Valence Electrons and Energy Levels of Atoms of Elements

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  • 0:08 Valence Electrons &…
  • 1:48 Valence Electrons
  • 2:53 Number of Valence Electrons
  • 6:11 Energy of Valence Electrons
  • 8:44 Lesson Summary
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Lesson Transcript
Instructor: Kristin Born

Kristin has an M.S. in Chemistry and has taught many at many levels, including introductory and AP Chemistry.

The periodic table contains a wealth of information. This lesson will explain how to use it to quickly determine the most useful information about the most important electrons. We will be focusing our discussion on valence electrons and energy levels.

Valence Electrons and Energy Levels

Valence electrons can greatly impact the properties of atoms of the same element
Valence Electrons Characteristics

The electron is one of the most important factors in determining how an atom will react with another atom or molecule. A single electron can make all the difference in the properties of an atom. For example, sodium has one outer electron, located in that 3s orbital. With that outer electron, sodium is very shiny, silvery and extremely explosive in water. It is so dangerous that you will likely never see it in its elemental, neutral form. And if you do find yourself coming across some sodium, you will probably see it being stored in some kind of oil so that it doesn't react with the moisture in the air.

So where have you seen sodium? You may have sprinkled some on your food this afternoon! If you put salt (or sodium chloride) on your food, you would have experienced what sodium is like without that outer 3s electron. Sodium in its silvery form easily loses that outer 3s electron, turning it into sodium ion with a positive 1 charge. Sodium with one less electron than proton will have a positive 1 charge because protons are positively charged and electrons are negatively charged. This sodium ion with only 10 electrons is completely different than neutral sodium metal with all 11 electrons. Sodium ion tastes salty and doesn't react with water at all. You consume it every day, and it's very important that you do, because it plays a major role in your body's nerve functions and fluid balance.

Valence Electrons

This was just a brief introduction into how the electronic structure will affect the function and reactivity (and even taste) of an atom. As I mentioned before, the location and quantity of electrons are important factors in determining how an atom will react. However, the most important information about the electrons has to do with the outermost electrons, or the valence electrons. The inner electrons in an atom are usually tightly held by the nucleus, and they aren't usually going to participate in very many reactions. The outer electrons are the key players in all chemical reactions. That little 3s electron in sodium is the most important electron in sodium. It will be the one that is either present (in explosive sodium metal) or absent (in the sodium ion in sodium chloride in your table salt). This lesson is going to focus on the two most important aspects of these valence electrons: the quantity of valence electrons and the energy of the valence electrons.

The final part in each electron configuration (3s^1 and 4s^1) refers to the valence electrons
Electron Configurations

Number of Valence Electrons

As mentioned before, sodium has one valence electron (that 3s electron), which is one reason why it is so reactive and unstable. If sodium has one valence electron, then how many does potassium have? The answer is also one! However, it is a 4s electron. In fact, all the atoms in the first column on the periodic table have one valence electron, and all of the atoms in the first column on the periodic table are extremely reactive and will have a tendency to lose that outer electron and become more stable.

Because the number of valence electrons is so important (as opposed to the inner ones), they are sometimes represented in Lewis dot diagrams as shown. Lewis dot diagrams show the symbols of atoms with their valence electrons. Sodium is represented by its symbol Na, and because it has one valence electron, that 3s electron, that electron is represented by a dot next to the symbol.

Moving on to the second column, you will notice that magnesium has an electron configuration that ends in 3s^2, meaning that there are two valence electrons in magnesium. Again, these two electrons are extremely important, so sometimes magnesium is represented as Mg with two dots around it. Notice how the dots are represented on opposite sides of each other in the symbol. So, all elements in the second column will have two valence electrons. Next, we are going to skip the d-block. The reason we are skipping over it is twofold: first, there is a less predictable pattern in numbers of valence electrons, which is beyond the scope of this lesson; and second (and most important), the d-electrons don't play as big of a part in the reactions as s and p electrons do.

Moving to the 13th column, which starts with boron, you will notice that there are three outer electrons: two s electrons and one p electron. All atoms in this family will have three valence electrons. Are you starting to see a pattern forming? The elements in the carbon family all will have four valence electrons, the elements in the nitrogen family will have five, the elements in the oxygen family will have six, the halogens will have seven valence electrons and aside from helium, the elements in the last column - the noble gases - will all have eight valence electrons (two s electrons and six p electrons).

The periodic table showing Lewis dot diagrams
Lewis Dot Diagrams

As you can see, the number of valence electrons an atom has is related to the column it is found in on the periodic table. When an atom has eight valence electrons it is said to have an octet of electrons. Atoms with a complete octet have s and p orbitals that are completely filled with electrons, so they are extremely stable. Notice that the Lewis dot diagrams fill the outer shells by first putting in four electrons alone on either side and then starting to pair them up with the addition of the fifth electron. This representation will help us later on when we discuss chemical bonding.

Energy of Valence Electrons

Aside from the number of valence electrons an atom has, the energy they have (or the energy level that they are in) is the last bit of information that helps predict how an atom will react. Let's take one look at the first column of elements. They all have one valence electron, but their valence electrons are located farther and farther away from the nucleus as you move down on the periodic table.

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