Presentation on theme: "CHAPTER 19 Chemical Bonds. COMBINING ELEMENTS Combining elements usually changes their properties. Example: Sodium (explosive) mixed with chlorine."— Presentation transcript:
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.
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.
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