Chapter 15 and Ionic Compounds Section 15.1 Electron Configuration in OBJECTIVES: Use the periodic table to infer the number of valence electrons in an atom, and draw it s s electron dot structure. (Already Done) Describe the formation of cations from metals, and of anions from nonmetals. Lets Summarize Who are the Valence Electrons? These are the electrons that are responsible for the chemical properties of atoms These are the electrons in the outer energy level. Valence electrons are the s and p electrons in the outermost, or highest energy level The Number of Valence Electrons is given by the A Group name. How Do we Determine the Valence Electrons?? So, the Valence Electrons are as follows: Group 1A 1 valence electron, s 1 Group 2A 2 valence electrons, s 2 Group 3A 3 valence electrons, s 2 p 1 Group 6A 6 valence electrons, s 2 p 4 Group 7A 7 valence electrons, s 2 p 5 Group 8A 8 valence electrons, s 2 p 6 Recall How to Predict the Ion Charge? Atoms will lose or gain electrons to complete the OCTET of electrons In other words, atoms will lose or gain electrons to have filled valence s & p orbitals, or the electron configuration of the nearest noble gas. Metals will lose electrons to become CATIONS. Nonmetals will gain electrons to become ANIONS,, not edible anions In case you ve been away or asleep? The Group 1A elements lose the s 1 electron to become +1 ion Similarly, Group 2A: loses s 2 to become a +2 ion Group 3A: loses s 2 p 1 to become a +3 ion Group 5A: gains 3 electrons to become s 2 p 6, and a -33 ion Group 6A: gains 2 electrons to become s 2 p 6, and a -22 ion Group 7A: gains 1 electron to become s 2 p 6, and a -11 ion 1
How do we represent the loss/gain of electrons with the Lewis Dot Structures? Remember, a Lewis Dot structure is written as follows. Write the symbol. Put one dot for each valence electron Don t t pair up until they have to (Hund( Hund s rule) X The Lewis Dot Structure for Nitrogen Nitrogen has 5 valence electrons. N First we write the symbol. Then add 1 electron at a time to each side. Until they are forced to pair up. Write electron dot diagrams: Na Mg C O F Ne V Let s s Look at Atoms to tions Metals lose electrons to attain noble gas configuration. If we look at the electron configuration, it makes sense to lose electrons: 1s 2 2s 2 2p 6 3s 2 3p 6 4s 2 (2 valence e - s) 2+ 1s 2 2s 2 2p 6 3s 2 3p 6, a noble gas configuration Electron Dots For tions Metals will have few valence electrons (usually 3 or less) These valence electrons are lost, creating a positive-charged ion. +2 +2 Making a Pseudo-noble gas configuration What about Anions? Nonmetals gain electrons to attain noble gas configuration, making negative ions (anions) S 1s 2 2s 2 2p 6 3s 2 3p 4 has 6 valence e s S 2-1s 2 2s 2 2p 6 3s 2 3p 6 a noble gas configuration. 2
Electron Dots For Anions Nonmetals will have many valence electrons (usually 5 or more) They will gain electrons to fill outer shell. P 3- Those Elements with Stable Electron Configurations Noble gases have an s 2 p 6 valence electron configuration. They have 8 valence electrons, to fulfill the octet rule. Ar Now I Know You re Wondering Do Metals and Nonmetals mate to form a compound? Metals want to lose e, while nonmetals want to gain e a perfect match. So how does this happen? Let s s Look at Ionic Bonds Section 15.2 Ionic Bonds OBJECTIVES: List the characteristics of an ionic bond. Use the characteristics of ionic compounds to explain the electrical conductivity of ionic compounds when melted and when in aqueous solution. The bond is formed through the transfer of electrons. Electrons are transferred to achieve noble gas configuration. The resulting anions and cations are held together by opposite charges. Ionic compounds are called salts. Simplest ratio is called the formula unit. 3
The Metal: 1s 2 2s 2 2p 6 3s 2 3p 6 4s 2 2+ 1s 2 2s 2 2p 6 3s 2 3p 6 And the Non-metal: S 1s 2 2s 2 2p 6 3s 2 3p 4 S 2-1s 2 2s 2 2p 6 3s 2 3p 6 2+ S S 2- This is the Lewis Dot Structure of the Ionic Compound + - Na Cl The Lewis Dot Structure for Sodium Chloride NaCl Let s s Check out this Web resource: All the electrons must be accounted for! P P 2+ P 4
2+ P 2+ P 3-2+ P 3- P 2+ P 3-2+ P 2+ P 3-2+ P 2+ P 3-2+ P 5
2+ 2+ P 3-2+ P 3- The Lewis Dot Structure for 3 +2 & 2 P 3-3 2+ 2 P 3-3 P 2 The Formula Unit What Have we Learned? Write the Ionic Combinations & Lewis Dot Structures of the Ionic Molecules for the following combinations: Ba + Cl Al + O K + Br Li + N Is There an Easier Way to Combine Ions? The CRISS CROSS Method: Al 3 + + Al O What would the Lewis Dot Structure look like? O 2 - Properties of Ionic Compounds Crystalline structure, usually solids A regular repeating arrangement of ions in the solid Properties of Ionic Compounds Ions are strongly bonded together. The Crystalline Structure is rigid, with long- range order. Hardness & Density are related to crystal composition and structure of the solid; i.e. how much space is between the ions in the crystal lattice These structures tend to melt at Specific & High melting points Amorphous Solids are those molecules that do not have an orderly arrangement. 6
Properties of Ionic Compounds What do Crystalline Solids look Like? Do Ionic Solids Conduct? Conducting electricity is allowing charges to move. In a solid, the ions are locked in place. Ionic solids are insulators. When melted,, the ions can move around. Melted ionic compounds conduct. NaCl: : must get to about 800 ºC. Dissolved in water they conduct (aqueous) Properties of Ionic Compounds The Coordination number- number of ions of opposite charge surrounding it Na + has a coordination number of six Cl - also has a coordination number of six. Properties of Ionic Compounds What about Cesium Chloride, CsCl? CsCl has the same atom ratio as NaCl,, but both Cs and Cl have coordination numbers of 8 Properties of Ionic Compounds Both NaCl & CsCl are arranged in the Cubic Crystal System A Simple or Primitive Cubic Atoms are at the corners of the cube # of atoms per cell: 8 corners/cell x (1/8) atom/corner = 1 atom/cell A Body Centered Cubic (BCC) Atoms at the CORNERS of a cube plus one in the CENTER of the body of the cube The corner atoms are shared by 8 unit cells, and the center atom is completely enclosed in one unit cell # of atoms per cell: 8 x (1/8) atom/corner + Center Atom = 2 atoms/cell Cubic Crystal System 7
Cubic Crystal System A Face Centered Cubic (FCC) Atoms at the CORNERS of a cube plus one in each face of the cube The corner atoms are shared by 8 unit cells, and the face atoms are shared by two unit cells # of atoms per cell: 8 x 1/8 + 6 x ½ = 4 atoms/cell The Face Centered Cube Revisiting NaCl & CsCl NaCl is a face-centered centered cube Revisiting NaCl & CsCl CsCl is a body-centered cube Na: (8 x 1/8)+(6 x ½) ) = 4 atoms Cl: : 1 (middle) + (12 x ¼) ) = 4 atoms Cs: (8 x 1/8) = 1 atom Cl: : 1 (middle) = 1 atom The Hexagonal Packed System Compounds with this Structure: AlSn, BIr, MoC, MoP, NbS,, WN, TaS, TiS, TeZr Section 15.3 Bonding in Metals OBJECTIVES: Use the theory of metallic bonds to explain the physical properties of metals. Describe the arrangements of atoms in some common metallic crystal structures. 8
Metallic Bonding How are atoms held together in a metallic solid? Metals have low ionization energies, and have a weak hold on their valence electrons. These outer electrons easily move around, as they do not "belong" to any one atom, but are part of the whole metal crystal. Sea of Electrons These outer electrons easily move around, as they do not "belong" to any one atom, but are part of the whole metal crystal. The negatively charged electrons act as a "cement" that hold the positively charged metal ions in their relatively fixed positions. Metals are Malleable Hammered into shape (bend). Also ductile - drawn into wires. Both malleability and ductility explained in terms of the mobility of the valence electrons Malleable Malleable Ionic solids are brittle Electrons allow atoms to slide by. + - + - - + - + + - + - - + - + 9
Ionic solids are brittle Strong Repulsion breaks crystal apart. + - + - - + - + + - + - - + - + Crystalline structure of metal If made of one kind of atom, metals are among the simplest crystals 1. Body-centered cubic: every atom has 8 neighbors Na, K, Fe, Cr, W 2. Face-centered centered cubic: every atom has 12 neighbors Cu, Ag, Au, Al, Pb 3. Hexagonal close-packed every atom also has 12 neighbors different pattern due to hexagonal Mg, Zn, Cd The Hexagonal Packed System A Simple Hexagonal Lattice: Packing the Hexagonal Pack A Hexagonal Close Packed Lattice: Elements with this Structure: Mg, Be, Sc, Te, Co, Zn, Y, Zr, Tc, Ru, Gd,, Tb, Py,, Ho, Er, Tm, Lu, Hf,, Re, Os, Tl Alloys We use lots of metals every day, but few are pure metals Alloys - mixtures of 2 or more elements, at least 1 is a metal made by melting a mixture of the ingredients, then cooling Brass: an alloy of Cu and Zn Bronze: Cu and Sn Why use alloys? Properties often superior to element Sterling silver (92.5% Ag, 7.5% Cu) is harder and more durable than pure Ag, but still soft enough to make jewelry and tableware Steels are very important alloys corrosion resistant, ductility, hardness, toughness, cost 10
Some Common Alloys Steel: Contains 99.5% Fe and 0.5% rbon. Steel is much harder than Iron. Stainless Steel: Ni & Cr is added to steel to make it shiny, strong and corrosion resistant. Brass: An alloy that contains ~80% Cu and 20% Zn. Brass is stronger and more malleable than Cu. Bronze: Contains 90% Cu and 10% Sn (tin) Why use alloys? Table 15.3, p.429 - common alloys What are the types? a) substitutional alloy- the atoms in the components are about the same size b) interstitial alloy- the atomic sizes quite different; smaller atoms fit into the spaces between larger 11