READ: Lewis Dot Diagrams

READ: Lewis Dot Diagrams

Chemical Bonds

 What do this lump of coal, diamond, and what we call pencil lead, which is not really lead at all, have in common? All three substances are forms of carbon. Are you surprised that one element can exist in forms that have such different properties? Do you know what explains it? The answer is chemical bonds. Carbon atoms chemically bond together in different ways to form these three substances.

When you look at everything around you and what it is made of, you will realize that atoms seldom exist on their own. More often, the things around us are made up of different atoms that have been joined together. This is called chemical bonding. Chemical bonding is one of the most important processes in chemistry because it allows all sorts of different molecules and combinations of atoms to form, which then make up the objects in the complex world around us.

A chemical bond is formed when atoms are held together by attractive forces. This attraction occurs when electrons are sharedbetween atoms, or when electrons are exchanged between the atoms that are involved in the bond. The sharing or exchange of electrons takes place so that the outer energy levels of the atoms involved are filled, making the atoms are more stable. If an electron is shared, it means that it will spend its time moving in the electron orbitals around both atoms. If an electron is exchanged it means that it is transferred from one atom to another. In other words one atom gains an electron while the other loses an electron.

The type of bond that is formed depends on the elements involved. In this lesson, we will be looking at ionic and covalent bonding.  

Electron Dot Diagrams

Diagrams contain a lot of useful information in a compact format.  What does the diagram above tell us? The football play diagrammed above describes the lineup of each player on the team and describes how they will move when the ball is snapped. Diagrams of electrons give similar information about where certain electrons are.  We can mark these electrons and indicate what happens to them when an element reacts.

Recall that the valence electrons of an atom are the electrons that reside in the highest occupied energy level. Valence electrons are primarily responsible for the chemical properties of various elements. The number of valence electrons can be easily determined from the position of the element on the periodic table. Remember from the previous module that within each column, or group, of the table, all the elements have the same number of valence electrons. For elements in groups 1-2 and 13-18, the number of valence electrons is easy to tell directly from the periodic table. This is illustrated in the simplified periodic table shown below.

When examining chemical bonding, it is necessary to keep track of the valence electrons of each atom. Electron dot diagrams (sometimes referred to as Lewis Dot Diagrams) are diagrams in which the valence electrons of an atom are shown as dots distributed around the element's symbol.  Since electrons repel each other, the dots for a given atom are distributed evenly around the symbol before they are paired. The table below shows the electron dot diagrams for the entire second period of elements on the periodic table.  Electron dot diagrams would be the same for each element in the representative element groups. 

The Octet Rule 

The noble gases are unreactive because of their electron configurations. American chemist, Gilbert Lewis (1875-1946), used this observation to explain the types of ions and molecules that are formed by other elements. He called his explanation the octet rule. The octet rule states that elements tend to form compounds in ways that give each atom eight valence electrons. There are two ways in which atoms can satisfy the octet rule. One way is by sharing their valence electrons with other atoms forming Covalent Bonds. The second way is by transferring valence electrons from one atom to another forming Ionic Bonds. Atoms of metallic elements tend to lose all of their valence electrons, which leaves them with an octet from the next lowest principal energy level. Atoms of nonmetallic elements tend to gain electrons in order to fill their outermost principal energy level with an octet.

Georgia Virtual, Bonding and Chemical Reactions, CC BY-NC-SA 3.0

After you have completed this part of the lesson, you can check the associated box on the main course page to mark it as complete