Chemistry Chapter 11 Bonding World of Chemistry Zumdahl Last revision Fall 2008

Bonding and Molecules Reaction – When an atom interacts with another atom forming a chemical bond. Chemical Bond – An attraction between two or more atoms that binds them together.

Types of Chemical Bonds Compounds are made from two or more atoms that share a chemical bond. 1)Found only in definite ratios. Examples: H 2 O, H 2 O 2 2) They can be broken down in chemical reactions. There are two groups of compounds and types of chemical bonds: Ionic and Covalent

Why form chemical bonds? 1)Atoms tend to be stable when they have 8 electrons in their outer energy level. Stable Unlikely to react. Unstable Likely to react.

Rule of Octet or The Octet Rule Stable energy levels are full energy levels and allow atoms to not react. The first energy level is stable with 2 electrons and other levels with 8. This is known as the rule of Octet or Octet rule. All elements want to have the same number of electrons as the nearest noble gas!

Na Cl 11 p + 12 n o 18 n o 17 p + 1e - 8e - 2e - 7e - 8e - 2e - 8e - + _ Bohr model and the octet rule

How do atoms become stable? a) Those with 1 to 3 outer electrons tend to lose those electrons because the next energy level in has 8 e -. b) Those with 5 to 7 outer electrons tend to want to gain extra electrons until they have 8 e -. c) Those with 4 outer electrons tend to share those electrons. Valence number = family number Can’t tell share Stable

p + e - n 0 Valence # Gain/lose Charge Na Ca H C O Cl Let’s find some parts of the atom: Lose 1e Lose 2e +1Lose 1e Share Gain 1e Gain 2e 0

Remember: Column number tells us the Valence electrons. We diagram the valence electrons with Lewis Dot diagrams Lewis Dot diagrams – dots around the symbol represent valence electrons around the nucleus. 1. Place two electrons together if there are more than four, otherwise spread them out around the symbol. 2. Spread the rest out before pairing.

Practice drawing Lewis Dots HMgCO FAr N B

Ionic Bonds This is the force of attraction between oppositely charged ions. a)Ion – an atom that gains or loses electrons.  Cation – positively charged ion = lost electrons  Anion – negatively charged ion = gained electrons

Covalent Bond In these bonds electrons are shared rather than lost or gained. Molecules – a group of atoms held together by covalent bonds. Electronegativity - This is the force of attraction that an atom has for a shared pair of electrons. This is what is responsible for the rules we looked at above. Note: Ionic compounds usually occur between a metal and a nonmetal while covalent bonds occur between two nonmetals.

Chemical Formulas Chemical Formula – This is the “recipe” of a compound. It contains the ratios of the atoms combined in the compound. Sometimes information is given above the normal line of information. In the example below, 12 and +4 are written as what we call a superscript C 6 1 Sometimes the information given is below the normal line of information. In the example below, 6 and 1 are written in subscripts.

1) Write the symbol for the positive ion first (determine the charge or oxidation number using your periodic table) 2) Write the symbol for the negative ion second (determine the charge or oxidation number using your periodic table). An oxidation number (or charge) indicates how many electrons are lost, gained or shared when bonding occurs. Writing formulas for ionic compounds:

3) Add subscripts so that the sum of the positive and negative oxidation numbers is zero. The Crisscross Method or Swap N’ Drop Method can also work. 4) All compounds are neutral so the oxidation numbers should combine in ratios that will add up to zero. The number of ions combining in the compound will be written as subscripts in the final formula. Example: Na +1 and Cl -1 (+1) + (-1) = 0 NaCl

Try this example on your own: Ca and Br Ca +2 Br -1 (+2) + (-1) =+1 Add more negative Ca +2 Br -1 (+2) + (-2) =0 CaBr 2 1 2

Al +3 O -2 Swap and drop short cut Al and O Charges can be crisscrossed to become subscripts (remember to remove the sign) 32 Al 2 O 3 Mg +2 Cl -1 2 Mg and Cl MgCl 2 A subscript of 1 is never written. The symbol stands for 1.

Polyatomic Ions – Some covalently bonded atoms function as a ion or charged particle. Two important ones are hydroxide (OH - ) and ammonium (NH 4 + ) Use parentheses to treat the group as a single unit when writing the chemical formula. Example: Al +3 OH -1 3 Al OH 3 () Whenever a subscript is used to indicate more than one polyatomic ion you must use parenthesis

Naming Ionic Compounds: 1) The first ion (cation or positive ion) keeps its name. Example: NaCl Sodium 2) The second ion (anion or negative ion) keeps the first syllable or the root of the name and then ends in the suffix –ide. 3) If any of the ions are polyatomic, use the their name from the list. Example: chlor NaCl Sodiumide

Fe +3 = Iron (III) Ion Fe +2 = Iron (II) Ion Transition metals may occur in a variety of oxidation states and a roman numeral must be included in the name to tell the reader which type of ion it is. You should include a roman numeral for all transition metals (except Silver [Ag + ] and Zinc [Zn +2 ]). Transition Metals

There are two steps for determining the charge on the transition metal. 1. Un-crisscross the subscripts (remember: no subscript indicates 1 atom) 2. Check the anion (-) to see if you must adjust the numbers

Example: FeO +1 1.Un-crisscross the charges. FeO +1 2.Check the anion. Adjust as needed. x 2=-2=2 x+2 The name for the compound is Iron (II) Oxide

Naming Covalent Compounds: 1) List the least electronegative first. First atom only keeps it’s name and only uses a prefix if there is more than one 2) List the most electronegative second. Second atom ends with “ide” and always uses a prefix to tell how many atoms of each there are: mono = 1 atomhexa = 6 atoms di = 2atomshepta = 7 atoms tri = 3atomsocto = 8 atoms tetra = 4 atomsnono = 9 atoms penta= 5 atomsdeca = 10 atoms 4) There are no charges in covalent compounds so there is no crisscrossing or uncrisscrossing or roman numerals.

Properties of Ionic compounds – Ionic compounds are usually solids at room temperature and we call them salts Melting and Boiling Points – Ionic compounds have much higher melting points and boiling points than covalent compounds –– Ionic compounds typically melt at several hundred degrees Celsius They exist in a crystal state so we refer to the smallest ratio of ions in the crystal as their formula unit When dissolved in water, ionic compounds break into ions (charged particles) and can conduct electricity through the solution.

Conductivity test for ionic solutions Ionic solutionCovalent solution Ionic substances dissolve into ions which will carry a charge and allow the flow of electrons through the solution and complete the circuit so the light comes on. Covalent substances dissolve into molecules which are not charged and will not allow the flow of electrons through the solution to complete the circuit so the light will not come on. NaCl Na +1 + Cl -1 C 6 H 12 O 6

Properties of Covalent compounds Covalent compounds are typically formed from nonmetals and we call them molecules Molecules is the single unit of compounds characterized by covalent bonding They tend to have low melting and boiling points. Exist as discrete molecules in the solid, liquid, and gas states. Their bonds can be characterized as polar or nonpolar When dissolved in water they stay together as a molecule and carry no charge