Molecules and Covalent Bonding

Molecules and Covalent Bonding Molecular compounds both occur naturally and are produced synthetically. Some are beneficial (nitrous oxide used by dentists to relax patients); others are not (nitrogen dioxide is emitted in car exhaust and contributes to smog). Molecular compounds are made up of molecules. A molecule is a group of two or more atoms joined by covalent bonds.

A covalent bond forms when two non-metallic atoms share electrons. Molecules that consist of only two atoms of either the same or different elements are called diatomic molecules Name of Element Chemical Symbol Formula of Molecule State at Room Temperature Hydrogen H H2 Gas Oxygen O O2 Fluorine F F2 Bromine Br Br2 Liquid Iodine I I2 Solid Nitrogen N N2 Chlorine Cl Cl2

Making Molecular Compounds from Elements One method of representing atoms is through Lewis dot diagrams. Unlike a Bohr-Rutherford diagram, in a dot diagram you only need to show the symbol for the element and the valence electrons (the electrons in the outermost energy level). You can use Lewis dot diagrams to represent both ionic bonds and covalent bonds.

Example 1: Draw the bond between the elements of Hydrogen gas (H2)

Example 2: Draw the bond between the elements of Carbon Dioxide (CO2)

Naming Molecular Compounds The name of a molecular compound includes prefixes to indicate how many atoms of each element are present. (e.g., dinitrogen pentoxide means there are two nitrogen atoms and five oxygen atoms). If there is only one atom of the first element, the prefix “mono” is omitted. The name of the last element ends in “ide ”.

Prefixes are: 1 = Mono (only place in front of the second non-metal) 2 = Di 3 = Tri 4 = Tetra 5 = Penta 6 = Hexa 7 = Hepta 8 = Octa

When you are asked to name a compound, first check the formula to see if it includes a metal. If the first element is a metal, then the substance is an ionic compound and should be named (with no prefixes).   If the compound consists only of non-metals, it is a molecular compound, and you should follow these steps to name it.

The compound N2O is called Dinitrogen monoxide Example 1: Name the compound with chemical formula N2O .   Step 1: Write the names of both elements in the same order as in the formula. Replace the ending of the second element with “ide”. Nitrogen Oxide Step 2: Add prefixes. Remember that the prefix “mono” is only used for the second element. Dinitrogen monoxide The compound N2O is called Dinitrogen monoxide

The compound PCl3 is called phosphorus trichloride Example 1: Name the molecular compound with the chemical formula PCl3.   Step 1: Write the names of both elements in the same order as in the formula. Replace the ending of the second element with “ide”. phosphorus chloride Step 2: Add prefixes. Remember that the prefix “mono” is only used for the second element. phosphorus trichloride The compound PCl3 is called phosphorus trichloride

Writing Chemical Formulas of Molecular Compounds Given the name, writing the chemical formula of the molecular compound is easy. The prefixes in the name become the subscripts in the formula. Example: Write the chemical formula for dinitrogen pentoxide. N O di = 2 penta = 5 Therefore, the chemical formula is N2O5.

Why are molecular compounds important? Most compounds are molecular. Living things and fossil fuels are both sources of molecular compounds.  Fossil fuels provide valuable energy and petrochemicals. Many important industrial chemicals and consumer products (plastics, pharmaceuticals, and synthetic fabrics) are made from petrochemicals.

Spills and Leaks of Molecular Compounds A variety of strategies are used to reduce the environmental damage resulting from spilled chemicals. Groundwater can be contaminated from oil spills and poor waste disposal methods. One toxin that can leak into groundwater is the molecular compound trichloroethene. Scientists have discovered a microbe (bacteria) that eats some pollutants. As microbes eat the trichloroethene, they convert the pollutant into ethane, a harmless gas.