1MSE 280: Introduction to Engineering Materials

Reading: Chapter 2Atomic structure and Bonding

g p

OverviewElectrons, protons and neutrons in atoms (Bohr and QM models).IP, EA, , and periodic trends.

MSE280 2007, 2008 Moonsub Shim, University of Illinois1

Bonding between atoms.Intermolecular forces.Relation to macroscopic properties.

Electrons in atomsorbital electrons:

n = principal quantum number

1

Nucleus: Z = # protons

Adapted from Fig. 2.1, Callister 6e.

Electrons in discrete orbitals.Bohr atom:

n=3 21

MSE280 2007, 2008 Moonsub Shim, University of Illinois2

N = # neutrons

Atomic mass A Z + N

1) electrons are particles that revolve around the nucleus.

2) quantized angular momentum.Quantum Mechanics:

Wave or matrix mechanics Probability.

2Comparison of Bohr and QM models

MSE280 2007, 2008 Moonsub Shim, University of Illinois3

Figs. 2.2 and 2.3 from Callister 6 ed.

Atomic orbitals

dyzdxy

MSE280 2007, 2008 Moonsub Shim, University of Illinois4

s px py pz dz2dx2-y2dxz

have discrete energy states (Quantized). tend to occupy lowest available energy state.

Electrons...

3Quantum numbers Principal: n = 1, 2, 3, 4 Angular momentum: l = 0, 1, 2, 3, n 1 = s, p, d, f

s = sharp, p = principal, d = diffuse, f = fundamental Magnetic: ml = 0, 1, 2, 3 , l

Determines the number of states in a given l subshell (2l +1 total)

Spin: ms = 1/2e g

MSE280 2007, 2008 Moonsub Shim, University of Illinois5

e.g.

1s n = 1, l = 0, ml = 0, ms = 1/2 n = 1, l = 0, ml = 0, ms = -1/2

2s n = 2, l = 0, ml = 0, ms = 1/2 n = 2, l = 0, ml = 0, ms = -1/2

Which atom is this? Be

Electron Configuration- Shorthand notation to represent which states electrons occupy in an atom (without specifying electron spin).

e.g. Carbon

1s

2s

Electron configuration: 1s22s22p2

2p

MSE280 2007, 2008 Moonsub Shim, University of Illinois6

Note- each energy level can only hold two electrons of opposite spin (Pauli exclusion principle).- for degenerate levels (e.g. 2p-orbitals), each orbital is filled with one electron before electrons are paired up.

4Electron configuration1 electron in the s-orbital: Alkali metals

Li, Na, K, Rb

2 electrons in the s-orbital: Alkaline earthsBe, Mg, Ca

Filled s-orbital and 4 electrons in p-orbital: ChalcogensO, S, Se

Filled s-orbital and 5 electrons in p-orbital: HalogensF Cl Br

MSE280 2007, 2008 Moonsub Shim, University of Illinois7

F, Cl, Br

Partially filled d-orbital: Transition metalse.g. Mn, Fe, Co

Valence electrons determine which group atoms belong to.

Stable configuration

have complete s and p subshellst d t b i t

Stable electron configurations...

tend to be inert.

Z Element Configuration 2 He 1s 2 10 Ne 1s 22s 22p 6 18 Ar 1s 2 2s 22p 63 s 23p 6

Adapted from Table 2.2, Callister 6e.

MSE280 2007, 2008 Moonsub Shim, University of Illinois8

36 Kr 1s 2 2s 22p 63 s 23p 63d 10 4 s 24p 6

Noble gases

5Valence electrons

2p

3s

2p

3s

Filled shell}

Valence electron

1s

2sp

Na

1s

2s

Na+

Filled shell leads to stability.

}Lose an electron

33p

3s3p Filled shell }

Valence electrons

MSE280 2007, 2008 Moonsub Shim, University of Illinois9

Gain an electron

1s

2s2p

3s

Cl1s

2s2p

3s

Cl-

leads to stability.

How much energy does it require to take an electron out of an atom?

Energy of an electron in vacuum

Ene

rgy

Valence electron

IP

MSE280 2007, 2008 Moonsub Shim, University of Illinois10

Ionization potential (IP): Energy required to pull out a valence electron (in vacuum).

By convention, IP is positive (i.e. need to put in energy to pull out the electron).

6How much energy does it require to place an electron in an atom?

Energy of electron in vacuum

Ener

gy

Valence electrons

EA

Lowest available state

MSE280 2007, 2008 Moonsub Shim, University of Illinois11

Electron Affinity (EA): Energy gained by putting an electron in (from vacuum).

By convention, EA is negative (i.e. electron goes from higher energy state in vacuum to lower energy state in atom).

How do we determine when an atom will accept an electron or give one up?

y EA IP

Vacuum level

Electronegativity (): a measure of how likely an atom will take up or give up an electron

EAIP +

Ene

rgy

Valence electrons

EA

Lowest available state

IP

MSE280 2007, 2008 Moonsub Shim, University of Illinois12

A simple (and intuitive) definition:

-When two atoms are brought together, the atom with larger will have higher electron density around its nucleus.

-Larger more ionic bond.

2~ EAIPx +

7MSE280 2007, 2008 Moonsub Shim, University of Illinois13

BondingPrimary

Ionic ECovalentMetallic

SecondaryDipole-dipoleH bonds

E

MSE280 2007, 2008 Moonsub Shim, University of Illinois14

H-bondsDipole-induced-dipoleFluctuating dipoles

Equilibrium bond length

8Ionic Bonding Occurs between + and - ions. Requires electron transfer.

Na (metal) unstable

Cl (nonmetal) unstable

electron

Large difference in electronegativity required. Example: NaCl

3s3p

MSE280 2007, 2008 Moonsub Shim, University of Illinois15

electron

+ - Coulombic Attraction

Na (cation) stable

Cl (anion) stable

Na( = 0.9)

3s3p

Cl( = 3.0)

Ionic BondingNa (metal) unstable

Cl (nonmetal) unstable

electron

+Na (cation) Cl (anion)ezzEA 4

221=

+ - Coulombic Attraction

Na (cation) stable

Cl (anion) stable ro

A 4Since z1 = +1 for Na+ and z2 = -1 for Cl-

rA

reE

oA == 4

2Negative energy means attraction only.Will the atoms collapse on themselves?

MSE280 2007, 2008 Moonsub Shim, University of Illinois16

No, there is also repulsive energy (e.g. e-e repulsion)

nR rBE = B and n depend on atoms involved.In many cases n ~ 8.

9Ionic BondingE

rAE

nR rBE =

Bond energy

MSE280 2007, 2008 Moonsub Shim, University of Illinois17

rEA =

Equilibrium bond length

gy

Note: Other types of bonds can also be described in a similar manner

Ionic Bonding: examples Predominant bonding in Ceramics

M ONaCl

He -

Ne -

Ar -

Kr -

Xe -

F 4.0Cl

3.0Br

2.8I

2.5

Li 1.0Na 0.9K

0.8Rb 0.8

H 2.1

Be 1.5Mg 1.2Ca 1.0Sr 1.0

Ti 1.5

Cr 1.6

Fe 1.8

Ni 1.8

Zn 1.8

As 2.0

Cs Cl

MgOCaF 2

O 3.5

MSE280 2007, 2008 Moonsub Shim, University of Illinois18

Give up electrons Acquire electrons

Rn -

At2.2

Cs 0.7Fr

0.7

Ba 0.9Ra 0.9

Adapted from Fig. 2.7, Callister 6e. (Fig. 2.7 is adapted from Linus Pauling, The Nature of the Chemical Bond, 3rd edition, Copyright 1939 and 1940, 3rd edition. Copyright 1960 by CornellUniversity.

From Callister 6e resource CD.

10

Covalent Bonding

1s 1s

HH

Molecular orbitals

Sharing of electrons Why do some atoms want to share electrons?

Example2: CH4C: has 4 valence e,

needs 4 more

Example1: H2

shared electrons from carbon atomHCH4

H HHH

Atomic orbitals

MSE280 2007, 2008 Moonsub Shim, University of Illinois19

H: has 1 valence e,needs 1 more

Electronegativitiesare same or comparable.

Adapted from Fig. 2.10, Callister 6e.

shared electrons from hydrogen atoms

HH

H

C

an s-orbital three p orbitalsAtomic Orbitals

atomic orbitals for carbon:

1s

2s2p 4 valence electrons but

two different types orbitals.Hs on CH4 should be equivalent.

xy

zpx py pz

an s-orbital p

Hybridization sp3 hybridization for C in CH4

MSE280 2007, 2008 Moonsub Shim, University of Illinois20x

y

z1s + 2p = sp2-orbitals 1s + 3p = sp3-orbitals

60

60x

y

z

y

xy

z1s + 1p = sp-orbitals

sp3 hybridization for C in CH4

11

EXAMPLES: COVALENT BONDING

He -

H 2 1 SiC

C(diamond)

H2OH2

Cl2

F2

colu

mn

IVA

Ne -

Ar -

Kr -

Xe -

Rn -

F 4.0Cl

3.0Br

2.8I

2.5At

2.2

Li 1.0Na 0.9K

0.8Rb 0.8Cs 0.7Fr

0.7

2.1Be 1.5Mg 1.2Ca 1.0Sr

1.0Ba 0.9Ra 0.9

Ti 1.5

Cr 1.6

Fe 1.8

Ni 1.8

Zn 1.8

As 2.0

SiCC

2.5

Cl2

Si 1.8

Ga 1.6

GaAs

Ge 1.8

O 2.0

c

Sn 1.8Pb 1.8

Adapted from Fig. 2.7, Callister 6e. (Fig. 2.7 is

MSE280 2007, 2008 Moonsub Shim, University of Illinois21

Molecules with nonmetals Molecules with metals and nonmetals Elemental solids (RHS of Periodic Table) Compound solids (about column IVA)

0.9 p g , ( gadapted from Linus Pauling, The Nature of the Chemical Bond, 3rd edition, Copyright 1939 and 1940, 3rd edition. Copyright 1960 by Cornell University.

% ionic character

Most bonds between two different types of atoms are somewhere in between ionic and covalent.between ionic and covalent.

% ionic character = ]})(25.0exp[1{ 2BA