Chapter 20 Chemical Bonds

Sec. 1: Stability in Bonding Atoms from different elements can combine to form compounds. When atoms combine, the compound has different properties than the elements. Ex. Sodium Chloride (table salt) is made of sodium & chlorine. Sodium is a soft metal that reacts violently with water. Chlorine is a poisonous greenish-yellow gas.

Formulas Chemical Formula—tells what elements a compound contains and the exact number of the atoms of each element. Ex. NaCl has 1 atom of sodium & 1 atom of chlorine Ex. H2O has 2 atoms of hydrogen and 1 atom of oxygen. A subscript (small # written below) is written after a symbol to tell how many atoms of that element are in the compound.

Atomic Stability Atoms combine to form a compound that is more stable than the separate atoms. Noble gases are already stable and do not form compounds. Their outer energy level is full of electrons. Atoms can gain, lose, or share electrons to get a full valence electron shell. Chemical Bond—the force that holds atoms together in a compound.

Sodium & Chlorine http://science360 Sodium has 1 valence electron. Chlorine has 7, so it needs 1 more electron to be stable. If sodium give its 1 valence e- to chlorine, they are both stable with 8 valence e-.

Water Molecule http://science360 In water, each hydrogen atom needs 1 e- to fill its outer shell. Oxygen needs 2 e- to fill its outer shell. To be stable, the atoms share electrons.

Types of Bonds Ionic Bonds Covalent Bonds Contain a metal & a nonmetal (at least 1 of each) Electrons are gained or lost Ex. Salt, NaCl Covalent Bonds Contain 2 or more nonmetals or hydrogen Electrons are shared Ex. Water, H2O

Ions When atoms gain or lose electrons in an ionic bond, they form ions. Ion—an atom with a positive or negative charge. You can tell the charge of the ion that an element will form based on its position on the periodic table. Polyatomic Ions—ions made of more than one atom.

Write the charges on you periodic table!!! 1+ 2+ 3+ 3- 2- 1-

Oxidation number—the charge of an ion 1+ 2+ 3+ 3- 2- 1-

Types of Ions Cations—ions that have a positive charge There are 2 types of ions: cations & anions. Cations—ions that have a positive charge They form by atoms losing electrons Metals form cations Anions—ions that have a negative charge They form by atoms gaining electrons Nonmetals form anions

Ionic Bonding In an ionic bond, one atoms gives electrons to another atom. When atoms combine in an ionic bond, it is called a formula unit. Ex. NaCl is a formula unit The charge of a formula unit is always 0. The positive and negative charges must always balance each other.

Multiple Oxidation Numbers Some metals (other metals & transition metals) can have more than 1 oxidation number. If an element can have more than 1 oxidation number, the charge of the ion is written as a roman numeral in parentheses. Ex. Copper (I) ion is Cu1+ Copper (II) ion is Cu2+

Writing Formulas for Ionic Compounds Write the symbol for the cation (either the metal element symbol or polyatomic ion) first Ex. Mg2+ or NH4+ Write the symbol for the anion (either the element symbol or polyatomic ion) second Ex. Cl- O2- Use subscripts to balance the charges of the ions. Ex. KCl, (NH4)2O, MgO, MgCl2

http://science360.gov/obj/tkn-video/dae5c20d-5b15-40dc-a080-e1a2003b6097

Balancing Chemical Formulas Practice http://www.chemfiles.com/flash/formulas.html Write the name of the chemical formula, ions and charges, and the chemical formulas when you get them balanced for 25 problems!!!!! Don’t forget subscripts and parenthesis!

Naming Ionic Compounds Write the name of the cation first (remember cations are metals) Ex. Ca2+ is calcium Some elements can have more than 1 charge (see table 2 p. 616) If it does put the charge of the ion using roman numerals and put it in parentheses. Ex. Cu2+ Copper (II)

Ionic Naming Continued Write the name of the anion second, but change the ending to –ide. Ex. Chlorine becomes Chloride If the anion is a polyatomic ion DO NOT CHANGE THE ENDING. Ex. NO3- is still named Nitrate Put the cation & anion together to get the full name. KI is Potassium Iodide CuSO4 is Copper (II) Sulfate

Making Ionic Compounds Precipitate- cause (a substance) to be deposited in solid form from a solution. ***Gravity is your friend! Keep the droppers above puddle!

Covalent Bonds Covalent bonds share electrons Composed of 2 or more nonmetals (and H) Covalent compounds are called molecules Molecules can be polar or nonpolar Polar molecule—slightly positive and negative at parts—but overall neutral e- are unevenly shared Nonpolar molecule—electrons are shared equally—completely neutral

Naming Covalent Compounds Mono—1 Di—2 Tri—3 Tetra—4 Penta—5 Hexa—6 Hepta—7 Octa—8 Nona—9 Deca—10 Use prefixes to tell how many of each element you have. This is done because you can have different covalent compounds containing the same elements. Change the ending of the 2nd element to -ide Ex. N2O is dinitrogen monoxide. You can leave off the prefix mono- on the first element. Ex. NO2 is nitrogen dioxide Not mononitrogen dioxide

Formulas for Covalent Compounds Use the prefixes as the subscript in the formula. Ex. Phosphorus Trichloride PCl3 Dinitrogen Tetrafluoride N2F4 http://science360.gov/obj/tkn-video/b6c46685-6a9f-4d79-9161-9bd42797cb37