Announcements & Agenda (01/29/07) Pick up graded quizzes (Average = 8.2/10) Note: Need more detailed/precise explanations This week s quiz moved to Friday Today (4.3-4.6): 4.6): More on ionic & covalent compounds Naming and writing ionic & covalent compounds Electronegativity & Bond Polarity (4.6) Last Time: Bonding Overview An amazing thing about the universe - Sometimes when things come together, they stick Protons and neutrons in a atomic nuclei Atoms in molecules H H H H 1 2 Last Time: Chemical Bonds: attractive force holding two or more atoms together TWO EXTREME CASES Ionic bonding: results when Atom A transfers an electron to Atom B (e.g. salts such as NaCl) Covalent bonding: results when atoms share electrons Polar-covalent bonding: everything in between (a good portion of reality ) Last Time: The Octet Rule An octet in the outer shell makes atoms stable Electrons are lost, gained or shared to form an octet Electronegativity (4.6): How tightly does an atom tend to hold on to electrons? 3 4 1
Last Time: Ions Atoms are electronically neutral they do not have a charge. WHY? Charge from protons and electrons cancel But an atom can lose or gain electrons (it cannot lose or gain protons ) An ion is a particle with an unequal number of protons and electrons. Why Do Ions Form? Ions tend to form octets. Periodic Table 2 Flavors: Cations positive charge, generally metal atoms result from loss of 1 or more electrons. Anions negative charge, generally nonmetals, result from gain of 1 or more electrons. Carbon and boron do not form ions 5 6 Ionic Charges - Cations Group 1A form cations with +1 charge. (Na +, K +, Cs + etc ) Group 2A form cations with +2 charge. (Ba 2+, Ca 2+, Mg 2 + etc ) Metals in the middle (transition metals) can be variable positive charge Ionic Charges - Anions Group 7A elements tend to form anions with 1 charge. (F -, Br -, I - etc..) Group 6A elements tend to form anions with 2 charge. (O 2-, S 2- etc ) Groups 5A elements can (but rare) form 33 anions (N 3- ) 7 8 2
Ionic Compounds A solid that contains cations and anions in a balanced whole-number ratio is called an ionic compound. Electrons lost by the cation must equal the electrons gained by the anion Formation of a Sodium Ion, Na + Sodium achieves an octet by losing its 1 valence electron. Usually it is between a metal & a nonmetal. Example: NaCl Important: When an ionic compound dissolves, it breaks apart into ions. Covalent compounds do not! 9 2, 8, 1 2, 8 By the way, cations are always smaller than their neutral counterparts! 10 Formation of a Chloride, Cl - Chlorine achieves an octet by adding an electron to its valence electrons. Charge Balance for NaCl, Salt In NaCl, a Na atom loses its valence electron. a Cl atom gains an electron. the symbol of the metal is written first followed by the symbol of the nonmetal. 2, 8, 7 2, 8, 8 By the way, anions are always bigger than their neutral counterparts! neutral counterparts! 11 12 3
In MgCl 2, Charge Balance In MgCl 2 a Mg atom loses 2 valence electrons. two Cl atoms each gain 1 electron. subscripts indicate the number of ions needed to give charge balance. From Lab: Naming Ionic Compounds Name the metal first, then the nonmetal as -ide. Use name of a metal if it can only have one fixed charge Groups 1A, 2A, 3A and Ag (+1), Zn (+2), and Cd (+2) Examples: NaCl ZnI 2 Al 2 O 3 sodium chloride zinc iodide aluminum oxide 13 14 The formula of the ionic compound of Na + and O 2- is 0% 1. NaO 0% 2. Na 2 O 0% 3. NaO 2 The formula of a compound of calcium and sulfur is: 0% 1. Ca 2 S 0% 2. CaS 2 0% 3. CaS 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 15 41 42 43 44 45 46 47 48 49 50 16 4
From Lab: Transition Metals Most form 2 or more positive ions 1+ 2+ 1+ or 2+ 2+ or 3+ Ag + Cd Cu +, Cu Fe, Fe silver cadmium copper(i) ion iron(ii) ) ion ion ion copper (II) ion iron(iii) ) ion Zn 2+ zinc ion Names of Variable Ions Use a roman number after the name of a metal that forms two or more ions Transition metals and the metals in groups 4A and 5A FeCl 3 CuCl SnF 4 PbCl 2 Fe 2 S 3 (Fe 3+ ) iron (III) chloride (Cu + ) copper (I) chloride (Sn tin (IV) fluoride (Pb ) lead (II) chloride (Fe ) iron (III) sulfide 17 18 Flowchart for Naming Ionic Compounds Covalent Bonds 19 Formed Formed between atoms of groups (columns) 4A- 7A (nonmetals( nonmetals) How How do you form octets, if neither atom particularly wants to give up electrons? Share Share Form Form single, double, and triple bonds to yield an octet Applies Applies to bonds between H, C, N, O, S, P, Halogens (F, Cl,, etc) not metals (usually). 20 5
Covalent Bonding: Considerations Diatomic Molecules Gases that exist as diatomic molecules are H 2, F 2, N 2, O 2, Cl 2, Br 2, I 2 octets Each electron in bonding pair has greater space available than in the unbonded individual atoms, and each gets to feel the positive charge of both nuclei. 21 N + N N::: :::N triple bond NOTE: For nitrogen to achieve its octet, it needs to form 3 bonds!!! 22 Lewis Structures: Covalent Bonds in NH 3 Bonding pairs H H : N : H Lone pair of electrons DO LEWIS STRUCTURE ON BOARD Carbon forms 4 covalent bonds In a CH 4, methane, molecule a C atom shares 4 electrons to attain an octet. each H shares 1 electron to become stable like helium. 23 24 6
More Examples Shifting Gears: Polyatomic Ions 25 26 Polyatomic Ions: A Group of Covalently- bonded Atoms with an Overall Charge. Some Compounds with Polyatomic Ions KNOW THESE & Table 4.7: NH + 4 ammonium OH - hydroxide NO - 3 nitrate NO - 2 nitrite CO 2-3 carbonate HCO - 3 hydrogen carbonate (bicarbonate) 27 28 7
Some Compounds with Polyatomic Ions Naming Ternary Compounds Contain at least 3 elements Name the nonmetals as a polyatomic ion Examples: NaNO NO 3 Sodium nitrate K 2 SO 4 Al(HCO 3 ) 3 Potassium sulfate Aluminum bicarbonate or aluminum hydrogen carbonate 29 30 Flowchart for Naming Ionic Compounds Connecting the Extremes: In Between Ionic and Covalent Bonds (4.6) A pure covalent bond occurs only when two identical atoms are bonded: N 2 Polar Covalent Bond: : Unequal sharing between two dissimilar atoms Therefore, the electrons are nearer to one of the atoms, and that atom acquires a partial negative charge (δ ).( And consequently the other atom has a partial positive charge (δ+). 31 32 8
KEY QUESTION: How do you determine which atom has the partial negative charge and which atom has the partial positive charge? 33 Electronegativity: : Ability of an Element to Draw Electrons to Itself The attraction of an atom for electrons is called its electronegativity. Can Can be represented with numerical values Larger Larger numbers mean a greater attraction for electrons Fluorine has the greatest electronegativity. THE TREND TO KNOW: Elements in the upper-right right corner have the greatest EN values. Elements in the lower-left left corner have the smallest EN values. 34 Some Electronegativity Values for Group A Elements Electronegativity `increases The Two Extremes Revisited Low values Electronegativity decreases ` High values 35 The greater the difference of the EN values, the more polar the bond! An EN difference 1.8 is considered an ionic bond. 36 9
Nonpolar Covalent Bonds occur between nonmetals. have equal or almost equal sharing of electrons. have almost no electronegativity difference (0.0 to 0.4). Examples: Electronegativity Atoms Difference Type of Bond N-N 3.0-3.0 = 0.0 Nonpolar covalent Cl-Br 3.0-2.8 = 0.2 Nonpolar covalent H-Si 2.1-1.8 = 0.3 Nonpolar covalent Polar Covalent Bonds occur between nonmetal atoms. have an unequal sharing of electrons. have a moderate electronegativity difference (0.5 to 1.7). Examples: Electronegativity Atoms Difference Type of Bond O-Cl 3.5-3.0 = 0.5 Polar covalent Cl-C 3.0-2.5 = 0.5 Polar covalent O-S 3.5-2.5 = 1.0 Polar covalent 37 38 Comparing Nonpolar & Polar Covalent Bonds Ionic Bonds occur between metal and nonmetal ions. form as a result of electron transfer. have a large electronegativity difference (1.8 or more). Examples: Electronegativity Atoms Difference Type of Bond Cl-K 3.0 0.8 = 2.2 Ionic N-NaNa 3.0 0.9 = 2.1 Ionic S-Cs 2.5 0.7 = 1.8 Ionic 39 40 10