Chemical Compounds
In what ways do compounds differ from their component elements?
In nature, most elements are found combined with other elements in compounds. A chemical compound is a substance formed by the chemical combination of two or more elements in definite proportions. Scientists show the composition of compounds by a kind of shorthand known as a chemical formula. Water, which contains two atoms of hydrogen for each atom of oxygen, has the chemical formula H2O. The formula for table salt, NaCl, indicates that the elements that make up table salt—sodium and chlorine—combine in a 1 : 1 ratio.
The physical and chemical properties of a compound are usually very different from those of the elements from which it is formed. For example, hydrogen and oxygen, which are gases at room temperature, can combine explosively and form liquid water. Sodium is a silver-colored metal that is soft enough to cut with a knife. It reacts explosively with water. Chlorine is very reactive, too. It is a poisonous, yellow-greenish gas that was used in battles during World War I. Sodium chloride, table salt, is a white solid that dissolves easily in water. As you know, sodium chloride is not poisonous. In fact, it is essential for the survival of most living things.
BUILD Vocabulary
RELATED WORD FORMS The verb react means to act in response to something. The adjective reactive describes the tendency to respond or react.
Chemical Bonds
What are the main types of chemical bonds?
The atoms in compounds are held together by various types of chemical bonds. Much of chemistry is devoted to understanding how and when chemical bonds form. Bond formation involves the electrons that surround each atomic nucleus. The electrons that are available to form bonds are called valence electrons. The main types of chemical bonds are ionic bonds and covalent bonds.
Quick Lab
GUIDED INQUIRY
You will be assigned to represent either a sodium atom or a chlorine atom.
Obtain the appropriate number of popcorn kernels to represent your electrons.
Find a partner with whom you can form the ionic compound sodium chloride—table salt.
In table salt, the closely packed sodium and chloride ions form an orderly structure called a crystal. With all your classmates, work as a class to model a sodium chloride crystal.
Model an Ionic Compound
Relate Cause and Effect Describe the exchange of popcorn kernels (electrons) that took place as you formed the ionic bond. What electrical charges resulted from the exchange?
Use Models How were the “ions” arranged in the model of the crystal? Why did you and your classmates choose this arrangement?
Analyze and Conclude
Table of Contents
- Formulas and Equations
- Applying Formulas and Equations
- Mean, Median, and Mode
- Estimation
- Using Measurements in Calculations
- Effects of Measurement Errors
- Accuracy
- Precision
- Comparing Accuracy and Precision
- Significant Figures
- Calculating With Significant Figures
- Scientific Notation
- Calculating With Scientific Notation
- Dimensional Analysis
- Applying Dimensional Analysis