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IUnit 3

How do we predict properties?

M1. Analyzing Molecular Structure Predicting properties based on molecular structure .

M4. Exploring Electronic Structure Predicting properties based on electron-configurations.

M3. Characterizing Ionic Networks Predicting properties based on ion charge and size.

M2. Considering Conformations Predicting properties based on spatial conformations.

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.

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IUnit 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.

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The Challenge

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.

ModelingHow do I predict it?

How can we make predictions about the properties of metals?

How do we take advantage of the properties of metals to

design novel materials?

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IThe Power of ClassificationMost 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).

BronzeCu-Sn

BrassCu-Zn

SteelFe-C

AmalgamHg-X

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Metals

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?

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ICrystal Structure

Atoms in metals are arranged in regular patterns.

Packing Density

Less ductile/ stronger More brittle

More ductile/ softer

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Room T

Less Rigid

High T

More Rigid

Phase Transitions

Pure metals and alloys can undergo solid-to-solid phase changes

that modify their properties.

A metal with “memory”? NiTi alloy

The high T form is “remembered”

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IElectrons 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

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How would you use the “electron sea” model to explain metals’ malleability

and ductility?

Let’s Think

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ILet’s Think

How would you use the “electron sea” model to explain metals’ high thermal

and electrical conductivity?

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LiNa K Rb Cs

Be

Mg

CaSr Ba

Let’s Think

How would you use the

“electron sea” model to explain these trends in

melting and boiling points for Alkali and Alkaline Earth

metals?

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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

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Magnetic PropertiesMetals 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

Diamagnetic Paramagnetic

Permanent Magnet

Ferromagnetic

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IMagnetism

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.

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Unpaired ElectronsTo 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 e-

2 p 6 e-

n = 3 3 s 2 e-

3 p 6 e-

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?

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Quantum Atomic Theory

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.

We cannot know the exact position and velocity of the electron at every instant (Uncertainty Principle). We can only predict probability densities.

Basic elements:

Electrons have a dual nature: particle-wave.

~ 1/mv

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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)

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

Orbitals

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Electron ConfigurationsBased on these ideas, we can take a new look to

atomic electron configurations:

1H

1s1

E2He

1s2

1s

E

1s1s

E4Be

1s

1s22s22s

3Li

1s22s1

E

1s

2s

2s

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IElectron Configurations

5B

1s22s22p1

1s

E

2s2px

2px 2pz 2py

x

2py

y

z2pz

6C

1s22s22p2

Let’s Think

Experimentally, it is found that the C-atom is paramagnetic.

How are its electrons distributed in energy levels?

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IHund’s Rule

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.

E

1s

2s

2px 2pz 2py

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?

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ILet’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?

Does this change affect your predictions about the magnetic properties of these atoms?

Cu, Ag, and Au atoms have unexpected electron configurations: [ ]ns2(n-1)d9 [ ]ns1(n-1)d10

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ILooking 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

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IFerromagnetism

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)

Fe, Co, Ni.

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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.

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.

Bonding Effects

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As atoms combine, the energy difference between the available electron energy levels

decreases.

Energy Bands

E

# of interacting atoms

1 2 3 4 20

In solid metals, with ~1023 atoms, the

energy difference becomes negligible,

and continuous “energy bands”

are formed.

E

Valence band(Lowermost filled)

Conduction band(Uppermost empty)

Energy Gap (Eg)

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IConductivity

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.

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I Assess what you know

Let′s apply!

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ILet′s apply!

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Explain something that you learned in this module to other

person in the class.

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Summary

Metals do not combine in definite proportions with other metals to form compounds.

They mostly form mixtures (alloys).

Exploring Electronic Structure

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

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Summary

Valence electrons in metals are delocalized, moving freely throughout the system

(electron sea model).

Exploring Electronic Structure

The existence of electrons that can freely move

throughout a metallic system is responsible for their high

electrical and thermal conductivities.

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Metals exhibit distinctive magnetic properties. Magnetism is a phenomenon associated with the

presence of unpaired electrons in an atom.

Exploring Electronic Structure

Summary

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)

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Are You Ready?

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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.

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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?

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Green SolventsThe 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 ChainN

N

N

Mp = 6.4 oC

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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?

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Magnetic CoolingParamagnetic 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?

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Lubricants

A company that produces lubricants is interested in selecting an inexpensive material

with high viscosity.

These are the cheapest options:

Which is best?

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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?

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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?