Electron Dot Symbol: What It Is and How to Write One for Any Element

I’m going to explain how to write an electron dot symbol. First, I’ll define what it is and what it visually communicates. Then I’ll explain the basic rules for writing one. Finally, I’ll show you how to use them to predict chemical bonding.

An electron dot symbol is a way of representing the valence electrons in an atom or ion. You write the element’s symbol, then put dots around it to represent the number of valence electrons that atom has. The dots are placed on the four sides of the atomic symbol, and each side can have up to two dots (one on top, one on bottom; or one on left, one on right). For example:

The electron dot symbol shows the total number of valence electrons that the atom has. This is also known as the electron dot diagram, Lewis diagram, or Lewis structure. The notation was devised by G. N. Lewis, an American chemist who lived from 1875 to 1946. He came up with this system at the beginning of the 20th century. His main purpose in devising the system was to be able to predict how many bonds most elements will form in compounds.

Counting Valence Electrons

The first thing you need to know about electron dot symbols is how to count the number of valence electrons in an atom. Main-group elements (which are all the nonmetals and most of the metals) have valence electrons that correspond directly to their group number. For example, elements in group 1A have one valence electron; elements in group 2A have two; etc. The groups go like this:

Group NumberValence Electron Count

1A1

2A2

3A3

4A4

5A5

6A6

7A7

8A8

So if you want to find out how many valence electrons a main-group element has, just look at its group number. For example, aluminum is in group 3A, so it has three valence electrons. Fluorine is in group 7A, so it has seven valence electrons. Helium is in group 8A, so it has eight valence electrons.

This works because the group number corresponds to the last digit of the element’s atomic number. The last digit of the atomic number tells you which subshell is being filled when the element is formed. In main-group elements, the valence electrons are found in the outermost s and p orbitals. So the group number gives you the number of electrons in these orbitals.

But wait! This doesn’t work for transition metals. Transition metals have valence electrons in both the outermost s orbital and the d orbitals. So the number of valence electrons isn’t just the group number. Instead, you have to add together the number of outermost s electrons and the number of d electrons. But since d orbitals fill after s orbitals, you have to consider both the s orbital and the d orbitals when counting valence electrons for transition metals. So the number of valence electrons for transition metals is not simply a read-off from the periodic table — you have to think about where the electrons are coming from.

Placing the Dots

Once you’ve counted the number of valence electrons, you need to place those electrons as dots around the atomic symbol. There are some physical rules for doing this. First, you should place the dots one at a time around the symbol. Second, there are four sides available for placing the dots: top, bottom, left, and right. Third, the order in which you place the dots doesn’t affect the meaning of the diagram. Fourth, each side can receive only one dot before any side receives a second dot.

For example, here’s how you’d draw the electron dot symbol for fluorine:

Fluorine has seven valence electrons, so we need to place seven dots around the F symbol. We place the first dot on the top of the symbol, the next on the bottom, the next on the left, the next on the right, and then we start putting more than one dot on each side. Here’s how it looks:

Electron Dot Symbols for Ions

Now let’s take a look at ions. When an atom becomes an ion, it either gains or loses electrons. So the number of valence electrons changes. If an atom gains electrons, it becomes a negative ion. If it loses electrons, it becomes a positive ion. The total number of electrons changes accordingly. So the dot symbol must change accordingly. Let’s look at an example. Take fluorine again. Fluorine has seven valence electrons. But when it forms the fluoride ion, it gains one electron, giving it a total of eight valence electrons. Here’s how we would draw the fluoride ion:

Notice that the fluoride ion has eight dots around the F symbol. That means it has eight valence electrons. Now take sodium. Sodium has one valence electron. When it forms the sodium ion, it loses that electron, giving it zero valence electrons. Here’s how we would draw the sodium ion:

Notice that the sodium ion has no dots around the Na symbol. That means it has no valence electrons.

What the Symbol Predicts — and What It Doesn’t

The number of unpaired dots in the electron dot symbol is important for predicting bonding. If an atom has a lot of unpaired dots, it tends to form a lot of bonds. For example, boron has three unpaired dots, so it tends to form three bonds. Carbon has four unpaired dots, so it tends to form four bonds. The octet rule says that atoms tend to lose, gain, or share electrons in order to reach eight valence electrons. For example, fluorine has seven valence electrons. It tends to gain one more electron to reach eight.

You might wonder why we care about valence electrons. After all, the electron dot symbol is just a simple visual representation of the number of valence electrons in an atom. But actually, the electron dot symbol is a bit more than just a simple device for counting electrons. In fact, it’s a fundamental model of how atoms bond. The electron dot symbol tells us how many bonds an atom will form in a compound. And it helps us understand the types of bonds that will form between different atoms.

In fact, the electron dot symbol is really just a way of counting electrons. It doesn’t tell us anything about the physical location of the electrons, or their motion, or their quantum behavior. It’s just a simple device that allows us to count electrons quickly and easily.