In chemistry, valence electrons are the electrons that are located in the last electron shell of an element. Knowing how to find the number of valence electrons for a particular atom is an important skill for chemists, as this information determines the chemical bonds it can form. Fortunately, all you need to find the valence electrons of an element is a standard periodic table.
Part 1 of 2: Finding Valence Electrons with a Periodic Table
For elements other than transition metals
Step 1. Find a periodic table of the elements
The periodic table is a color-coded table that contains several different tables and shows all the chemical elements known to mankind. The periodic table reveals a great deal of information about the elements. You will use some of this information to determine the number of valence electrons for the atom you want to investigate. You can usually find this table on the cover of chemistry books. You can also find an excellent interactive table online available on this page.
Step 2. Label each column of the periodic table of the elements from 1 to 18
Generally, in a periodic table, all the elements in a single vertical column have the same number of valence electrons. If your periodic table doesn't have every column numbered, assign each a number starting with 1 for the far left and 18 for the far right. In scientific terms, these columns are known as "groups" Of elements.
For example, if you are going to work with a periodic table where the groups are not numbered, you should write a 1 above the hydrogen (H), a 2 above the beryllium (Be) and so on until writing an 18 above the helium (He)
Step 3. Find your element in the table
Now find the element for which you want to find the valence electrons in the table. You can do it through its chemical symbol (the letters that appear in each box), its atomic number (the number that is in the upper left part of each box) or any other information available in the table.
- As an example, you can look up the valence electrons for a very common element: carbon (C). This element has an atomic number of 6. It is located at the top of group 14. In the next step, we will explain how to find its valence electrons.
- In this subsection we will not take into account transition metals, which are the elements of the rectangle-shaped block made up of groups 3 to 12. These elements are slightly different from the rest, so in the steps of this subsection we will not work with them. You can see how to work with them in the next subsection.
Step 4. Use the group numbers to determine the number of valence electrons
You can use the group number of elements other than transition metals to find the number of valence electrons in an atom of that element. The units in the group number is the number of valence electrons in an atom of these elements. In other words:
- group 1: 1 valence electron
- group 2: 2 valence electrons
- group 13: 3 valence electrons
- group 14: 4 valence electrons
- group 15: 5 valence electrons
- group 16: 6 valence electrons
- group 17: 7 valence electrons
- group 18: 8 valence electrons (except for helium, which has 2)
For the example of carbon, because that element belongs to group 14, you can say that a carbon atom has four valence electrons.
For transition metals
Step 1. Find an item from group 3 to 12
As we have explained before, the elements of groups 3 to 12 are called "transition metals" and they behave differently from the rest of the elements with respect to their valence electrons. In this section we will explain how, to some extent, it is often not possible to assign valence electrons to these atoms.
- As an example, we will take tantalum (Ta) which is element 73. In the next steps, we will find its valence electrons (or at least try to do so).
- Note that transition metals include the lanthanide and actinide series (also known as "rare earths"). They are the two rows of items that normally fall below the rest of the table, beginning with lanthanum and actinium. All these elements belong to the Group number 3 of the periodic table.
Step 2. Understand that transition metals do not have "traditional" valence electrons
To understand why transition metals don't actually "work" the same way as the rest of the periodic table, you need a brief explanation of how electrons behave in atoms. Read the next few paragraphs for a quick look at this topic, or skip them to go straight to the answers.
- As electrons are added to an atom, they are arranged into different "orbitals," which are basically different areas around an atom in which electrons congregate. Generally, the valence electrons are the atoms belonging to the last shell. In other words, they are the last atoms added.
- For reasons that are a bit complex to explain in this article, when atoms are added to the last layer of a transition metal (we will explain it in more depth later), the first atoms that go in that layer tend to act like normal valence electrons, but after that, they stop and electrons from other orbital shells sometimes act as valence electrons instead. This means that an atom can have several numbers of valence electrons depending on how they are manipulated.
- For a more detailed explanation, read this excellent page from Clackamas Community College on valence electrons.
Step 3. Determine the number of valence electrons based on the group number
Again, the group number of the element you are examining can tell you its number of valence electrons. However, for transition metals there is no pattern you can follow. The group number usually corresponds to a range of possible amounts of valence electrons. These are:
- group 3: 3 valence electrons
- group 4: 2 to 4 valence electrons
- group 5: 2 to 5 valence electrons
- group 6: 2 to 6 valence electrons
- group 7: 2 to 7 valence electrons
- group 8: 2 or 3 valence electrons
- group 9: 2 or 3 valence electrons
- group 10: 2 or 3 valence electrons
- group 11: 1 or 2 valence electrons
- group 12: 2 valence electrons
- For the example of tantalum, as it belongs to group 5, you can say that it has between 2 and 5 valence electrons depending on the situation.
Part 2 of 2: Finding Valence Electrons Through Their Electronic Configuration
Step 1. Learn to read the electronic configuration
Another way to find the valence electrons of an element is through something called an "electron configuration." This may seem somewhat complicated at first glance, but it is just a way to represent the electron orbitals of an atom with letters and numbers. Once you understand what you see, it will be easy for you.
Look at the example of the electron configuration of the element sodium (Na):
Note that this electron configuration is just a repeating chain formed like this:
(number) (letter)(high number) (number) (letter)(high number)…
- …and so on. The first fragment of (number) (letter) is the name of the electron orbital and the (high number) is the number of electrons in that orbital. That is all!
- So for the example above, you could say that sodium has 2 electrons in the 1s orbital plus 2 electrons in the 2s orbital plus 6 electrons in the 2p orbital plus 1 electron in the 3s orbital. That adds up to 11 electrons in all. Sodium is element number 11, so it makes sense.
Step 2. Find the electron configuration for the element you are going to examine
Once you know its electron configuration, finding its valence number is a breeze (except, of course, for transition metals). If they give you the settings out of the box, you can skip the next step. If you have to find it yourself, keep reading:
Here is the complete electron configuration of ununoctium (Uuo), which is element 118:
1s22s22 P63s23p64s23d104p65s24d105 p66s24f145 d106p67s25f146d107p6
Now that you have this, all you need to do to find the electron configuration of another atom is to simply complete this pattern from scratch until there are no more electrons left. This is easier than it sounds. For example, if you want to make the orbital diagram of chlorine (Cl), element 17, which has 17 electrons, you would do it this way:
- Note that the number of electrons must add up to 17: 2 + 2 + 6 + 2 + 5 = 17. You only need to change the number in the last orbital. The rest is the same, since the orbitals that are before the last one are completely filled.
- To learn more about electronic settings, read this article as well.
Step 3. Assign electrons to the orbital shells with the octet rule
As electrons are added to an atom, they fall in various orbitals in the order mentioned above. The first two go in the 1s orbital, the next two go in the 2s orbital, the next six go in the 2p orbital, and so on. When working with atoms that are not transition metals, these orbitals are said to form "orbital shells" around the atom and each successive layer is further apart than the previous ones. In addition to the first shell, which can only house two electrons, each shell can have eight electrons (except, we reiterate, when you work with transition metals). This is known as the octet rule.
- For example, suppose you want to examine the element boron (B). Since its atomic number is 5, you know that it has 5 electrons and that its electronic configuration is: 1s22s22 P1. Because the first shell of orbitals has only two electrons, you know that boron has two shells: one with two 1s electrons and one with three electrons in the 2s and 2p orbitals.
- As another example, an element like chlorine has three orbital shells: one with two 1s electrons; another with two 2s electrons and six 2p electrons; and another with two 2s electrons and five 3p electrons.
Step 4. Find the number of electrons in the last shell
Now that you know the element's electron shells, finding its valence electrons is easy - just use the number of electrons in the last shell. If the last shell is full (in other words, if you have 8 electrons or 2 if it is the first shell), the element is inert and will not easily react with other elements. However, we repeat: these rules may not be followed for transition metals.
- For example, if you are going to work with boron, since it has three electrons in the second layer, you can say that boron has three valence electrons.
Step 5. Use the rows in the table as shortcuts to find out how many layers it has
The horizontal rows of the periodic table are known as "periods" of the elements. Starting from the top of the table, each period corresponds to the number of electron shells possessed by the atoms of that period. You can use that number as a shortcut to determine how many valence electrons an element has. Just start from the left side of the period when you go to count electrons. Once again: with this method you must ignore the transition metals.
For example, you know that the element selenium has four layers of orbitals because it is in the fourth period. Because it is the sixth element from the left in the fourth period (ignoring the transition metals), you know that the furthest fourth shell has six electrons and therefore selenium has six valence electrons.
- Note that electron configurations can be written in abbreviated form using the noble gases (group 18 elements) to replace the orbitals at the beginning of the configuration. For example, the electron configuration of sodium can be written as [Ne] 3s1. Essentially, it is the same as the neon one but with one more electron in the 3s orbital.
- Transition metals can have valence subshells that are not completely filled. Determining the exact number of valence electrons for transition metals requires knowledge of principles of quantum theory that are beyond the scope of this article.
- Keep in mind that the periodic tables differ between different countries. So make sure you use the correct one to avoid any kind of confusion.