In Table 1, some symbols of the elements are black and some red. The ones in black are for those elements that exhibit only one valence. For example, the valence of hydrogen, fluorine, and sodium is always 1. For oxygen (O), calcium (Ca), and zinc (Zn), it is always 2 and for aluminum (Al), it is always 3. However, iron (Fe) can be divalent and trivalent, and copper can be monovalent and divalent. We know this from the facts that both CuCl and CuCl2
are well-known stable compounds, and so are FeCl2
. Both CuCl and CuCl2
are copper chlorides. To distinguish between the two when writing their chemical names, the valence is specified as a Roman numeral in parentheses after the name of the element exhibiting more than one valence. Consequently, the name for CuCl is "copper (I) chloride" and for CuCl2
"copper (II) chloride". Similarly, "iron (II) chloride" is the name for FeCl2
and "iron (III) chloride" for FeCl3
. The names are pronounced as follows: copper-one-chloride, copper-two-chloride, iron-two-chloride, and iron-three-chloride.
There are also fascinating old common names for copper and iron chlorides, reflecting the valences of Cu and Fe. Copper (I) chloride is called cuprous
chloride and copper (II) chloride cupric
chloride. Similarly, the old names for FeCl2
chloride and ferric
chloride, respectively. These names are still in use today.
Why do different elements have a different number of "hands", or, more scientifically, different valences? We will learn that in the next module (Volume 2) of our course. Meanwhile, let us do a few simple exercises. 1.9.2. Exercises.
When doing these exercises, think in terms of "hands" the elements have (Table 1).
1. The valence of indium (In) is always 3. Knowing the valences of chlorine (1) and oxygen (2), what are the subscripts x, y, and z in the formulas of (a) indium chloride InClx
and (b) indium oxide Iny
2. Using the valence numbers in Table 1, write chemical formulas for compounds of oxygen with the following elements: K, Ba, Al, C, P, and Cr for all valences of these elements. Answer