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Mapping It Out
To draw a dot diagram, you need to know which element you are diagramming. For example, if you are mapping out carbon, you need to first determine how many electrons the element has. An element's atomic number determines how many protons and electrons an element possesses. Carbon has six electrons. The first electron shell can only hold two electrons, so this means that four electrons are left in the outer electron shell. To map out the dot diagram, write the symbol for carbon (the letter "C") and place four dots around the symbol, one on each side.
Outer Electrons and the Periodic Table
Where the elements are on the periodic table will determine how many electrons the element contains in its outer shell. For example, elements in Group 2 (called alkali earth metals) will always have two electrons in the outer shell, so there would be two dots in the diagram. Those elements in Group 13 will have three electrons in the outer shell and therefore, three dots in the dot diagram.
More Than Four Electrons
Once you get to Group 15, there will be more than four electrons in the outer shell. For example, oxygen is in Group 16 and therefore has six outer electrons. To diagram this, you place a dot on each side of the element's symbol and then double up as needed. Each "side" is only able to hold up to two dots. Two sides of oxygen would have two dots and two sides would have one dot. The sides with one dot indicate that oxygen will have two electrons available for a covalent bond.
Covalent Bonds and Dot Diagrams
A covalent bond is the sharing of outer shell electrons between two atoms. For example, water (symbolized as H2O) has two single covalent bonds between the molecules. The two hydrogen molecules each have one electron in their outer shells to share, while the oxygen molecule has two electrons in its outer shell. The two free electrons in the hydrogen molecules will bond with the two electrons of the oxygen molecule, giving the water molecule its bent shape.