In Chemistry, valence electrons are the electrons located in the outermost electron shell of an element. Knowing how to find the number of valence electrons in a given atom is an important skill for chemists because this information determines the types of chemical bonds that can be formed. Fortunately, all you need to find the valence electrons is the regular periodic table of the elements.
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Part 1 of 2: Finding Valence Electrons with the Periodic Table
Non-Transition Metals
Step 1. Find the periodic table of the elements
This table is a color coded table made up of many different boxes containing all the chemical elements known to man. The periodic table provides a wealth of information about the elements - we will use some of this information to determine the number of valence electrons in the atom we are studying. Usually, you can find this information on the cover of a chemistry textbook. There are also good interactive tables available online here.
Step 2. Label each column in the periodic table of elements from 1 to 18
Usually, in the periodic table, all elements in a vertical column have the same number of valence electrons. If your periodic table doesn't already have a number in each column, number it from 1 in the leftmost column to 18 in the rightmost column. In scientific terms, these columns are called "group" element.
For example, if we were to use the periodic table where the groups have no numbers, we would write 1 above Hydrogen (H), 2 above Beryllium (Be), and so on up to 18 above Helium (He)
Step 3. Find your element in the table
Now, find the element for which you want to know the valence electrons on the table. You can do this by using the chemical symbol (the letter in each box), the atomic number (the number at the top left of each box), or any other information available to you in the table.
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For demonstration purposes, let's look for the valence electrons for a very frequently used element: carbon (C).
This element has an atomic number of 6. This element is located above group 14. In the next step, we will look for its valence electrons.
- In this subsection, we will ignore the transition metals, which are elements in square blocks of groups 3 through 12. These elements differ slightly from the others, so the steps in this subsection do not apply to that element. See how to do this in the subsection below.
Step 4. Use group numbers to determine the number of valence electrons
The group number of a non-transition metal can be used to find the number of valence electrons in the element's atom. Unit place of group number is the number of valence electrons in the element's atom. In other words:
- Group 1: 1 valence electrons
- Group 2: 2 valence electrons
- Group 13: 3 valence electrons
- Group 14: 4 valence electrons
- Group 15: 5 valence electrons
- Group: 6 valence electrons
- Group: 7 valence electrons
- Group: 8 valence electrons (except for helium, which has 2 valence electrons)
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In our example, since carbon is in group 14, we can say that one carbon atom has four valence electrons.
Transition Metal
Step 1. Find the elements from groups 3 to 12
As noted above, the elements in groups 3 through 12 are called transition metals and behave differently from the other elements in terms of valence electrons. In this section, we will explain the difference, to some extent, it is often not possible to assign valence electrons to these atoms.
- For demonstration purposes, let's take Tantalum (Ta), element 73. In the next few steps, we'll look for its valence electrons (or, at least, try).
- Note that the transition metals include the lanthanide and actinide (also called the rare earth metals) series - two rows of elements usually located at the bottom of the rest of the table, starting with lanthanum and actinium. All of these elements include group 3 in the periodic table.
Step 2. Understand that transition metals do not have traditional valence electrons
Understanding that the reason transition metals don't really work like the rest of the periodic table requires a little explanation of how electrons work in atoms. See below for a quick understanding or skip this step to get straight to the point.
- As electrons are added to atoms, these electrons are sorted into different orbitals - essentially different regions around the atom where the atoms are assembled. Usually, the valence electrons are the atoms in the outermost shell - in other words, the last atoms added.
- For reasons that are a bit complicated to explain here, when atoms are added to the outer d shell of a transition metal (more on that below), the first atoms to enter the shell tend to act like ordinary valence electrons, but after that, electrons it doesn't behave like that, and electrons from other orbital layers sometimes even act like valence electrons. This means that an atom can have multiple valence electrons depending on how it is manipulated.
- For a more detailed explanation, take a look at Clackamas Community College's good valence electrons page.
Step 3. Determine the number of valence electrons based on their group number
Again, the group number of the element you're looking at can tell you how many valence electrons it has. For transition metals, however, there's no pattern you can follow - the group number will usually correspond to a number of possible valence electrons. The numbers 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 to 2 valence electrons
- Group 12: 2 valence electrons
- In our example, since Tantalum is in group 5, we can say that Tantalum has between two and five valence electrons, depending on the situation.
Part 2 of 2: Finding Valence Electrons by Electron Configuration
Step 1. Learn how to read electron configurations
Another way to find the valence electrons of an element is with something called the electron configuration. The electron configuration may seem complicated, but it's just a way of representing the electron orbitals in an atom with letters and numbers, and it's easy if you know what you're doing.
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Let's look at an example configuration for the element sodium (Na):
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- 1s22s22p63s1
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Note that this electron configuration is simply repeating a pattern like this:
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- (number)(letter)(number above)(number)(letter)(number above)…
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- …etc. Pattern (number)(letter) first is the name of the electron orbital and (number above) is the number of electrons in that orbital - that's it!
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So, for our example, we say that sodium has 2 electrons in 1s. orbital added 2 electrons in 2s. orbital added 6 electrons in 2p. orbitals added 1 electron in the 3s orbital.
The total is 11 electrons - sodium is element number 11, so it makes sense.
Step 2. Find the electron configuration for the element you are studying
Once you know the electron configuration of an element, finding the number of valence electrons is fairly easy (except, of course, for transition metals.) If you are given the configuration from the problem, you can move on to the next step. If you have to look it up yourself, take a look below:
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Here is the complete electron configuration for ununoctium (Uuo), element number 118:
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- 1s22s22p63s23p64s23d104p65s24d105p66s24f145d106p67s25f146d107p6
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Now that you have the configuration, all you have to do to find the electron configuration of another atom is to fill this pattern from scratch until you run out of electrons. This is easier than it sounds. For example, if we wanted to create an orbital diagram for chlorine (Cl), element number 17, which has 17 electrons, we would do it like this:
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- 1s22s22p63s23p5
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- Notice that the number of electrons adds up to 17: 2 + 2 + 6 + 2 + 5 = 17. You just need to change the amount in the final orbital - the rest is the same because the orbitals before the last orbital are full.
- For other electron configurations, see also this article.
Step 3. Add electrons to the orbital shells with the Octet Rule
When electrons are added to an atom, they fall into various orbitals in the order listed above - the first two electrons go into the 1s orbital, the next two electrons go into the 2s orbital, the next six electrons go into the 2p orbital, and so on. When we work with atoms outside of the transition metals, we say that these orbitals form orbital shells around the atom, with each successive shell further away from the previous shell. In addition to the first shell, which can only hold two electrons, each shell can hold eight electrons (besides, again, when working with transition metals.) This is called Octet Rule.
- For example, let's say we look at the element Boron (B). Since the atomic number is five, we know that the element has five electrons and its electron configuration looks like this: 1s22s22p1. Since the first orbital shell has only two electrons, we know that Boron has only two shells: one shell with two 1s electrons and one shell with three electrons from the 2s and 2p orbitals.
- As another example, an element such as chlorine would have three orbital shells: one with 1s electrons, one with two 2s electrons and six 2p electrons, and one with two 3s electrons and five 3p electrons.
Step 4. Find the number of electrons in the outer shell
Now that you know the electron shell of your element, finding the valence electrons is very easy: just use the number of electrons in the outer shell. If the outermost shell is full (in other words, if the outermost shell has eight electrons, or for the first shell it has two), the element becomes inert and will not react easily with other elements. However, again, this rule does not apply to transition metals.
For example, if we use Boron, since there are three electrons in the second shell, we can say that Boron has three valence electrons.
Step 5. Use table rows as a shorthand way to find orbital shells
The horizontal rows in the periodic table are called "period" element. Starting at the top of the table, each period corresponds to the number of electron shells the atom has in that period. You can use it as a shorthand way to determine how many valence electrons an element has - just start on the left side of the period when counting electrons. Again, you need to ignore the transition metals for this method.
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For example, we know that the element selenium has four orbital shells because it is in the fourth period. Since this element is the sixth element from the left in the fourth period (ignoring the transition metals), we know that its outer fourth shell has six electrons, and thus selenium has six valence electrons.
Tips
- Note that the electron configuration can be written in a concise way using the noble gases (elements in group 18) to replace the orbitals at the beginning of the configuration. For example, the electron configuration of sodium can be written as [Ne]3s1 - actually, the same as neon, but with one extra electron in the 3s orbital.
- Transition metals may have valence subshells that are not completely filled. Determining the exact number of valence electrons in transition metals involves principles of quantum theory which are not covered by this article.
- Note that the periodic table differs from country to country. So check if you are using the correct periodic table to avoid confusion.
