In chemistry, valence electrons are those located in the outermost electronic 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 type of chemical bonds that atom can form. Luckily, you just need a standard periodic table to find this number.
Part 1 of 2: Finding the valence electrons with a periodic table
Step 1. Find a periodic table
It is a table categorized by color and made up of several different squares that list all the chemical elements known to mankind. It reveals a lot of information about the elements, and we'll use some of it to determine the number of valence electrons in the atom we're investigating. These tables can often be found on the inside front cover of chemistry books. There is also an excellent interactive table available online here.
Step 2. Label each column 1-18
In general, in the periodic table, all elements in the same vertical column will have the same number of valence electrons. If the table does not have numbered columns, give each column a number starting with 1 on the far left and 18 on the far right. In scientific terms, columns are called "groups" of elements.
For example, if we are working with a table in which the groups are not numbered, we will write 1 for Hydrogen (H), 2 for Beryllium (Be) and so on, until we end up with 18 for Helium (He)
Step 3. Find the element in question in the table
For this, you can use the chemical symbol (the letters in each box), the atomic number (the number at the top left of each box) or any other available information.
- For example, let's find the number of valence electrons of a well-known element: o carbon (C), whose atomic number is 6. It's located at the top of the group 14. In the next step, we'll find its valence electrons.
- In this subsection, we will ignore the transition metals, which are the elements of the rectangular block formed by groups 3 to 12. These elements are a little different from the rest, so the steps in this subsection will not apply to them. See how to deal with these elements in the subsection below.
Step 4. Use the group numbers to determine the number of valence electrons
You can use the group number of a non-transition metal to find out how many valence electrons an atom of that element has. THE group number unit is the number of valence electrons of 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).
- In our example, since carbon is in group 14, we can say that a carbon atom has four valence electrons.
Step 1. Find an element from groups 3 to 12
As stated above, the elements in groups 3 to 12 are called "transition metals" and behave differently from the rest of the elements when it comes to valence electrons. In this section, we'll explain how, to some extent, it's generally not possible to assign valence electrons to these atoms.
- As an example, let's use Tantalus (Ta), element 73. In the next steps, we'll find, or try to find, its valence electrons.
- Note that transition metals include the lanthanide and actinide series (also called "rare earth metals"), the two rows that are usually positioned below the rest of the table and that start with lanthanum and actinium. These elements all belong to the group 3 of the periodic table.
Step 2. Understand that transition metals do not have "traditional" valence electrons
Understanding why transition metals don't "work" like the rest of the periodic table requires a brief explanation of the way electrons behave in atoms. See below for a quick summary or skip this step to get right to the answers.
- As they are added to an atom, electrons are distributed into different "orbitals," which are basically different areas around the atom where electrons congregate. In general, the valence electrons are those from the outermost shell, that is, the last ones added.
- For reasons too complex to explain here, when electrons are added to the outermost d-shell of a transition metal (see below), the first ones that enter tend to act like normal valence electrons, but after that they no longer act like that. shape, 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 it is manipulated.
- For a more detailed explanation in English, see Clackamas Community College's excellent valence electrons page.
Step 3. Find the number of valence electrons based on the group number
Again, the group number of the element you are examining can tell its valence electrons. However, for transition metals, there is no pattern you can follow, as the group number will usually correspond to a range of possible valence electron numbers. 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.
- In our example, as Tantalus is in group 5, we can say that it has between two and five valence electrons, depending on the situation.
Part 2 of 2: Finding valence electrons with an electron configuration
Step 1. Learn to read an electronic configuration
This is another way to find the valence electrons of an element. It may seem complicated at first, but it's usually just a way of representing the electron orbitals in an atom using letters and numbers, and it's easy to understand once you know what you're looking at.
- Let's see, for example, the configuration of the element sodium (Na):
- Note that this electronic configuration is just a repeated line that goes like this:
(number)(letter)(high number)(number)(letter)(high number)…
- … and so on. the first block (number)(letter) is the name of the electronic orbital, and the (high number) is the number of electrons in that orbital. That's it!
- So in our example, we would say that sodium has 2 electrons in 1s orbital, most 2 electrons in 2s orbital, most 6 electrons in 2p orbital, most 1 electron in 3s orbital. In total there are 11 electrons. Sodium is element #11, so it makes sense.
Step 2. Find the electronic configuration of the element you are examining
Once you know the electron configuration of an element, finding the number of its valence electrons is pretty straightforward (except, of course, for transition metals). If you receive the configuration, you can skip straight to the next step. If you need to find it, see below:
- Here is the complete electronic configuration for the Ununoctio(Uuo), element 118:
- Now that you have it, all you need to do to find the electron configuration of another atom is fill in that pattern from scratch until it runs out of electrons. It's easier than it sounds. For example, if we want to make the orbital diagram of Chlorine (Cl), an element with 17 electrons, we will do the following:
- Note that the sum of the electrons equals 17: 2 + 2 + 6 + 2 + 5 = 17. You just need to change the number of the final orbital; the rest will be the same, since the previous orbitals will be completely full.
- To learn more about electronic configuration, also read this article.
Step 3. Place the electrons in the orbital shells with the Octet Rule
As electrons are added to any atom, they enter various orbitals in the order given above: the first two enter 1s, the next two enter 2s, the next six enter 2p, and so on. When we are dealing with atoms that are not transition metals, we say that these orbitals form layers around the atom, with each successive layer being further apart than the previous ones. Apart from the first shell, which can only have 2 electrons, each can have up to 8 electrons (except, again, in the case of transition metals). this is the call Octet Rule.
- For example, let's say we're looking at the Boro element (B). Since its atomic number is five, we know that it has 5 electrons and that its electron configuration is as follows: 1s22s22p1. Since the first orbital shell has only 2 electrons, we know that Boron has two shells: one with two 1s electrons and one with three electrons from the 2s and 2p orbitals.
- As another example, an element like chlorine will have three orbital shells: one with two 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 outermost shell
Now that you know the electron shells of your element, finding the valence electrons is easy: just use the number of electrons in the outermost shell. If this shell is full (that is, if it has eight electrons or, in the case of the first shell, 2), the element is inert and will not easily react with others. Again, however, the rules don't apply so well to transition metals.
- For example, if we are working with Boron, since there are three electrons in the second shell, we can say that this element has three valence electrons.
Step 5. Use the table lines as shortcuts to the orbital layer
The horizontal lines of the periodic table are called periods of the elements. Starting at the top, each period corresponds to the number of electron shells the atoms in that row have. You can use this information as a shortcut to determine how many valence electrons an element has. Just start on the left side of the period when counting the electrons. Again, ignore transition metals when using this method.
- For example, we know that the element selenium has four orbital layers because it is in the fourth period. Since it is the sixth element from the left in this period (ignoring the transition metals), we know that the outer fourth shell has six electrons and therefore that Selenium has six valence electrons.
- Note that electronic configurations can be written in a summarized form using noble gases (the elements of group 18) to serve as orbitals at the beginning of the configuration. For example, the electron configuration of sodium can be written as [Ne]3s1. Essentially, it's the same as neon, but with an extra element in the 3s orbital.
- Transition metals may have incompletely filled valence sublayers. Determining the exact number of valence electrons in these metals involves principles of quantum theory that are beyond the scope of this article.
- Be aware that periodic tables differ from country to country, so make sure you're using the correct one to avoid confusion.