Unit 3 How do we predict properties? The central goal of this unit is to help you develop ways of thinking that can be used to predict the physical properties of chemical compounds based on their submicroscopic structure. M1. Analyzing Molecular Structure Predicting properties based on molecular structure . M2. Considering Conformations Predicting properties based on spatial conformations. Every Course Unit includes a brief description of the central learning objectives of each Module in the Unit. M3. Characterizing Ionic Networks Predicting properties based on ion charge and size. M4. Exploring Electronic Structure Predicting properties based on electron-configurations.

How do we predict properties? Module 4: Exploring Electronic Structure Unit 3 How do we predict properties? Module 4: Exploring Electronic Structure Central goal: To explain and predict the physical properties of metallic systems based on the crystalline arrangement and electron-configurations of their atoms. Each Module begins with a description of its central learning objective.

The Challenge Modeling How do I predict it? Not all of the substances in our surrounding world are molecular or ionic. In fact, many of the materials that we use on a regular basis are composed by metals. How can we make predictions about the properties of metals? How do we take advantage of the properties of metals to design novel materials? “The Challenge” describes the types of problems or questions that we would like to be able to answer by the end of the Module. Questions in green are rhetorical questions. They are included to invite students to think and brainstorm initial ideas.

The Power of Classification Most of the elements in our world are metallic. However, metals do not tend to combine in definite proportions with other metals to form compounds. They mostly form mixtures (alloys). Bronze Cu-Sn Brass Cu-Zn Steel Fe-C AmalgamHg-X

Metals WHY? In general, metals: are solid, hard, and shiny; have high melting and much higher boiling points; conduct electricity and heat; can be pounded into shapes (Malleable); can be pulled into wires (Ductile); can have magnetic properties. WHY?

Crystal Structure Atoms in metals are arranged in regular patterns. More ductile/ softer Less ductile/ stronger More brittle Packing Density

The high T form is “remembered” Phase Transitions Pure metals and alloys can undergo solid-to-solid phase changes that modify their properties. A metal with “memory”? NiTi alloy Click on the “HOT” word to open the movie that illustrates the behavior of a memory metal. Room T Less Rigid High T More Rigid The high T form is “remembered”

Electrons in Metals In general, metal atoms are relatively large, and their valence electrons are well shielded from the nucleus by core electrons. Thus, they lose outer electrons relatively easily but do not gain them very readily. Metal atoms tend to share their valence electrons with all of the other atoms in the structure (metallic bonding). Valence electrons are delocalized moving freely throughout the system (electron sea model). Na+ [Ne] Na [Ne]3s1

Let’s Think How would you use the “electron sea” model to explain metals’ malleability and ductility? “Let’s think” designed to have students brainstorm ideas based on their knowledge and understanding.

Let’s Think How would you use the “electron sea” model to explain metals’ high thermal and electrical conductivity? “Let’s think” designed to have students brainstorm ideas based on their knowledge and understanding.

Let’s Think Li Na K Rb Cs Be Mg Ca Sr Ba How would you use the “electron sea” model to explain these trends in melting and boiling points for Alkali and Alkaline Earth metals? “Let’s think” to have students use a model to explain observed properties and behaviors.

These two factors increase the strength of the metallic bonding. Periodic Trends For Alkali and Alkaline Earth Metals, the boiling and melting points tend to increase the larger the # of valence electrons and the smaller the atomic radius. These two factors increase the strength of the metallic bonding. However, these trends change with the transition metals: Periods

Not attracted or slightly repelled Magnetic Properties Metals exhibit distinctive magnetic properties. Some of them are attracted to magnets, some of them are slightly repelled, and others are magnets themselves. Not attracted or slightly repelled Strongly Attracted Permanent Magnet Diamagnetic Paramagnetic Ferromagnetic

In atoms and ions, electrons (-) are constantly moving and spinning. Magnetism In general, magnetism is a phenomenon associated with the presence of moving + or – charges. In atoms and ions, electrons (-) are constantly moving and spinning. A spinning electron behaves like a tiny magnet. When electrons with different spins are paired, their magnetic effects cancel out. It is the presence of unpaired electrons that leads to paramagnetic or ferromagnetic behaviors.

How do we know if the electrons at each level are paired or unpaired? Unpaired Electrons To explain and make predictions about magnetic properties, we need to find a way to determine the number of unpaired electrons in atoms or ions. Shell Subshell # of e- n = 1 1 s 2 e- n = 2 2 s 2 p 6 e- n = 3 3 s 3 p 3 d 10 e- According to our shell model, electrons in an atom occupy different energy levels. How do we know if the electrons at each level are paired or unpaired?

Quantum Atomic Theory Basic elements: The particular distribution of electrons in shells, as well as the magnetic properties of atoms and ions, were first explained by what is known as the Quantum Theory of the atom. Basic elements: Electrons have a dual nature: particle-wave. l ~ 1/mv We cannot know the exact position and velocity of the electron at every instant (Uncertainty Principle). We can only predict probability densities.

Orbitals According to quantum theory, the state of every electron in an atom can be characterized by a mathematical function (atomic orbital). This function can be used to calculate both the energy state and the probability density for the electron. E To explain the magnetic properties of atoms, it was proposed that the state of only two electrons could be described by the same orbital, and that these electrons must have opposite spins . (Pauli Exclusion Principle)

Electron Configurations Based on these ideas, we can take a new look to atomic electron configurations: E 1s E 2He 1s2 1s 1H 1s1 E 1s 2s E 4Be 1s 1s22s2 2s 3Li 1s22s1

Electron Configurations 2px 2pz 2py x y z 5B 1s22s22p1 6C 1s22s22p2 Let’s Think Experimentally, it is found that the C-atom is paramagnetic. How are its electrons distributed in energy levels? “Let’s think” designed for students to apply what they have learned.

Hund’s Rule E 1s 2s 2px 2pz 2py The electron distribution that minimizes the energy of an atom is that in which electrons occupy different same-energy orbitals until forced to paired up. While unpaired, these electrons have the same spin. Let’s Think How would you apply Hund’s rule to build the electrons configurations of N, O, F and Ne? How would you extend all these ideas to build the electron configuration of Mn, a transition metal? Would you expect this metal to be diamagnetic? “Let’s think” designed for students to apply what they have learned.

There is a flash movie (ec. swf) embedded in this page There is a flash movie (ec.swf) embedded in this page. You can add electron (up and down arrows) by clicking on the orbital boxes. The arrows disappear when you click on a full box (2 electron in it). You can show the atomic number of each atom by clicking on its symbol. We recommend to use this tool to build the electron configurations of some transition metals (in preparation for next slide).

Which of these metal atoms would you expect to be diamagnetic? Let’s Think The magnetic properties of transition metals can be predicted by determining the number of unpaired electrons in the d-subshell. Which of these metal atoms would you expect to be diamagnetic? “Let’s think” designed for students to make a prediction based on what they have learned. Cu, Ag, and Au atoms have unexpected electron configurations: [ ]ns2(n-1)d9  [ ]ns1(n-1)d10 Does this change affect your predictions about the magnetic properties of these atoms?

Looking at Ions The ions of many metallic elements are paramagnetic, particularly those of transition metals. Thus, many ionic compounds (salts) of involving these elements have magnetic properties. Cu: [Ar]4s13d10 Cu+: [Ar]3d10 Diamagnetic Cu2+: [Ar]3d9 Paramagnetic Note: Electrons in the most external shells go first. Let’s Think Which of the following ionic compounds have paramagnetic cations? KCl FeCl3 ZnS TiCl4

Ferromagnetism When the interactions among unpaired spins are strong, they can align spontaneously in a given direction. The cooperative effect of all the spins creates a strong magnetic field (permanent magnet) There is a flash movie embedded in this slide. Use the sliders to control the different parameters. Set the exchange interaction to -1 if you want to observe ferromagnetism. Set it to +1 if you want to observe antiferromagnetism. In case you need it, click on the square in the upper right corner to open the simulation in a larger screen (dipolem.swf). Fe, Co, Ni.

Bonding Effects The electron configuration in metals can also be used to explain why the conduct electricity. When a bond between two atoms is formed, there is a dramatic change in the distribution of valence electrons. There is a flash movie embedded in this slide. You can click on and drag the atom on the right to illustrate the change in charge distribution when a bond is formed. The energy state and the probability density of the electrons changes as the bond is formed. Their behavior is better described by “molecular orbitals,” instead of atomic orbitals.

Conduction band (Uppermost empty) Valence band (Lowermost filled) Energy Bands E As atoms combine, the energy difference between the available electron energy levels decreases. 20 4 3 2 1 # of interacting atoms In solid metals, with ~1023 atoms, the energy difference becomes negligible, and continuous “energy bands” are formed. E Conduction band (Uppermost empty) Energy Gap (Eg) Valence band (Lowermost filled)

Conductivity Based on their band structure, solid materials can be conductors, semiconductors, or insulators. E Metal The energy cost for e- to jump from the VB to the CB is negligible. VB CB Semiconductor The Eg can be overcome by thermal vibrations or UV-vis-IR light. Eg ~ 60-300 kJ/mol Insulator Eg > 300 kJ/mol Very large Eg.

Let′s apply! Assess what you know

Let′s apply!

Explain something that you learned in this module to other person in the class.

Exploring Electronic Structure Summary Metals do not combine in definite proportions with other metals to form compounds. They mostly form mixtures (alloys). Atoms in metals are arranged in regular patterns. The crystalline structure of metals has an important effect on physical properties such as ductility, brittleness, and density. Less ductile/ stronger More ductile/ softer

Exploring Electronic Structure Summary Valence electrons in metals are delocalized, moving freely throughout the system (electron sea model). The existence of electrons that can freely move throughout a metallic system is responsible for their high electrical and thermal conductivities.

Exploring Electronic Structure Summary Metals exhibit distinctive magnetic properties. Magnetism is a phenomenon associated with the presence of unpaired electrons in an atom. In solid metals, electrons occupy almost continuous energy levels or “energy bands.” The relative energy of the valence band and the conduction band, determines whether the metal’s conductivity. E Valence band (Lowermost filled) Conduction band (Uppermost empty) Energy Gap (Eg)

Are You Ready?

Materials Advisor Imagine that you work in a consulting agency specialized in providing advice to chemical industries and companies involved in materials design and production. Your task is thus to provide the best possible counsel to different companies and justify your suggestions based on your chemical knowledge.

Solvents An industry specialized in the production of organic solvents for chemical synthesis is interested in designing three light weight hydrocarbons with the same number of carbons but different boiling points. Which molecular structures would you propose in order of increasing boiling point?

Green Solvents The same company is interested in producing derivatives of the ionic liquid shown below by changing the structure of the side chain. They want you to make predictions about changes in the melting point of the substance for three different side chains (higher or lower?). Side Chain N N N Mp = 6.4 oC

Green Solvents Following your suggestions, the company synthesized ionic liquid with lower melting points by increasing the length of the side chain. However, at some point the melting points starts to increase rather than decreasing with side chain length. How do you explain it?

Which of them would you recommend them to eliminate from their list? Magnetic Cooling Paramagnetic salts can be used for generating the very low temperatures needed to produce liquid helium. A company that produces liquid helium for chemical equipment is interested in testing these new salts: CrCl3 CdSO4 MnO2 Which of them would you recommend them to eliminate from their list?

These are the cheapest options: Lubricants A company that produces lubricants is interested in selecting an inexpensive material with high viscosity. These are the cheapest options: Which is best?

How would you justify your selection? Alloys A company is interested in producing alloys for spacecraft applications using metals with high melting points. Which two of the following set of available metals would you recommend they use? K, Ca, Sc, Ti, V, Rb, Sr How would you justify your selection?

What type of protein structure would you propose? Protein Design A pharmaceutical company is experimenting with a new set of drugs based on protein chains. They are interested in designing a protein that will have a coiled section and an un-coiled section when dissolved in water. What type of protein structure would you propose? Show the flash simulation, and ask students to choose a sequence of aminoacids using the six aminoacids in the movie.