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Using Atoms and Ions to Determine Molecular Formulas

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  • 0:05 Atoms Determine…
  • 2:08 Molecular Formula Examples
  • 3:24 Coefficients in…
  • 4:30 Ions Also Determine Formulas
  • 5:28 Writing the Formula…
  • 8:12 Lesson Summary
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Lesson Transcript
Instructor: Laura Nappi
In this lesson, you will learn how atoms and ions determine molecular formulas. You will understand how molecular formulas tell information about the type and quantity of atoms involved in a molecule. Lastly, we will discuss how to write a molecular formula for an ionic compound.

Atoms Determine Molecular Formulas

Do you enjoy cooking? If so, you know it is very important to add the right type and quantity of ingredients to get your desired food item. If you forget to add one key ingredient, you could mess up the entire recipe. For example, one time when I was making cookies, I totally forgot to add eggs and the cookies tasted horrible.

Elements are just like the ingredients used in cooking, and the type and quantity is crucial to getting your desired result. Molecular formulas are like a step-by-step recipe that tells information about what is involved in making a certain molecule. Molecular formulas express what a molecule is comprised of by identifying the type of elements involved and quantity of atoms present. Remember, a molecule is comprised of at least two atoms joined together.

A molecular formula includes the chemical symbols of elements involved in a molecule and a subscript to indicate the quantity of atoms of each element. If you are like me, one of your favorite things to do is breathe, right? Did you know that the air you breathe has the molecular formula O2? The subscript 2 indicates there are two oxygen atoms present in this molecule.

What do you think would happen if you added one more oxygen atom? Would we still get an oxygen molecule? The addition of one more oxygen atom could totally change the result. Just like in cooking, if you change the quantity of ingredients, you get something completely different. If you combined three oxygen atoms, you would not get an oxygen molecule, you would get ozone. Ozone is very different than an oxygen molecule. It provides a protective layer over Earth that shields the sun's dangerous rays. The molecular formula for ozone is O3. The subscript of 3 identifies that there are three oxygen atoms involved in the formation of this molecule.

Examples of Molecular Formulas

Has all of this talking about molecular formulas made you thirsty, perhaps for a beverage like soda? Have you ever wondered what ingredients make up soda? The recipe for soda has many ingredients, such as sugar, caffeine, and water.

Sugar is also known as glucose. The molecular formula of glucose is C6H12O6. The formula identifies there are six carbon atoms, twelve hydrogen atoms, and six oxygen atoms present. Another ingredient in soda is caffeine, which has the molecular formula C8H10N4O2. This molecular formula identifies there are eight carbon atoms, ten hydrogen atoms, four nitrogen atoms, and two oxygen atoms present. Lastly, soda also contains water, which has the molecular formula H2O. This formula identifies there are two hydrogen atoms and one oxygen atom present. Did you notice there is no subscript on the oxygen atom? If an atom does not have a subscript, it is implied to be 1.

The molecular formula of glucose
Chemical Formula of Glucose

Coefficients in Molecular Formulas

Sometimes a molecular formula has a numerical coefficient in front of the entire molecule. For example, if a molecular formula is written as 2H2O, the numerical coefficient of 2 means there are two water molecules present, with each water molecule having two hydrogen atoms and one oxygen atom.

To determine the quantity of atoms involved in this molecule, simply multiply the coefficient by the subscript of each atom. To find the total quantity of hydrogen, multiply the numerical coefficient 2 by the subscript of 2. This quick calculation reveals that four hydrogen atoms are present, because 2 multiplied by 2 equals 4.

Then to find the total quantity of oxygen atoms present, multiply the numerical coefficient of 2 by the subscript of oxygen. Since there is no subscript of oxygen, remember, it is implied to be 1, so the calculation is 2 multiplied by 1, which equals 2. This calculation reveals there are two oxygen atoms present.

Ions Also Determine Formulas

Sometimes the atoms that make up a molecular formula are ions. Remember, an ion is an atom that carries a charge because it lost or gained one or more electrons. Recall there are two types of ions, a cation and anion. An anion is a type of ion with an overall negative charge because it gained one or more electrons, and a cation is a type of ion with an overall positive charge because it lost one or more electrons. Lastly, recall when an anion and cation form a bond it is called an ionic compound.

Molecular formulas are also written for ionic compounds. For example, the ionic compound sodium chloride has the molecular formula NaCl. The atoms involved in this compound are ions - sodium is a cation and chloride is an anion.

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