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):    More on ionic & covalent compounds Naming and writing ionic & covalent compounds Electronegativity & Bond Polarity (4.6) 1

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 2

Last Time: Chemical Bonds: attractive force holding two or more atoms together •

TWO EXTREME CASES Ionic bonding:

results when “Atom A” transfers electron to “Atom B” (e.g. salts such as NaCl) an •

Covalent bonding:

electrons results when atoms share •

Polar-covalent bonding:

good portion of ‘reality’) everything in between (a 3

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?

4

Last Time: Ions

Atoms are electronically neutral  not have a charge.

they do  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.

5

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 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 7

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 –3 anions (N 3 ) 8

Ionic Compounds

A solid that contains cations and anions in a balanced whole-number ratio is called an ionic compound.

 Electrons lost by the electrons gained cation must equal the by the anion 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

Formation of a Sodium Ion, Na

+ Sodium achieves an octet by losing its 1 valence electron.

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.

2, 8, 7 2, 8, 8

By the way, anions are always bigger than their neutral counterparts!

11

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

symbol of the nonmetal.

followed by the 12

Charge Balance In MgCl

2 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.

13

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 14

The formula of the ionic compound of Na

+

and O

2-

is

0% 0% 0%

1. NaO 2. Na 2 O 3. NaO 2

1 21 41 2 22 42 3 23 43 4 24 44 5 25 45 6 26 46 7 27 47 8 28 48 9 29 49 10 30 50 11 31 12 32 13 33 14 34 15 35 16 36 17 37 18 38 19 39 20 40

15

The formula of a compound of calcium and sulfur is:

0% 0% 0%

1. Ca 2 S 2. CaS 2 3. CaS

1 21 41 2 22 42 3 23 43 4 24 44 5 25 45 6 26 46 7 27 47 8 28 48 9 29 49 10 30 50 11 31 12 32 13 33 14 34 15 35 16 36 17 37 18 38 19 39 20 40

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From Lab: Transition Metals

Most form 2 or more positive ions 1+ 2+ 1+ or 2+ 2+ or 3+ Ag + Cd 2+ Cu + , Cu 2+ Fe 2+ , Fe 3+ silver cadmium copper(I) ion ion ion iron(II) ion copper (II) ion iron(III) ion Zn 2+ zinc ion 17

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+ ) (Cu + ) (Sn 4+ ) (Pb 2+ ) (Fe 3+ ) iron (III) chloride copper (I) chloride tin (IV) fluoride lead (II) chloride iron (III) sulfide 18

Flowchart for Naming Ionic Compounds Copyright © 2005 by Pearson Education, Inc.

Publishing as Benjamin Cummings 19

Covalent Bonds

 Formed between atoms of groups (columns) 4A 7A ( nonmetals )  How do you form octets, if neither atom particularly wants to give up electrons?

 Share   Form single, double, and triple bonds to yield an octet  Applies to bonds between H, C, N, O, S, P, Halogens (F, Cl, etc) – not metals (usually).

20

Covalent Bonding: Considerations

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

Diatomic Molecules

Gases that exist as diatomic molecules are H 2 , F 2 , N 2 , O 2 , Cl 2 , Br 2 , I 2    N   +    N   octets      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 H : H   Bonding pairs N   : H DO LEWIS STRUCTURE ON BOARD Lone pair of electrons 23

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.

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More Examples…

25

Shifting Gears: Polyatomic Ions…

26

Polyatomic Ions: A Group of Covalently bonded Atoms with an Overall Charge.

KNOW THESE & Table 4.7: NH 4 + ammonium OH NO 3 CO 3 2 HCO 3 nitrate carbonate hydroxide NO 2 nitrite hydrogen carbonate (bicarbonate) 27

Some Compounds with Polyatomic Ions Copyright © 2005 by Pearson Education, Inc.

Publishing as Benjamin Cummings 28

Some Compounds with Polyatomic Ions Copyright © 2005 by Pearson Education, Inc.

Publishing as Benjamin Cummings 29

Naming Ternary Compounds

 Contain at least 3 elements  Name the nonmetals as a polyatomic ion  Examples: Na NO 3 Sodium nitrate K 2 SO 4 Potassium sulfate Al (HCO 3 ) 3 Aluminum bicarbonate or aluminum hydrogen carbonate 30

Flowchart for Naming Ionic Compounds Copyright © 2005 by Pearson Education, Inc.

Publishing as Benjamin Cummings 31

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 (

d-

)

.  And consequently the other atom has a

partial positive charge

(d+).

32

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 be represented with numerical values  Larger numbers mean a greater attraction for electrons  Fluorine has the greatest electronegativity.

THE TREND TO KNOW: Elements in the upper-right corner have the greatest EN values.

Elements in the lower-left corner have the smallest EN values.

34

Low values

Some Electronegativity Values for Group A Elements

High values

Copyright © 2005 by Pearson Education, Inc.

Publishing as Benjamin Cummings 35

The Two Extremes Revisited…

The greater the difference of the EN values, the more polar the bond! An EN difference ≥ 1.8 is considered an ionic bond.

36

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 N-N Cl-Br H-Si 3.0 - 3.0 = 3.0 - 2.8 = 2.1 - 1.8 = 0.0

0.2

0.3

Type of Bond Nonpolar covalent Nonpolar covalent Nonpolar covalent 37

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 O-Cl Cl-C O-S Difference 3.5 - 3.0 = 0.5

3.0 - 2.5 = 3.5 - 2.5 = 0.5

1.0

Type of Bond Polar covalent Polar covalent Polar covalent 38

Comparing Nonpolar & Polar Covalent Bonds Copyright © 2005 by Pearson Education, Inc.

Publishing as Benjamin Cummings 39

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 Cl-K N-Na S-Cs Difference 3.0 – 0.8 3.0 – 0.9 2.5 – 0.7

= = = 2.2

2.1

1.8

Type of Bond Ionic Ionic Ionic 40