Pearson Prentice Hall Physical Science: Concepts in Action Chapter 6 Chemical Bonds

6.1 Ionic Bonding Objectives: 1. Explain when an atom is unlikely to react 2. Indicate one way in which elements can achieve a stable electron configuration 3. Express how the structure of an ionic compound affects its properties

When Atoms are Unlikely to React When the highest occupied energy level (the outer shell) is filled with electrons (valence electrons), the atom is stable and unlikely to react The valence electrons can be represented by a special drawing called an electron dot diagram

Achieving Stable Electron Configurations Atoms can become stable by filling their outer shells with electrons Some elements achieve stable electron configurations by transferring electrons between atoms The gain or loss of electrons between the atoms then produces full shells & stability

The gain or loss of electrons also produces an atom with a net positive charge (lost one or more electrons) or a net negative charge (gained one or more electrons) Definition: an ion is an atom with a net positive or net negative charge Definition: an anion is an ion with a net negative charge Definition: a cation is an ion with a net positive charge

Definition: a chemical bond is the force that holds atoms or ions together as a unit Definition: an ionic bond is the force that holds cations and anions together Definition: an ionic bond is the bond that forms when electrons are transferred from one atom to another Definition: a chemical formula is a notation that shows which elements a compound contains & the ratio of atoms or ions of the elements that make up the compound

Cations form when electrons gain enough energy to escape from atoms Electrons must overcome the natural attraction to the protons in the nucleus Definition: ionization energy is the amount of energy used to remove an electron from an atom The lower the ionization energy is, the easier it is to remove an electron Ionization generally increases in the periodic table from left to right and decreases from the top down

Structure and Properties Many ionic compounds are solid at room temperature IONIC COMPOUNDS GENERALLY CONSIST OF A METAL + A NONMETAL Their molecular structure has an orderly repeating three dimensional structure called a lattice Definition: solids whose particles are arrange in a lattice are called crystals The properties of an ionic compound can be explained by the strong attractions among the ions in a crystal lattice

6.2 Covalent Bonding Objectives: 1. Explain how atoms are held together in a covalent bond 2. Describe what happens when atoms don’t share electrons equally 3. Discuss what factors determine whether a molecule is polar 4. Compare the attractions between polar molecules with the attractions between nonpolar molecules

How Atoms Are Held Together in a Covalent Bond Definition: a covalent bond is a chemical bond in which two atoms share a pair of valence electrons Definition: a molecule is a neutral group of atoms that are joined together by one or more covalent bonds The attractions between the shared electrons & the protons in each nucleus hold the atoms together in a covalent bond It is possible for more than one pair of electrons to be shared producing a double or triple covalent bond

Polar Attractions Attractions between polar molecules are stronger than attractions between nonpolar molecules “Like attracts like”: polar molecules are attracted to polar molecules while nonpolar molecules are attracted to nonpolar molecules Q: What does the subscript 2 in the formula for the hydrogen molecule (H2) mean?

Unequal Sharing of Electrons IN GENRAL, COVALENT MOLECULES CONSIST OF TWO NONMETALS Definition: a polar covalent bond is a covalent bond in which the electrons are not shared equally When atoms form a polar covalent bond, the atom with the greater attraction for electrons has a partial negative charge (written δ-) The other atom has a partial positive charge (written δ+) Water is an example

Factors that Determine Polarity The type of atoms in a molecule and its geometric shape are factors that determine whether a molecule is polar or nonpolar Example: CO2 is linear, while the water molecule is bent

6.3 Naming Compounds & Writing Formulas Objectives: 1. Explain what information the name and formula of an ionic compound provide 2. Describe the information that the name and formula of a molecular compound provide

Ionic Compound Formulas The name of an ionic compound (usually a metal plus a nonmetal) must distinguish the compound from other ionic compounds containing the same elements The formula of an ionic compound describes the ratio of the ions in the compound

Definition: a binary compound is a compound made of only 2 elements The name of a binary compound follows a pattern The cation (+ charge, usually the metal) is named first followed by the anion (- charge, usually the nonmetal) The cation states its name There is a special suffix for the anion The anion uses the stem of its name plus –ide ex: chlorine becomes chloride (-ide means it’s an ion)

Many transition metals can form more than one ion Ex: iron can be Fe2+ or Fe3+ & is called “iron two ion” or “iron three ion” The name of transition metal ions will have a Roman numeral after the name that states the charge Ex: CuO is copper(II) oxide because copper can be Cu+ or Cu2+ Definition: a polyatomic ion is a covalently bonded group of atoms acting as a single unit that carries a charge Compounds with polyatomic ions are ionic compounds

If you know the name, you can write the formula The total charges of the cations and anions must add up to zero Sodium ion is Na+ while sulfide is S2- You will need 2 sodium ions to make the charges add up to zero It is written Na2S and is called sodium sulfide

Molecular Compound Formulas The name and formula of a molecular compound describe the type and number of atoms in a molecule of that compound Molecular compounds are made from nonmetals (upstairs to the right in the periodic table) To name the compounds, the most metallic element, farthest to the left on the table is named first If the elements are in the same group, the element closest to the bottom of the group is named first

The suffix of the second element is changed to –ide It is possible to for 2 elements to combine in different amounts to produce a molecular compound such as N2O4 and NO2 Prefixes are used to tell the amount of each except when there is only one atom of the first element Do not use –mono on the first element only 1=mono- 2=di- 3=tri- 4=tetra- 5=penta- 6=hexa- 7=hepta- 8=octa- 9=nona- 10=deca-

To write the formula for a molecular compound, write the symbols of the elements in the name Then add the appropriate subscript to the element Ex: diphosphorus tetrafluoride is P2F4 Q: What is a polyatomic ion? Q: What is the formula for calcium chloride? Q: Is calcium chloride an ionic or molecular compound? How do you know?