Double Dot Symbol: Meanings Across Chemistry, Math, and Typography
Explore what the double dot symbol means in chemistry as a lone pair marker in Lewis dot structures, in typography as the diaeresis diacritical mark, and in math education as a teaching tool for inequalities.
Chemistry
A symbol that means different things in different contexts.
In chemistry, the double dot symbol is used to represent paired valence electrons in Lewis dot structures. In typography, it’s the diaeresis — a diacritical mark over a letter. And in math teaching, it’s the “dot method” for remembering how to use inequality symbols.
In chemistry, the double dot symbol is used to represent paired valence electrons in Lewis dot diagrams. These are also called lone pairs.
Lewis dot notation is a visual way to represent an atom’s or ion’s valence electrons. The number of dots around an element symbol represents the number of valence electrons that atom has. Dots are placed around the element symbol like this:
The placement of single or double dots around the symbol isn’t critical, as long as you don’t put more than two dots on any one side. Paired electrons can alternatively be represented as a line instead of a double dot.
The number of dots around an element depends on the number of valence electrons it has. For example, oxygen has 6 valence electrons, so we draw 6 dots around O:
Oxygen’s valence electron count is 6, so its Lewis dot diagram shows six dots. Two of those dots are paired (as shown by the double dot), and the other four are unpaired.
Sodium has only 1 valence electron, so we draw just one dot next to Na:
Chlorine has 7 valence electrons, so we draw seven dots around Cl:
Each atom wants to have eight valence electrons (4 electron pairs) around it. This is known as the Octet Rule. So atoms will gain, lose, or share electrons until they’re surrounded by eight electrons. For example, each Cl atom here has 7 valence electrons, so they’ll share 2 electrons:
Typography
In typography, the double dot is the diaeresis — a diacritical mark over a letter. It’s used in German, Dutch, and other languages. You insert it into Microsoft Word by going to Insert > Symbols, then changing the Subset box to Combining Diacritical Marks. If you want it to show up automatically when you type a certain letter, you can create an AutoCorrect entry. Jay Freedman, a Microsoft Word MVP, recommends typing the letter with the diaeresis together, then creating an AutoCorrect entry that pairs them. Alternatively, you can make a keyboard shortcut for the combining diacritic character alone.
Math
In math teaching, the double dot is part of the “dot method”, which helps students reliably distinguish greater-than from less-than symbols. When teaching inequalities, I always draw two dots on the open/wider end of the inequality symbol, and one dot on the pointed end. Students learn quickly that two dots always face the bigger number, and one dot always faces the smaller number. This reasoning works because two dots is bigger than one dot.
I learned about the dot method from a math coach. In my second-grade class, we recognized and applied the connection between the number of dots and the size of the numbers. We positioned the dot method as a more “mathy” alternative to the alligator mnemonic.