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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.
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.
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.
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).
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.
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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For example, that one valence electron in lithium is in the 2s orbital. That number 2 is the principal quantum number that represents the size of the orbital. The 2s orbital is going to be a lot smaller than the 4s orbital in potassium that holds its valence electron. What that means is the valence electron in potassium is going to have more energy and be farther away from the nucleus than the valence electron in lithium.
What difference does that make? Well, as you may have noticed from the sodium example, those elements in the first column are going to get rid of their outer electrons as quickly as possible. Having that one outer electron flying around alone out there makes that atom very chemically unstable. The ability of it to chemically react is directly dependent upon how easily it can get rid of that outer electron.
Potassium is way more likely to get rid of its outer electron than lithium is because its outer electron is in the 4s orbital, which is much farther away from the inner pull of the positively charged nucleus. Lithium will hang on to its 2s electron more tightly than potassium will hang on to its 4s electron because the 2s electron is closer to the inward pull of the positively charged nucleus. This makes potassium much more reactive than lithium. If you put a tiny chunk of lithium in water it may just fizz, but if you put the same amount of potassium in water it will probably pop or explode.
As you may have noticed, the row an element is in will represent the energy level the valence electrons will have. Elements in the first row (hydrogen and helium) will have outer electrons in the first energy level. Their principal quantum number is 1. Elements in the second row (lithium through neon) will have valence electrons in the second energy level with a principal quantum number of 2. The trend continues all the way down to the seventh row. Remember, those last two rows really belong squeezed into the sixth and seventh rows.
The most important feature of an atom that helps predict its chemical properties is the location and quantity of its electrons - more specifically, its valence electrons or outer electrons. The outer electrons are the ones that participate in the chemical reactions, changing the properties of an atom or molecule. The column an element is in on the periodic table will indicate how many valence electrons it has, and for now, when we count across columns, we will skip over the d-block. The row an element is in will indicate the energy level of the outer electrons. Finally, because valence electrons are so important, they can be represented symbolically in Lewis dot diagrams.
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