CHAPTER 19 Chemical Bonds

COMBINING ELEMENTS  Combining elements usually changes their properties.  Example: Sodium (explosive) mixed with chlorine (poisonous gas) gives us table salt (NaCl)

CHEMICAL FORMULAS  Like a recipe  Tells us what elements are in a compound and exactly how many atoms of each.  H 2 O is 2 hydrogen and 1 oxygen ( No subscript = 1 atom)

Why do atoms form compounds? To be stable!

NOBLE GASES  Chemically Stable atoms: the outer energy level is full (8 electrons)  Don’t need to bond with other atoms to be “happy”

OTHER ENERGY LEVELS  If the energy levels are not full, the atoms look for other atoms to bond with so that each ends up “happy” with 8 electrons.  They can share electrons or take electrons from other atoms.

GETTING STABLE  Sometimes electrons are shared, sometimes they are transferred.  Goal: have 8 electrons in the outer level.  Examples:

WHAT IS AN ION?  A charged particle …Has more or less electrons than protons. …Sometimes made in chemical reactions.

IONIC BONDS  Bonds where electrons are transferred.  Some atoms have an electron to give away; hydrogen, sodium (Na), lithium (Li) (Group 1A or 2A)  Some atoms want to gain electrons; chlorine (Cl), iodine (I); oxygen (O) (Group 7A)

IONIC BONDS  There should always be a zero net charge.  Happens more with metallic elements.

IONIC BONDS

COVALENT BONDS  Electrons are shared between 2 or more atoms; creates a molecule.  Best for atoms with 3-5 electrons in the outer energy level.  Can be like a tug-of-war. The bigger “team” (atom) keeps the electrons closer to itself.  Happens more in non-metals.

COVALENT BONDS  Sometime the tug-of-war causes the molecule to be polar …Has one more negative end and one more positive end.

COVALENT BONDS

CHAPTER 24 CHEMICAL REACTIONS

WHAT IS A CHEMICAL REACTION?  One or more substances are changed into new substances.

Reactants Produce Products

 Chemical equations are used to describe a chemical reaction.  Uses: Chemical Formulas Ex. NiCl 2 or NaOH (letters and subscripts) Coefficients Ex. 2NaCl or 3H 2 O (numbers—tell # of molecules)

BALANCING EQUATIONS  Balanced chemical equations: have the same number of atoms on both sides.

FEBRUARY 23, 2009  Please answer these questions on the left side of your notes…. …What are isotopes? …What is the difference between C-14 and C-12? …What do all balanced equations need?

HgO → Hg + O 2 Atoms: Hg: O:

Li + H 2 O → LiOH + H 2 Atoms: Li: H: O:

Mg + O 2 → MgO Atoms: Mg: O:

COMBUSTION Substance + Oxygen produce energy (as heat and light) H 2 + O 2 = H 2 O

SYNTHESIS REACTIONS  2 substances combine to form a different substance.  A + B → AB

DECOMPOSITION REACTIONS  A substance breaks down into 2 or more substances.  AB → A + B

SINGLE DISPLACEMENT REACTION  One element replaces another in the equation.  A + BC → AC + B

DOUBLE DISPLACEMENT  + ion of one compound replaces the + ion of another compound  Sometimes get a precipitate—a solid that separates from the liquid.  AB + CD → AD + CB

CONSERVATION OF MASS  Mass of the products = mass of the reactants  Matter is not created or destroyed, just changes form. …Have to have an equal number of atoms of each element on both sides.

EXOTHERMIC VS. ENDOTHERMIC Exothermic: Heat released due to exergonic reaction. Example: burning wood; iron rusting Energy comes from chemical bonds. Endothermic Heat is absorbed due to endergonic reaction. Example: cold packs

CATALYST VS. INHIBITORS  Catalyst: Speeds up a reaction  Inhibitor: Slows or prevents a reaction