Chapter 4: Formation of Compounds

The Variety of Compounds Section 4.1 The Variety of Compounds

Table Salt Also known as sodium chloride. Occurs naturally in large, solid, underground deposits throughout the world and is dissolved in the world’s oceans.

Salt Salt is used to preserve foods, enhance the flavor of food, send signals through the nervous system, and melt ice on roadways.

Physical Properties of Salt White solid at room temperature. Crystals are cube shaped and brittle. Solid sodium chloride does not conduct electricity but melted sodium chloride does. Salt easily dissolves in water which results in a solution which is an excellent conductor of electricity. TEXTBOOK PAGE 119

Chemical Properties of Salt Does not easily react with other substances. Does not have to be handled in a special way or stored in a special container. Is referred to as stable or unreactive.

Sodium Sodium is a metal but it is soft enough to be cut with a knife. When cut, it has a silvery luster. It must be stored in oil because it reacts with oxygen and water vapor in the air.

Chlorine Is a pale green poisonous gas. Is an excellent disinfectant. Is among the most reactive elements.

= + Pass the salt please… Sodium Chlorine Salt = + Soft, Reactive Metal Poisonous Gas How do the properties of the individual elements compare to the compound produced? TEXTBOOK PAGE 121

Carbon Dioxide Colorless, odorless gas. Gas released when you exhale. Green plants take in carbon dioxide and give off oxygen during photosynthesis. CO2

Carbon Dioxide Solid carbon dioxide is called dry ice. It is used to ship perishable items. The solid form of carbon dioxide does not melt to a liquid. The solid turns into a gas.

Carbon Dioxide Fire extinguishers are filled with carbon dioxide. Carbon dioxide is denser than air so it displaces air and deprives a fire of its supply of oxygen.

Carbon Is a nonmetal and fairly nonreactive at room temperature. At high temperatures it does react with many elements. Carbon forms a huge variety of compounds.

Oxygen Is a colorless, odorless, and tasteless gas. It makes up 21 percent of the air you breathe. Oxygen is the most abundant element in the earth’s crust. TEXTBOOK PAGE 124

Water Covers 70 percent of the Earth’s surface. Makes up almost 70 percent of the mass of the average human body. It is often called “the universal solvent” because many substances dissolve in it. Pure water does not conduct electricity.

Hydrogen Is a colorless, odorless, and tasteless gas. Is the lightest and most abundant element in the universe. Is a reactive element.

How Elements Form Compounds Section 4.2 How Elements Form Compounds

Atoms Collide When substances react, particles of the substance must collide. The collisions are what determines what kind of product will form. Reactions between atoms only involve their valence electrons.

Chemical Stability The Noble Gases: Do not react with other elements to form compounds (are nonreactive & very stable). Each element gives off a different color of light when high-voltage current passes through it. Were previously called the inert gases.

Noble Gases Each noble gas has 8 valence electrons, except helium which has 2. The word inert means nonreactive

The Octet Rule An atom becomes stable by having 8 valence electrons (some smaller atoms only need 2). Atoms become more stable by combining. Elements become more stable by achieving the same configuration of valence electrons as the noble gases. This is called a noble gas configuration.

Stability If atoms collide with enough energy, their outer electrons may rearrange to achieve a stable octet and the atoms will form a compound. There are two ways to achieve stability: To transfer valence electrons between atoms. To share valence electrons between atoms.

Ions An ion is an atom or group of combined atoms that has a charge because of the loss or gain of electrons. Ions always form when valence electrons rearrange by electron transfer between atoms. TEXTBOOK PAGE 132

Cation If an atom loses an electron it becomes positive ion. A positive ion is called a cation. Metals lose electrons to become positively charged ions. Example: By losing its lone valence electron, sodium will now have a stable octet.

Anion If an atom gains an electron it becomes negative ion. A negative ion is called an anion. Nonmetals gain electrons to become negatively charged ions. Example: Chlorine had 7 valence electrons. It wants eight to become stable. It gains 1 electron and then has a negative charge.

Ionic Compound An ionic compound is a compound composed of ions. It consists of a metal ion (positive charge) and a nonmetal ion (negative charge). The strong attractive force between ions of opposite charge is called an ionic bond.

Crystal A crystal is a regular, repeating arrangement of atoms, ions, or molecules.

Chemical Formula A formula tells you which elements make up the compound and how many atoms of each element are present in one unit of the compound. H2O 2 Hydrogen atoms (grey) 1 Oxygen atoms (red)

Sharing Electrons When atoms collide with enough energy to react, but neither attracts electrons strongly enough to take electrons from the other atom, the atoms combine by sharing valence electrons. Each oxygen atom has 6 valence electrons. They each want 8 to become stable. They share two electrons so they can both now have 8 valence electrons.

Covalent Bonds & Compounds A covalent bond is the attraction of two atoms for a shared pair of electrons. A covalent compound is a compound whose atoms are held together by covalent bonds. Covalent compounds consist of all nonmetals.

Molecules A molecule is an uncharged group of two or more atoms held together by covalent bonds. Sometimes chemists refer to covalent compounds as molecular compounds. The terms mean the same thing.

Multiple Bonds Single Bond (two electrons) Double Bond (four electrons) Triple Bond (six electrons)

Ionic vs. Covalent Compounds Ionic Compounds Crystalline solids at room temp. Generally hard, rough and brittle. High melting and high boiling points (due to the strong attraction between ions of opposite charge). Conducts electricity when melted or dissolved in water. Soluble (dissolves) in water. Covalent Compounds Gases, or liquids at room temp. Low melting and low boiling points. Poor electrical conductors in all phases. Not soluble (does not dissolve) in water.

Electrolyte An electrolyte is a compound that when dissolved in water or melted conducts electricity. Ionic compounds are electrolytes. Dissolving ionic compounds in water separates the compound into ions. When covalent compounds dissolve in water, the ions do not separate from the compound. Therefore, is does not conduct electricity. Ions Separate Ions Do Not Separate

Electrolytes

Interparticle Forces Interparticle forces are the forces between particles that make up a substance. When these forces are between molecules, they are called intermolecular forces.