 Welcome to this module on Lewis dot structures of covalent compounds. Atoms are made up of protons, neutrons, and electrons. Protons and neutrons are located at the center of the atom in the nucleus. Electrons surround the nucleus at varying distances, and they can be divided into core electrons and valence electrons. Core electrons are tightly bound to the nucleus, and valence electrons are the outermost electrons. They're also involved in all chemical reactions. The number of valence electrons varies by element. For the main group elements, the number is equal to the group number that the element belongs to. For example, sodium belongs to group 1A, so it has one valence electron. Bromine belongs to group 7A and has seven valence electrons. We represent valence electrons using a Lewis dot symbol. Each element symbol is surrounded by dots, which represent its valence electrons. For example, oxygen has six valence electrons, and this is its Lewis dot symbol. Each dot represents one of oxygen's six valence electrons. Neon has eight valence electrons, and this is its Lewis dot symbol. Carbon has four valence electrons, so its Lewis dot symbol looks like this. Now it's your turn. How many valence electrons does potassium have? Potassium is in group 1A. Therefore, it has one valence electron. How many valence electrons does antimony have? Antimony, Sb, is in group 5A. Therefore, it has five valence electrons. How many valence electrons does phosphorus have? Phosphorous is in group 5A. Therefore, it has five valence electrons. Finally, how many valence electrons does magnesium have? Magnesium is in group 2A. Therefore, it has two valence electrons. Next, we'll explore the octet rule. Noble gas elements in group 8A have either two valence electrons, like helium, or eight valence electrons, like neon, argon, krypton, xenon, and radon. These elements are extremely stable because they have full valence shells. Two electrons for helium in the first row and eight electrons in each of the latter rows for the other noble gases. All of these gases represent the octet rule, which states that elements tend to react in a way to attain the same electron configuration as group 8A, the noble gases. Put another way, each atom wants to combine with other atoms until it reaches eight electrons in its valence shell. Atoms reach eight electrons by sharing electrons through covalent bonds. Metallic elements at the left side of the periodic table tend to lose one or more electrons and form positive ions, such as sodium and magnesium. Each of these has the same electron configuration as the noble gas that precedes it. Nonmetals at the right side of the periodic table tend to either gain electrons to form negative ions, such as fluorine, oxygen, and nitrogen, or to share electrons in covalent bonds. When nonmetallic elements react with each other, they share electrons in order to reach eight valence electrons. Let's see this in action. One fluorine atom has seven valence electrons, so it requires one more to satisfy the octet rule. Fluorine gas, or F2, shares electrons between two fluorine atoms. The left fluorine atom now has a total of eight electrons around it, as does the right fluorine atom. The shared electrons form a covalent bond. Any unshared electrons are called lone pairs. The two electrons that form the covalent bond are often represented by a single line. We can represent the fluorine gas molecule by using a line and dots to show the bonding pair and the six lone pairs, respectively. This is called a Lewis dot structure. Some atoms have to share more than one electron in order to satisfy the octet rule. Let's look at oxygen. Each oxygen atom has six valence electrons, so each one requires two more electrons. The left oxygen atom now has a total of eight electrons around it, as does the right oxygen atom. The four shared electrons form a double bond, which is represented by two single lines. Remember, each line in the Lewis dot structure represents two electrons. Hydrogen is an exception to the octet rule, because it only needs two electrons to be stable instead of eight. In this example, this hydrogen atom has one valence electron, so it requires one more. The fluorine atom has seven valence electrons. It requires one more to satisfy the octet rule. The hydrogen atom now has a total of two electrons, so it's stable, and the fluorine atom now has a total of eight electrons around it, so it's also stable. The Lewis dot structure of this hydrofluoric acid molecule shows a line and six dots. This represents the bonding pair and the three lone pairs of the electrons. Now let's review the rules for writing Lewis dot structures. Rule number one is, add the number of valence electrons for each atom in the molecule together. Let's use carbon tetrafluoride as an example. Carbon has four valence electrons and fluorine has seven valence electrons. Therefore, the total number of valence electrons is four plus four times seven, which equals 32. The second rule is, write out the elements of the molecules so that the least electronegative element is in the center and is surrounded by the other elements. Again, we'll use carbon tetrafluoride as our example. Rule three, place the covalent bond between the central atom and the outside atoms. Remember, each covalent bond contains two electrons. The four covalent bonds use eight of the 32 valence electrons in carbon tetrafluoride. The fourth rule is, add electrons to the outer atoms as lone pairs to satisfy the octet rule. This uses 24 electrons. There are no electrons left, so this is the Lewis dot structure for carbon tetrafluoride. Next, we'll apply the fifth rule. In this example, we're going to use NH3, commonly known as ammonia. We'll start by going through the first four rules. Now we can apply rule five, which states, if there are still unused electrons, add the lone pairs to the center atom. So we'll place the remaining two valence electrons on the central nitrogen atom. This creates the Lewis dot structure for ammonia. Finally, we'll apply rule six, which states, check all atoms in the molecule to make sure each one has eight electrons, with the exception being hydrogen, which only requires two. If an atom has fewer than eight electrons, we'll need to create double or triple bonds. It's also important to point out that double bonds only exist between carbon, nitrogen, oxygen, and sulfur. Now, let's apply rule six to CH4, or methane. Hydrogen has one bond, which equals two electrons, and carbon has four bonds, which equals eight electrons, so both molecules are stable. Let's see how rule six applies to CF4, or carbon tetrafluoride. Fluorine has one bond with three lone pairs. This equals two plus three times two electrons for a total of eight electrons. It's now stable. Carbon has four bonds, equaling eight electrons, and is also stable. Finally, we'll look at formaldehyde. We'll start, as always, by applying the first five rules. Finally, we'll apply rule six. Oxygen shares one of its lone pairs with carbon. This creates a double bond between carbon and oxygen, and gives carbon the needed eight electron total. This is the Lewis dot structure for formaldehyde. The octet rule applies to groups 4A through 7A in the second row of the periodic table, but there are exceptions to the rule. Let's take a look at two examples. BF3, or boron trifluoride, and PF5, or phosphorus pentafluoride. We'll start by applying the first five rules to boron trifluoride. Next, we'll move on to rule six, which is where we find the exception. Check the number of electrons around each atom. Each fluorine atom has eight electrons, but boron only has six, which makes it an exception to the octet rule. A boron fluorine double bond isn't an option, because double bonds only exist between carbon, nitrogen, oxygen, and sulfur atoms. The result of this exception is the Lewis dot structure for boron trifluoride. Next, we'll look at phosphorus pentafluoride. Just with boron trifluoride, we'll apply the first five rules. Then we come to rule six, which is again where we find the exception to the octet rule. Check the number of electrons around each atom. Each fluorine atom has eight electrons, but the phosphorus atom has ten. This is the Lewis dot structure for PF5. Lewis dot structures show how electrons are distributed in molecules. There are six rules. Now, let's check your understanding by answering the following questions. What is the total number of valence electrons in the molecule? Arsenic is in group five, and there are three hydrogen. Therefore, the total is eight valence electrons. Which atom is at the center of the molecule? Arsenic is at the center. Remember, hydrogen cannot be at the center of any molecule. Where should the bond be placed between the atoms? All three places need a bond. How many valence electrons do you have left? There are two valence electrons left. On which atom do the remaining two electrons go? The remaining two electrons get applied to the arsenic molecule to complete its five electrons. This is the Lewis dot structure for arsenic trihydride. You've completed Lewis dot structures of covalent compounds.