Molecules and Molecular Formula Help

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By — McGraw-Hill Professional
Updated on Aug 28, 2011


Though many forms of matter like wood, rock, or soap appear solid upon first inspection, most matter is composed of a combination of atoms in a specific geometrical arrangement. The force that binds two or more atoms together is known as a chemical bond . A molecule is the basic joining of two or more atoms held together by chemical bonds. In a covalent bond , electrons are shared equally and in an ionic bond , electrons are transferred. We will see how this happens in Chapter 6.

Several elements occur naturally as two-atom or diatomic molecules. Of these, oxygen, nitrogen, hydrogen, fluorine, chlorine, bromine, and iodine occur in pairs at room temperature. These are in groups IA, VA, VIA, and VIIA of the Periodic Table. Such groupings make it easier for a researcher to tell a lot of specifics about an element at a glance. For example, those elements with one electron short of the noble gas structure are generally very electronegative. This means they really want to find or share an electron to form compounds by gaining an electron and completing a stable outer orbital.

Other molecules such as elemental phosphorus (P) are composed of four atoms, while sulfur (S) has eight atoms. Knowing the normal state of elements becomes important in predicting the outcome of chemical reactions.

If all the atoms within a molecule are composed of the same atoms, then it is a pure element. However, different elements combine with each other all the time. When this happens, then the product is called a molecular compound .

A molecule is composed of the same kinds of atoms, chemically bonded by attractive forces.

One familiar compound is composed of two atoms of hydrogen and one atom of oxygen. This compound, water, written as H 2 O is held together by covalent bonds. Compounds of combined element symbols are called formulas of the compound.

The formula for water is H 2 O. The number of atoms of each element is written as a subscript in the formula, such as the 2 in H 2 O. Later, when we take a closer look at the way atoms and elements combine, the importance of listing these subscript numbers will be more easily seen. (When no subscript is written, it is understood that only one atom of the element is involved.) The following example shows a few other molecular ratios in different compounds.


Some simple examples of the formulas of compounds are given.

Sodium chloride (NaCl) = 1 atom of sodium and 1 atom of chlorine.

Nitrous oxide (NO 2 ) 2 = 2 atoms of nitrogen and 4 atoms of oxygen.

Sucrose (C 6 H 12 O 6 ) = 6 atoms of carbon, 12 atoms of hydrogen, and 6 atoms of oxygen.

Since the earth contains many different forms of matter, solid, liquid, and gas, it is easy to see that atoms can combine in nearly infinite ways to form molecular compounds. However, there are only a certain number of discovered elements and sometimes chemical formulas are the same for different compounds. The way chemists keep these formulas straight is through their molecular and structural formulas.

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