CHAPTER INTRODUCTION Lesson 1Lesson 1Electrons and Energy Levels Lesson 2Lesson 2Compounds, Chemical Formulas, and Covalent Bonds Lesson 3Lesson 3Ionic and Metallic Bonds Chapter Wrap-Up

A chemical bond is a force that holds two or more atoms together in a compound.chemical bond Atoms contain protons, neutrons, and electrons. Each proton has a positive charge; each neutron has no charge; and each electron has a negative charge. Atoms Bond

Protons and neutrons are in an atom’s nucleus. Electrons move around the nucleus.

The atomic number of an element is the number of protons in each atom of that element. An electron moves around the nucleus at a distance that corresponds to its amount of energy. Areas of space in which electrons move around the nucleus are called energy levels. Electrons closest to the nucleus have the least amount of energy. Electrons farthest from the nucleus have the greatest amount of energy. Atoms Bond (cont.)

Electron Energy Levels

The attraction between the positive nucleus of one atom and the negative electrons of another atom is what creates a chemical bond. A valence electron is an outermost electron of an atom that participates in chemical bonding.valence electron The number of valence electrons in each atom of an element can help determine the type and number of bonds that an atom can form. Atoms Bond (cont.)

The periodic table can tell you how many valence electrons an atom has.

In 1916 an American chemist named Gilbert Lewis developed the electron dot diagram, a model that represents valence electrons in an atom as dots around the element’s chemical symbol.electron dot diagram Atoms Bond (cont.)

Atoms with eight valence electrons are chemically stable and do not easily react with other atoms. Atoms that have between one and seven valence electrons are reactive or chemically unstable and easily bond with other atoms to form chemically stable compounds. The elements in group 18 are called noble gases. With the exception of helium, noble gases have eight valence electrons and are chemically stable. Atoms Bond (cont.)

Atoms gain, lose, or share valence electrons and become chemically stable.

Electrons are less strongly attracted to a nucleus the farther they are from it, similar to the way a magnet attracts a paper clip. Electrons in atoms are in energy levels around the nucleus. Valence electrons are involved in chemical bonding. All noble gases, except He, have four pairs of dots in their electron dot diagrams. Noble gases are chemically stable. LESSON 1 SUMMARY

Compounds are chemical combinations of different types of atoms. Chemical bonds join atoms together. From Elements to Compounds

A covalent bond is a chemical bond formed when two atoms share one or more pairs of valence electrons.covalent bond A compound formed from many covalent bonds is called a covalent compound. Covalent Bonds—Electron Sharing

Atoms with less than eight valence electrons become chemically stable by forming a chemical bond.

A single covalent bond exists when two atoms share one pair of valence electrons. A double covalent bond exists when two atoms share two pairs of valence electrons A triple covalent bond exists when two atoms share three pairs of valence electrons. Covalent Bonds—Electron Sharing (cont.)

The more valence electrons that two atoms share, the stronger the covalent bond is between the atoms.

When two or more atoms share valence electrons, they form a stable covalent compound. Covalent compounds usually have low melting points and low boiling points. They are usually gases or liquids at room temperature, but they can also be solids. Covalent compounds are poor conductors of thermal energy and electricity. Covalent Compounds

A molecule is a group of atoms held together by covalent bonding that acts as an independent unit.molecule A molecule that has a partial positive end and a partial negative end because of unequal sharing of electrons is a polar molecule.polar molecule Covalent Compounds (cont.)

Atoms of a polar molecule share their valence electrons unequally.

Atoms of a nonpolar molecule share their valence electrons equally.

A chemical formula is a group of chemical symbols and numbers that represent the elements and the number of atoms of each element that make up a compound.chemical formula A chemical formula describes the types of atoms in a compound or a molecule, but it does not explain the shape or appearance of the molecule. Covalent Compounds (cont.)

Chemical formulas and molecular models provide information about molecules.

LESSON 2 SUMMARY A chemical formula is one way to show the elements that make up a compound. A covalent bond forms when atoms share valence electrons. The smallest particle of a covalent compound is a molecule. Water is a polar molecule because the oxygen and hydrogen atoms unequally share electrons.

An ion is an atom that is no longer electrically neutral because it has lost or gained valence electrons.ion Understanding Ions ion from Greek ienai, means “to go”

Because electrons have a negative charge, losing or gaining an electron changes the overall charge of an atom. Atoms that lose valence electrons become ions with a positive charge. Metal atoms, such as sodium, become more stable when they lose valence electrons and form a chemical bond with a nonmetal. Understanding Ions (cont.)

Sodium atoms have a tendency to lose a valence electron. Chlorine atoms have a tendency to gain a valence electron.

Atoms are electrically neutral because they have the same number of protons and electrons. Once an atom gains or loses electrons, it becomes a charged ion. Understanding Ions (cont.)

When forming a compound, the nonmetal atoms gain the electrons lost by the metal atoms. The attraction between positively and negatively charged ions in an ionic compound is an ionic bond.ionic bond Ionic Bonds—Electron Transferring

An ionic bond forms between Na and Cl when an Na atom transfers an electron to a Cl atom.

Individual ions in an ionic compound are strongly attracted to each other. Covalent compounds are made up of many molecules. When nonmetal ions bond to metal ions in an ionic compound there is a large collection of oppositely charged ions and no molecules. Ionic Compounds

A metallic bond is a bond formed when many metal atoms share their pooled valence electrons.metallic bond Valence electrons in metals are not bonded to one atom. Instead, a “sea of electrons” surrounds the positive ions. Metallic Bonds—Electron Pooling

Valence electrons are free to move among all the aluminum (Al) ions.

Metals are good conductors of thermal energy and electricity. Metals are shiny because the valence electrons at the surface of a metal interact with light. Metallic Bonds—Electron Pooling (cont.)

LESSON 3 SUMMARY Metal atoms lose electrons and nonmetal atoms gain electrons and form stable compounds. An atom that has gained or lost an electron is an ion. An ionic bond forms between positively and negatively charged ions. A metallic bond forms when many metal atoms share their pooled valence electrons.

Electrons with more energy are farther from the atom’s nucleus and are in a higher energy level. Atoms with fewer than eight valence electrons gain, lose, or share valence electrons and form stable compounds. Atoms in stable compounds have the same electron arrangement as a noble gas. Lesson 1: Electrons and Energy Levels

A compound and the elements it is made from have different chemical and physical properties. A covalent bond forms when two nonmetal atoms share valence electrons. Common properties of covalent compounds include low melting points and low boiling points. They are usually gas or liquid at room temperature and poor conductors of electricity. Water is a polar compound because the oxygen atom pulls more strongly on the shared valence electrons than the hydrogen atoms do. Lesson 2: Compounds, Chemical Formulas, and Covalent Bonds

Ionic bonds form when valence electrons from a metal atom transfers to a nonmetal atom. An ionic compound is held together by ionic bonds, which are attractions between positively and negatively charged ions. A metallic bond forms when valence electrons are pooled among many metal atoms. Lesson 3: Ionic and Metallic Bonds