Atomic structure and bonding

Click here to load reader

Embed Size (px)

Transcript of Atomic structure and bonding

  • 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