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Chemical Formula

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Chemical Formula
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You can make a simple salad dressing using just the two ingredients pictured above: oil and vinegar. Recipes for oil-and-vinegar salad dressing vary, but they typically include about three parts oil to one part vinegar, or a ratio of 3:1. For example, if you wanted to make a cup of salad dressing, you could mix together  \frac{3}{4} cup of oil and  \frac{1}{4} cup of vinegar. Chemical compounds also have “ingredients” in a certain ratio. However, unlike oil-and-vinegar salad dressing, a chemical compound always has exactly the same ratio of elements. This ratio can be represented by a chemical formula.

Representing Compounds

In a chemical formula , the elements in a compound are represented by their chemical symbols, and the ratio of different elements is represented by subscripts. Consider the compound water as an example. Each water molecule contains two hydrogen atoms and one oxygen atom. Therefore, the chemical formula for water is:

H 2 O

The subscript 2 after the H shows that there are two atoms of hydrogen in the molecule. The O for oxygen has no subscript. When there is just one atom of an element in a molecule, no subscript is used in the chemical formula.

Formulas for Ionic and Covalent Compounds

The Table below shows four examples of compounds and their chemical formulas. The first two compounds are ionic compounds, and the second two are covalent compounds. Each formula shows the ratio of ions or atoms that make up the compound.

Name of Compound Type of Compound Ratio of Ions or Atoms of Each Element Chemical Formulas
Sodium chloride ionic 1 sodium ion (Na + ) 1 chloride ion (Cl - ) NaCl
Calcium iodide ionic 1 calcium ion (Ca 2+ ) 2 iodide ions (I - ) CaI 2
Hydrogen peroxide covalent 2 hydrogen atoms (H) 2 oxygen atoms (O) H 2 O 2
Carbon dioxide covalent 1 carbon atom (C) 2 oxygen atoms (O) CO 2

There is a different rule for writing the chemical formula for each type of compound.

Ionic compounds are compounds in which positive metal ions and negative nonmetal ions are joined by ionic bonds. In these compounds, the chemical symbol for the positive metal ion is written first, followed by the symbol for the negative nonmetal ion. The video at the following URL gives a fuller explanation of ionic compounds and their formulas: http://www.youtube.com/watch?v=vscoYh6m46M .

  • Q: The ionic compound lithium fluoride consists of a ratio of one lithium ion (Li + ) to one fluoride ion (F - ). What is the chemical formula for this compound?
  • A: The chemical formula is LiF.

Covalent compounds are compounds in which nonmetals are joined by covalent bonds. In these compounds, the element that is farther to the left in the periodic table is written first, followed by the element that is farther to the right. If both elements are in the same group of the periodic table, the one with the higher period number is written first. The following URL gives a fuller explanation of covalent compounds and their formulas: http://www.youtube.com/watch?v=ZTMHbKtgI-M .

  • Q: A molecule of the covalent compound nitrogen dioxide consists of one nitrogen atom (N) and two oxygen atoms (O). What is the chemical formula for this compound?
  • A: The chemical formula is NO 2 .

Summary

  • Compounds are represented by chemical formulas. Elements in a compound are represented by chemical symbols, and the ratio of different elements is represented by subscripts.
  • There are different rules for writing the chemical formulas for ionic and covalent compounds.

Vocabulary

  • chemical formula : Symbol of a chemical compound using element symbols and subscripts to represent the ratio of atoms in the compound.

Practice

At the following URL, do the online learning activity on chemical formulas. Then take the quiz at the end of the activity to check your comprehension.

http://www.wisc-online.com/Objects/ViewObject.aspx?ID=GCH603

Review

  1. Complete the following analogy: A chemical symbol is to an element as a chemical formula is to a(n) __________.
  2. The compound sodium sulfide consists of a ratio of one sodium ion (Na + ) to two sulfide ions (S 2- ). Write the chemical formula for this compound.
  3. A molecule of sulfur dioxide consists of one sulfur atom (S) and two oxygen atoms (O). What is the chemical formula for this compound?
  4. Identify the ratio of atoms in the compound represented by the following chemical formula: N 2 O 5 .

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