Xe Files - Case 3

The Octet Rule

The Octet rule states that atoms tend to gain, lose, or share electrons in order to acquire a full set of valence electrons. This full set usually consists of 8 electrons, hence the term octet.

The amount of electrons an atom will lose or gain can be determined by looking at its location on the periodic table. Elements in group 1A have one valence electron. Since they would need to gain seven more to have a full octet, it is easier for them to lose one electron and go down to a lower energy level, which already has a full octet. Elements in group 2A will lose two electrons; elements in 3A will generally lose three. In groups 3A, 4A, 5A, 6A, and 7A, the metals in that group will want to lose electrons, nonmetals will want to gain electrons, and semimetals (or metalloids) can go either way. Atoms located in family 8A, called noble gases, already have a full octet and, therefore, so not usually do not want to bond with other atoms.

Lewis Dot Diagrams

A Lewis Dot Diagram, also known as an electron dot diagram, is a diagram used to represent the outer most electron shell of an atom. Dots are used to represent the valence electrons by being placed around the element’s symbol. These diagrams are also used to show how electrons rearrange themselves during a chemical reaction. They can show how many electrons are lost, gained, or shared when they bond to other elements. See the figure below.

The Lewis Dot Diagram above represents Cl₂. The circled dots represent shared electrons while the others represent unshared electrons. In a Lewis Dot Diagram, shared electrons can be represented by dots between the symbols of two atoms or as a line to represent one shared pair of electrons.

Ionic Bonds

Ionic bonds are bonds in which an electron is transferred from one atom to another, usually metal to nonmetal, in order to satisfy the octet rule for both atoms. A group of atoms ionicly bonded is called a compound or a salt—for example, sodium chloride. The sodium, with one outer electron, transfers that electron to the chlorine, with seven outer electrons. The sodium now has no electrons in that shell, so the outer shell is the previous one, which is full, and the chlorine now has eight outer electrons making it a complete shell. Both atoms have satisfied the octet rule.

In the dot diagram above, the red dots represent electrons that were original chlorine electrons while the blue one represents the one donated by sodium.

Covalent Bonds

A covalent bond is a bond formed by a shared pair of electrons between two atoms, usually a non-metal and a non-metal. A group of atoms covalently bonded is called a molecule—for example, ammonia. Ammonia (NH₃), as any other molecule with hydrogen, has a special bonding property: hydrogen only needs to have two electrons to fill its outer energy level. That is due to the fact that hydrogen’s highest principle energy level is n = 1. Therefore, in the NH₃ molecule, the three hydrogen atoms share their one valence electron with nitrogen, which then in turn shares three of its valence electrons, one with each hydrogen atom.

In the dot diagram above, the red dots represent electrons that were original nitrogen electrons while the blue ones represent the ones that were originally hydrogen electrons. There are three shared pairs of electrons in this molecule.

Multiple Bonds

Up until now, all of the molecules we have looked at have had single covalent bonds (bonds with only one pair of electrons shared between two particular atoms). Atoms can, however, form multiple bonds (bonds where two or more pairs of electrons are shared). They can form double bonds where two pairs of electrons are shared or triple bonds where three pairs of electrons are shared.

Double Bonds:
One example of a molecule with double bonds is carbon dioxide. The carbon atom has four outer electrons and the two oxygens each have six outer electrons. The carbon shares two of its outer electrons with each of the oxygens and the oxygens share two of theirs with the carbons as well. There is a total of four electrons being shared between the carbon and each of the oxygens. This gives the carbon 8 electrons, and the oxygens have four extra electrons left, and the four sharing giving them 8 as well. The octet rule is satisfied again.

In the carbon dioxide molecule, carbon has a double bond with both oxygen atoms.

Triple Bonds:
Ethyhe, C2H2, has a triple bond between the two carbon atoms. Each hydrogen atom shares one pair of electrons with one carbon atom. Then, the carbon atoms share three pairs of electrons with one another, giving them a full octet (the 6 electrons they are sharing with each other and the 2 electrons they are sharing with the hydrogen atoms).

A triple bond exitst between the two carbon atoms in order to give each one a full octet.

Resonance:
Sometimes, atoms have a double bond with one atom and a single bond with another atom of the same type. So, which atom has the double bond and which has the single bond? The answer, believe it or not, is that they both have the double and the single bond. That is, both have somewhere between a single and a double bond. One example of a molecule with resonance is sulfur dioxide (SO₂). Here is how you would draw a Lewis Dot Diagram of SO₂.

Since there is only one double bond between two of the same type of atom, one regular and one dotted line is drawn between the two to show that they both have somewhere between a single and a double bond.

Exceptions to the Octet Rule

You have already learned that hydrogen does not follow the octet rule, but there are other elements that sometimes do not have complete octets after bonding. Boron trifluoride (BF₃), for example, gives each fluorine atom a complete octet but boron is left with only 6 valence electrons. Some elements also take more than 8 valence electrons. In sulfur tetrafluoride (SF₄), each fluorine atom shares one pair of electrons with the sulfur atom, completing their octets but leaving sulfur with 10 valence electrons.

Diatomic Elements

Diatomic Elements are elements always found bonded to themselves in nature. The diatomic elements are Hydrogen, Nitrogen, Oxygen, and the five elements in group 7A, the halogens.