Chemical Reactions and Bonding

1. Chemical Reactions and Bonding

2. Chemical Reactions • A change in matter that produces new substances is called a chemical change. • The new substances are made of the same elements as the original substance, but now in different combinations. • Elements and compounds rearrange to make new materials. • Elements may combine to make compounds, compounds may be broken down into elements, or compounds may change into other compounds.

3. Think about elements and compounds as if they were letters and words. • Every word is made of specific letters in a certain combination. • Likewise, every compound is made of specific elements in a certain combination. • A physical change is like printing the same word in a different style of type: • Stampede stampede • A chemical change, or chemical reaction, is like scrambling the letters of a word to make new words • Stampede made + steps

4. You have seen the result of many chemical reactions • Rusting turns the strong iron metal of a car body into iron oxide, a compound you can easily knock a hole through. • When wood burns, the compounds that make up the wood combine with oxygen in the air to make carbon dioxide and water.

5. Chemical Bonds and Chemical Reactions • The force that holds atoms together is called a chemical bond. • You can think of chemical bonds as the “glue” that makes atoms “stick” to each other. • The types of atoms and how they are joined by chemical bonds determine the properties of a substance. • Chemical reactions occur when chemical b onds are either formed or broken apart. • When bonds are broken and new ones are formed, atoms are shuffled, making new substances with different properties.

6. Some chemical bonds are strong and hard to change. • Others break apart easily. • Glass is unreactive because the chemical bonds that hold it together are strong. • Windows in buildings hundreds of years old show no damage from weather, while wood frames around the glass have rotted away. • The wood is made of compounds that react easily with other substances in the environment. • It can be softened by water and rotted by fungi. • Or it can burn in a fire.

7. When atoms bond, electrons may be transferred from one atom to another, or they may be shared between the atoms. • In either case, the change results in a chemical reaction—that is, new substances form. • Certain elements are more likely to gain electrons, while others are more likely to give up electrons or share electrons.

8. Chemical Bonds and Stability • Atoms that react easily with other atoms are referred to as unstable. • The more stable an atom is, the less likely it is to react with another atom. • An atom’s stability depends on its valance electrons. • Most atoms are more stable—less likely to react—when they have eight valence electrons.

9. For example, atoms of neon, argon, krypton, and xenon all have eight valence electrons and are very unreactive. • These elements do not easily react with other atoms to form compounds. • Some small atoms, such as helium, are stable with just two valence electrons in their first and only energy level.

10. When atoms react, they usually do so in a way that makes each atom more stable. • One of two things can happen: • Either the number of valence electrons increases to eight (or two, in the case of hydrogen). • Or, the atom gives up its most loosely held valence electrons. • Atoms that react this way become chemically combined, or bonded together. • A chemical bond is the force of attraction that holds two atoms together as a result of the rearrangement of electrons between them.