In this tutorial, we learn about valence electrons, what they are, and why they are significant. We will also learn how to tell how many valence electrons an element has.
Topics Covered in Other Articles
- How to read the periodic table
- Periodic trends made easy
- Ionic vs. covalent bonds
- Electron shell configurations
- Electron orbitals & orbital shapes
What are valence electrons? Why are they significant?
Valence electrons are electrons that located in the outermost electron shell of an atom. These electrons, being the furthest from the nucleus and thus the least tightly held by the atom, are the electrons that participate in bonds and reactions. This also means that the number of valence electrons that an element has determines its reactivity, electronegativity, and the number of bonds it can form.
For example, in the figure below showing a simplified diagram of sodium’s electrons, the valence electron is shown in red. It is located on the outermost shell (in this case, the shell resembles a ring).
The term valence refers to the ability of an element to form bonds with other atoms. An element’s valence was historically determined by how many hydrogen atoms it could bond to (which is determined by how many valence electrons it has available for bonding): for example, carbon can form CH4 so it has a valence of 4, and 4 valence electrons. On the other hand, nitrogen can form NH3 so it has a valence of 3, and 3 valence electrons.
How many valence electrons does an element have?
You can use the periodic table to help you determine how many valence electrons an element (specifically, a neutral atom of the element) has. Look at the group that the element is in, as the group number indicates the number of valence electrons that the element has.
Note, however, that this rule only applies to elements that are not transition metals. Transition metals have more complicated electron configurations. Thus you should take a look at the element’s specific electron shell configuration to figure it out.
This also means that when looking at a group number, exclude the transition metals. They are located in the block in the middle of the periodic table. In this sense, groups 1 and 2 in the diagram below stay the same, but group 13 is our new “group 3”, group 14 is our new “group 4,” and so on.
For example, sodium is in group 1, indicating it has one valence electron. To confirm this, we can check its electron shell configuration: 1s22s22p63s1. As we can see, there is one electron in the 3s orbital, which is the outermost one.
Another example is that of chlorine. Chlorine is in group 7, indicating it has seven valence electrons. To confirm this, we can take a look at its electron shell configuration: 1s22s22p63s23p5. As we can see, there are seven electrons in the 3s and 3p orbitals combined, which together make up the outermost third orbital.
Lastly, we can take a look at carbon, which is in group 4. From this, we infer it has four valence electrons. To confirm, we check with its electron shell configuration: 1s22s22p2. As we can see, there are four electrons in the 2s and 2p orbitals combined, which together make up the outermost second orbital.
A noteworthy case is that of group 8: elements in this group have eight electrons, which makes a full octet. That makes these elements special, known as the noble gases. They have virtually no valence electrons available for bonding or reacting because the full octet of their electron shell is so stable.