition elements, the outermost energy sublevel can hold 10 electrons. A completely filled 4th level can hold 32 electrons, and the 5th level can hold 50. Because the electrons in the highest energy level (the ones furthermost away from the nucleus are the valance electrons, the transition elements frequently have valances such up to +3, such as Fe.

I hope you were able to follow this, its not as complicated as it may initially seem. Keep up your interest in chemistry, and consider a career in the materials field if you really like chemistry.

Greg Dries