Chemical Symbols

• Each element has been assigned a unique one, two, or three letter symbol for its identification.

• The First letter of a symbol is always capitalized and the rest are lowercase

• Some common elements that have single letter symbols include O for oxygen and H for hydrogen.

• Symbols are often shortened from the name of the elements such as He for helium and AI for aluminum.

• Sometimes the letters of the symbol correspond to Latin or Greek names such as K (Latin kalium) for potassium and Na(latin natrium) for sodium.

Diatomic Molecules

• Subscript: the number to the right and slightly below a symbol that tells the number of atoms present.

• When writing symbols of uncombined elements almost all are written without a subscript.

  • Subscript is not written if only one atom is present.

  • Iron is Fe, Neon is Ne, Carbon is C

• However, some elements exist in nature as two identical atoms covalently bonded into a diatomic molecule.

  • Oxygen normally exists as O₂. Other diatomic molecules include hydrogen(H₂), nitrogen(N₂) and group 17 of the periodic table

Chemical formulas

• Compounds are composed of combinations of elements chemically combined.

• The formula CO tells the reader that the compound consists of carbon and oxygen.

  • Notice the difference between CO and Co. CO shows a combination of two elements in a compound. Co is a symbol for the element cobalt.

    *Remember the rules of chemical symbols listed above.

• Subscripts in formulas: the use of subscripts shows the relative number of atoms of each type in a compound

  E.g.

  • CO (one carbon atom, one oxygen atom)

  • CO₂ (one carbon atom, two oxygen atoms)

Types of Formulas

There are two basic types of formula

1. Empirical formula: represents the simplest ratio in which atoms combine to form a compound

2. Molecular formulas: represents the ratio in which atoms combine to form a compound

In many cases the molecular formula will be the empirical formula or can be multiple of the empirical formula

E.g.

When molecular formula = empirical formula.

Molecular formula for water = H₂O.

• This tells us that there are two hydrogen ions for every oxygen ion.  This ratio also represents the formula for water in simplest ratio form, therefore it is also the empirical formula.

*when a molecular formula cannot be reduced anymore, it will be the empirical formula

When molecular formula ≠ empirical formula:

The molecular formula of glucose is C₆H₁₂O₆

• The reduced and simplest ratio form of glucose is CH₂O, therefore its empirical formula is CH₂O.