Chemistry: the Periodic Table and Ionic Compounds

When you tutor high school chemistry, these basics come up every year.  Let’s sort them out.

From the high school chemistry point of view, there are two types compounds:  ionic and covalent.  Ionic names start with a metal; covalent names contain only nonmetals.

What is a metal and what is a nonmetal?  There are two ways to answer that question.  Talking about elements on the periodic table, the metals are on the left.  They stretch all the way to a boundary that starts at the 13th column and zigzags down to the right.  Usually it’s a red line, but if it’s not on yours, here’s the boundary:

boron is a nonmetal, but aluminum is a metal;

silicon is a nonmetal, but germanium is a metal;

arsenic is a nonmetal, but antimony is a metal;

tellurium is a nonmetal, but polonium is a metal.

Anything to the right of that boundary is a nonmetal.  Once again, anything to its left is a metal.

The important concept about forming an ionic compound – such as calcium chloride – is that the nonmetal “charges” (or combining capacities) must equal the metal charges (or combining capacities, depending on how you see it).

Some periodic tables have these charges right on them:  they’re either positive or negative, and most are between 1 and 5.  However, old ones often don’t.  If yours doesn’t, here’s how to tell what the combining capacity is:

Column H, Li, Na, etc Be B C N O F
Combining Capacity 1 2 3 4 3 2 1

To our example:  calcium chloride.  Calcium is under Be, so its combining capacity is 2.  On the other hand, chlorine is under F, so it has a combining capacity of 1.  Since the nonmetal combining capacity has to equal the metal, we need two chlorines.  2×1=2, which equals the calcium’s 2 (calcium being the metal, of course).  The formula for calcium chloride is, therefore, CaCl2.

By that same reasoning, the formula of sodium sulfide is Na2S. Finally, the formula of magnesium oxide is MgO (since both Mg and O have the same combining capacity, you only need one each).

We’ll make some observations:

1)  When written in words, the names are in all lower case.

2)  When written as formulas, only the first letter of each atomic symbol is capitalized.  For example, hydrogen is written H, but sodium is written Na.

3)  When naming a simple ionic, the second atom (the nonmetal) gets an -ide ending.

4)  The sunken middle portion of the table (which starts on the left with Sc) we didn’t discuss this time.  Its members get a slightly different treatment, which we’ll address soon.

Jack of Oracle Tutoring by Jack and Diane, Campbell River, BC.

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