In chemistry, the valence electrons of an element are found in the outermost electron shell. The number of valence electrons in an atom determines the types of chemical bonds that atom will be able to form. The best way to find valence electrons is to use the table of elements.
Steps
Method 1 of 2: Finding the Valence Electrons with the Periodic Table
Elements that do not belong to the Transition Metals Group
Step 1. Get a periodic table of elements
It is a colored and coded table made up of numerous boxes that lists all the chemical elements known so far. The periodic table provides a lot of information that we can use to find the number of valence electrons of each atom that we want to examine. Most of the time, chemistry texts show it on the back cover. However, you can also download it from the internet.
Step 2. Label each column of the periodic table with numbers 1 through 18
Usually, elements belonging to the same vertical column have the same number of valence electrons. If your table does not have numbered columns, do it yourself starting from left to right. In scientific terms the columns are called "Groups".
If we consider a periodic table where the groups are not numbered, start by assigning the number 1 to the column where you find hydrogen (H), 2 to that of beryllium (Be) and so on up to column 18 of helium (He)
Step 3. Find the item you are interested in on the table
Now you have to identify the atom you have to study; inside each square you will find the chemical symbol of the element (of the letters), its atomic number (top left in each square) and any other information available, based on the type of periodic table.
- As an example, let's consider the element carbon (C). This has an atomic number of 6, is in the upper part of group 14 and in the next step we will calculate the number of valence electrons.
- In this section of the article we do not consider transition metals, the elements collected in a rectangular block consisting of groups between 3 and 12. These are particular elements that behave differently than the others. We will address them later.
Step 4. Use the group numbers to determine the number of valence electrons. The unit digit of the group number corresponds to the number of valence electrons of the 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 belongs to group 14, it possesses 4 valence electrons.
Transition Metals
Step 1. Find an item from groups 3 to 12
As described above, these elements are called "transition metals" and behave differently when it comes to calculating valence electrons. In this section we will explain how, in a given range, it is often not possible to assign the number of valence electrons to these atoms.
- As an example, we consider tantalum (Ta), element 73. In the next steps we will find the number of valence electrons or at least we will try.
- Remember that the set of transition metals also includes or lanthanoids and actinoids (also called "rare earths"). The two lines of elements that are usually written under the periodic table begin with lanthanum and actinium. These belong to the group 3.
Step 2. Remember that transition metals do not have the "traditional" valence electrons
Understanding why this requires a little explanation of how atoms behave. Read on if you want to know more, or skip to the next section if you just want to have the solution to this problem.
- When electrons are added to atoms, they arrange themselves in different "orbitals"; in practice they are different areas surrounding the atom, in which electrons are grouped. The valence electrons are those that are placed in the outermost shell, those that are involved in the bonds.
- For reasons that are a little more complex and beyond the scope of this article, when atoms bind to the outermost electron shell d of a transition metal, the first electron entering the shell behaves like a normal valence electron., but the others do not and the electrons that are present in other shells act as if they were valence. This means that an atom can have a variable number of valence electrons based on how it is manipulated.
- For more details, you can do some research online.
Step 3. Determine the number of valence electrons based on the group number
However, for transition metals there is no logic pattern you can follow; the number of the group can correspond to a wide variety of 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 to 3 valence electrons.
- Group 9: 2 to 3 valence electrons.
- Group 10: 2 to 3 valence electrons.
- Group 11: 1 to 2 valence electrons.
- Group 12: 2 valence electrons.
- In the example of tantalum, we know that it is in group 5, therefore it has 2 to 5 valence electrons, according to the situation in which it is found.
Method 2 of 2: Finding the Number of Valence Electrons Based on the Electronic Configuration
Step 1. Learn how to read the electronic configuration
Another method to find the number of valence electrons is through the electron configuration. At first glance it seems a complex technique, but it is the representation of the orbitals of an atom by means of letters and numbers. It is a simple notation to understand, once you have studied it.
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Take for example the electron configuration of sodium (Na):
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- 1s22s22p63s1
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Note that this is a line of repeating letters and numbers:
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- (number) (letter)(exponent)(number) (letter)(exponent)…
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- …and so on. The first set of (number) (letter) represents the name of the orbital e (the exponent) the number of electrons that are present in the orbital.
- So, for example, we can say that sodium has 2 electrons in the 1s orbital, 2 electrons in the 2s more 6 electrons in the 2p more 1 electron in the 3s orbital. In total there are 11 electrons; sodium has element number 11 and the accounts add up.
Step 2. Find the electronic configuration of the element you want to study
Once you know it, finding the number of valence electrons is pretty straightforward (except, of course, for transition metals). If the configuration was given to you in the problem data, skip this step and read the next one directly. If you need to write the configuration, here's how:
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This is the electronic configuration for the ununoctio (Uuo), element 118:
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- 1s22s22p63s23p64s23d104p65s24d105p66s24f145d106p67s25f146d107p6
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Now that you have this example model, you are able to find the electron configuration of another atom by simply filling in the schematic with available electrons. It's easier than it looks. Let's take as an example the orbital diagram of chlorine (Cl), element number 17 which has 17 electrons:
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- 1s22s22p63s23p5
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- Note that by adding together the number of electrons present on the orbitals you get: 2 + 2 + 6 + 2 + 5 = 17. You just have to change the number in the final orbital; the rest will remain unchanged, since the previous orbitals are completely full.
- If you want to know more read this article.
Step 3. Assign electrons to the orbital shell with the octet rule
When electrons bind to an atom, they fall inside various orbitals following a precise order; the first two are in the 1s orbital, the next two in the 2s orbital and the next six in the 2p one and so on. When you consider atoms that are not part of the transition metals, you can say that the orbitals form "orbital shells" around the atom and that the next shell is always external to the previous one. Except for the very first shell, which contains only two electrons, all the others contain eight (except in the case of transition metals). This is called octet rule.
- Let's consider boron (B). Its atomic number is 5, so it has 5 electrons and its electron configuration is: 1s22s22p1. Since its first orbital shell has only two electrons, we know that boron has only two orbital shells: 1s with two electrons and one with three electrons from 2s and 2p.
- Take chlorine as a second example, which has three orbital shells: one with two electrons in 1s, one with two electrons in 2s and six electrons in 2p, and finally a third with 2 electrons in 3s and five in 3p.
Step 4. Find the number of electrons in the outermost shell
Now that you know the electronic shells of the atom, it is not difficult to find the number of valence electrons, which is equal to the number of electrons present in the outermost shell. If the outer shell is solid (in other words it has 8 electrons or, in the case of the first shell, 2), then it is an inert element that does not react with others. Always remember that these rules only apply to elements that are not transition metals.
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If we still consider boron, since it has three electrons in the second shell, we can say that it has
Step 3. valence electrons.
Step 5. Use the lines of the periodic table as a shortcut
The horizontal lines are called "Periods". Starting from the top of the table, each period corresponds to the number of "Electronic shells" that an atom possesses. You can use this "trick" to find out how many valence electrons an element has, starting from the left of the period when you count electrons. Do not use this method for transition metals.
For example, we know that selenium has four orbital shells because it is in the fourth period. Since it is also the sixth element from the left in the fourth period (ignoring the transition metals), we know that the outermost shell has six electrons and therefore selenium has six valence electrons.
Advice
- Note that electronic configurations can be written in a shortened form using that of noble gases (the elements of group 18) to represent orbitals starting with it. For example, the electron configuration of sodium can be referred to as [Ne] 3s1. In practice, it shares the same configuration as neon but has an extra electron in the 3s orbital.
- Transition metals may have valence sub-shells (sublevels) that are not totally complete. Calculating the exact number of valence electrons in transition metals requires knowledge of quantum theory principles that are far beyond the scope of this article.
- Remember that the periodic table changes slightly from country to country. So check the one you are using to avoid mistakes and confusion.
