Unit B Lesson 7: Chemical Formulas
Completion requirements
Unit B Lesson 7: Chemical Formulas
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Learning Targets |
Big Question: How do I interpret chemical formulas?
Chemical formulas like H2O and CCl4 reveal the type and number of atoms there are in a compound.
Chemical formulas like H2O and CCl4 reveal the type and number of atoms there are in a compound.
At the end of this inquiry, you should be able to answer the following questions:
- What is a chemical formula?
- What is a subscript?
- In a chemical formula, what does a subscript represent?
- What does the term 'aqueous' mean?
- In a chemical formula, how are the four states of matter represented?
Page 139 to 142 in your text will help you answer these questions about chemical formulas.
Periodic Table links:
Introduction
Chemical Symbols and Formulas
In lesson 6, you learned about chemical symbols for elements, such as C for carbon, O for oxygen, and H for hydrogen. Elements join together like building blocks to form larger structures called compounds. We write the chemical formulas for compounds by combining their element symbols together.
In lesson 6, you learned about chemical symbols for elements, such as C for carbon, O for oxygen, and H for hydrogen. Elements join together like building blocks to form larger structures called compounds. We write the chemical formulas for compounds by combining their element symbols together.
Which elements are in carbon monoxide (CO), carbon dioxide (CO2), and methane (CH4)? The periodic table is used to identify the elements present in compounds, and certain rules are used to determine how many of each element
appear in a compound.
In this lesson, you will learn to interpret chemical formulas. Chemical formulas can be interpreted by translating symbols and numbers into words that can be spoken.
Symbols are the alphabet of the language of chemistry. Chemical symbols identify the elements present in the molecule. Click here if you need a review of the Symbols of Elements in lesson 6.
In this lesson, you will learn to interpret chemical formulas. Chemical formulas can be interpreted by translating symbols and numbers into words that can be spoken.
Symbols are the alphabet of the language of chemistry. Chemical symbols identify the elements present in the molecule. Click here if you need a review of the Symbols of Elements in lesson 6.
The chemical formula for water is H2O. H is the chemical symbol for hydrogen. (Find it on the periodic table. Its atomic number is 1). O is the symbol for oxygen. (Find it on the periodic table. Its atomic number is 8.) Therefore, a water molecule contains both hydrogen and oxygen.
As you can see in Figure 1, water, H2O, contains two hydrogen atoms and one oxygen atom.
Chemical formulas are used to show:
- the elements that make up a molecule
- the number of atoms of each element in a molecule
Figure 1 – A water molecule, H2O.
Watch
Chemical Compounds and Formulas
Watch the following videos to learn more about chemical compounds and using symbols in chemical formulas.
Watch the following videos to learn more about chemical compounds and using symbols in chemical formulas.
Subscripts
Water
The chemical formula for water is H2O. Subscripts are numbers written to the lower right of an element and are used to identify how many atoms of an element are present. Because the subscript for hydrogen is 2, you know two hydrogen atoms are in a water molecule. When no subscript occurs, as in the case of oxygen in the water molecule, only one of that type of atom is present.
The chemical formula for water is H2O. Subscripts are numbers written to the lower right of an element and are used to identify how many atoms of an element are present. Because the subscript for hydrogen is 2, you know two hydrogen atoms are in a water molecule. When no subscript occurs, as in the case of oxygen in the water molecule, only one of that type of atom is present.
| Symbol | Elements | How Many Atoms? |
| H | Hydrogen | 2 |
| O | Oxygen |
1
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Table 1 - Type and number of atoms in a molecule of water.
Ammonia
The ammonia molecule has the formula NH3. According to the periodic table, which elements are present in the ammonia molecule? Complete the blanks in Table 2 and answer the following questions:
The ammonia molecule has the formula NH3. According to the periodic table, which elements are present in the ammonia molecule? Complete the blanks in Table 2 and answer the following questions:
1. What elements are present?
The periodic table shows that N is nitrogen (N has an atomic number of 7) and H is hydrogen (H as an atomic number of 1).
2. How many of each type of atom is present?
The formula for ammonia is NH3. Because N does not have a subscript, there is one atom of nitrogen in every ammonia molecule. Because the subscript for hydrogen is 3, three atoms of hydrogen are in every ammonia molecule.
| Symbol | Elements | How Many Atoms? |
| N
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| H
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Methanol
The methanol molecule has the formula CH4O. According to the periodic table, which elements are present in the methanol molecule? Complete the blanks in Table 3 and answer the following questions:
The methanol molecule has the formula CH4O. According to the periodic table, which elements are present in the methanol molecule? Complete the blanks in Table 3 and answer the following questions:
1. What elements are present?
On the periodic table, C is carbon (C has an atomic number of 6), H is hydrogen (H as an atomic number of 1), and O is oxygen (O has an atomic number of 8).
2. How many of each type of atom is present?
The formula for methanol is CH4O. Because C and O do not have subscripts, one atom of carbon and one atom of oxygen are in every methanol molecule. Because the subscript for hydrogen is 4, four atoms of hydrogen
are in every methanol molecule.
| Symbol | Elements | How Many Atoms?
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carbon
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hydrogen
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oxygen
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Table 3- Type and number of atoms in a molecule of methanol
Perchloric Acid
The perchloric acid molecule has the formula HClO4. On the periodic table, find which elements are present in the perchloric acid molecule. Complete the blanks in Table 4 and answer the following questions:
The perchloric acid molecule has the formula HClO4. On the periodic table, find which elements are present in the perchloric acid molecule. Complete the blanks in Table 4 and answer the following questions:
1. What elements are present?
Look for capital letters. Each time you see a new capital letter, a new element starts. A lowercase letter is part of the symbol for the capital letter to the left of it.
On the periodic table, you will find that H is hydrogen (H as an atomic number of 1), Cl is chlorine (Cl has an atomic number of 17), and O is oxygen (O has an atomic number of 8).
On the periodic table, you will find that H is hydrogen (H as an atomic number of 1), Cl is chlorine (Cl has an atomic number of 17), and O is oxygen (O has an atomic number of 8).
2. How many of each type of atom is present?
The formula for perchloric acid is HClO4. Because H and Cl do not have subscripts, one atom of hydrogen and one atom of chlorine are in every perchloric acid molecule. Because the subscript for oxygen is 4, four atoms
of oxygen are in every perchloric acid molecule.
| Symbol | Elements | How Many Atoms?
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1
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1
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4
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Table 4– Type and number of atoms in a molecule of perchloric acid
Indicating the Physical State of a Compound
In Lesson B2 the three states of matter (solid, liquid, and gas) were discussed. These states are written at the end of the chemical name for elements and compounds. The style of the state should be written in italics and subscript, but it will sometimes simply appear at the end of the formula in parentheses. For example, oxygen gas might be written as O2(g) or O2(g).
This lesson introduces a fourth term called aqueous. An aqueous solution is formed when a substance is dissolved in water. For example, dissolving salt, NaCl, in water would have the chemical formula NaCl(aq)
Thus, four types of state symbols can be used when writing chemical formulas; solid (s), liquid (l), gas (g), and aqueous (aq).
In Lesson B2 the three states of matter (solid, liquid, and gas) were discussed. These states are written at the end of the chemical name for elements and compounds. The style of the state should be written in italics and subscript, but it will sometimes simply appear at the end of the formula in parentheses. For example, oxygen gas might be written as O2(g) or O2(g).
This lesson introduces a fourth term called aqueous. An aqueous solution is formed when a substance is dissolved in water. For example, dissolving salt, NaCl, in water would have the chemical formula NaCl(aq)
Thus, four types of state symbols can be used when writing chemical formulas; solid (s), liquid (l), gas (g), and aqueous (aq).
| Symbol | Meaning |
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(s)
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solid |
| (l) | liquid |
| (g) | gas |
| (aq) | aqueous |
An aluminum can is a solid; therefore, it would be identified by the symbol Al(s). Have you ever wondered how the can is made?
To make a can, the aluminum must be melted and pressed into sheets. Then, it is molded into cans. Liquid aluminum is represented as Al(l).
Water is a liquid at room temperature, H2O(l). It will freeze into H2O(s) belong 0°C, and will evaporate to H2O(g) at 100°C.
The chemical formula for salt is NaCl. The solid salt in the spoon is identified as NaCl(s). The salt that is dissolved in water is identified as NaCl(aq).
Try It!
Practice Worksheet: Working with Chemical Formulas
- DOWNLOAD this practice worksheet (S9_UB_S3_L7_chemical_formulas). If you prefer to use a Google Drive or PDF version of the worksheet, click here.
- Referring to the formulas listed in the table on the worksheet, complete the following:
- List the elements (symbols and names) present in the formula
- State the number of atoms of each element in the formula
- Identify the state of the substance
- When you are satisfied with your responses you can check your work by clicking on the "SUGGESTED ANSWERS" button below.
Wait! Don't view the suggested answers first. This practice work is not for marks, it is meant to help you check your understanding. Check the answers AFTER doing the questions! Keep the practice worksheet for study purposes. If you don't understand something, contact your teacher!
