Lecture 2 - Stanford Universitydionne.stanford.edu/MatSci152_2013/Lecture2_ppt.pdf · Lecture 2...

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Lecture 2 Lecture 2 Atoms & Their Interactions

Transcript of Lecture 2 - Stanford Universitydionne.stanford.edu/MatSci152_2013/Lecture2_ppt.pdf · Lecture 2...

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Lecture 2Lecture 2

Atoms & Their Interactions

Page 2: Lecture 2 - Stanford Universitydionne.stanford.edu/MatSci152_2013/Lecture2_ppt.pdf · Lecture 2 Atoms & Their Interactions. Si: the heart of electronic materials ... The diamond crystal

Si: the heart of electronic materials

Twin Creeks Technologies San Jose Intel 300mm Si wafer 200 μm thick Twin Creeks Technologies, San Jose, Si wafer, 20 μm thick

Intel, 300mm Si wafer, 200 μm thick and 48-core CPU (“cloud computing

on a chip”)

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Atomic Configuration

The shell model of the atom: electrons are confined within certain

Carbon: 1s22s22p2 or [He]2s22p2

The shell model of the atom: electrons are confined within certain shells and in subshells within shells

insulator, semiconductor, conductor

Silicon: 1s22s22p63s23p2 or [Ne]3s23p2

Aluminum: [Ne]3s23p1 metal

semiconductor

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Atomic bonding influences material ti

q

properties

q1 q2r

1. rinfinity:y

• Particles don’t interact• Potential energy E(r)=0Potential energy E(r) 0• Force F=dE/dr=0

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Atomic bonding influences material tiproperties

qq1 q2r

2. If the two particles are close enough, they will attract. This g yattraction is governed by electrostatic interactions:

• Potential energy EA(r) = -Cq1q2/r• C α 1/(4πε0)C α 1/(4πε0)

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Atomic bonding influences material tiproperties

qq1 q2r

3. If the two particles are too close, they will repel. y• Potential energy ER(r) = +B/rm

• m an integer, usually large (for Na+ and Cl-, m=8)

For all separations, the net force exerted on the particles is the sum of attractive and repulsive forces:p

F=dE/drFnet=FA+FR

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Force, F(r)

•Net Force FN=FA+FR

E ilib i h F 0•Equilibrium when FN=0•r0=bond length

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Potential energy, E(r)

•E0: bond energy or cohesive energy (energy required to separate the two atoms)

ER

separate the two atoms)

• In general: mn rB

rArE )(

Err

E0

EEA

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Types of bonds

Covalent

Metallic

Ionic

Secondary Bonding (Van der Waals)Secondary Bonding (Van der Waals)

d dMixed Bonding

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Covalent Bonding

Formation of a covalent bond between two hydrogen atoms leads to the H2 molecule Electrons spend majority of their time between the two nucleimolecule. Electrons spend majority of their time between the two nuclei

which results in a net attraction between the electrons and the two nuclei.

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Covalent Bonding in Methane

Covalent bonding in methane, CH4, involves four hydrogen atoms Covalent bonding in methane, CH4, involves four hydrogen atoms sharing bonds with one carbon atom. Each covalent bond has two

shared electrons. The four bonds are identical and repel each other.

In three dimensions, due to symmetry, the bonds are directed towards the corners of a tetrahedron.

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Covalent Bonding in Diamond

The diamond crystal is a covalently bonded network of carbon atoms Each carbonThe diamond crystal is a covalently bonded network of carbon atoms. Each carbonatom is covalently bonded to four neighbors forming a regular three dimensional

pattern of atoms which constitutes the diamond crystal.

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Properties of covalently-bonded materials

• due to the strong Coulombic interaction between the shared electrons and the positive nuclei, the covalent shared electrons and the positive nuclei, the covalent

bond energy is usually the highest among all bond types

• very high melting temperatures•very hard solids (like diamond)• insoluble in nearly all solvents•Non-ductile (or non malleable)

•Exhibit brittle fracture under a strong forceExhibit brittle fracture under a strong force

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Properties of covalently-bonded materials

• due to the strong Coulombic interaction between the shared electrons and the positive nuclei, the covalent shared electrons and the positive nuclei, the covalent

bond energy is usually the highest among all bond types

• very high melting temperatures•very hard solids (like diamond)• insoluble in nearly all solvents•Non-ductile (or non malleable)

•Exhibit brittle fracture under a strong forceExhibit brittle fracture under a strong force

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Properties of covalently-bonded materials

• due to the strong Coulombic interaction between the shared electrons and the positive nuclei, the covalent shared electrons and the positive nuclei, the covalent

bond energy is usually the highest among all bond types

• very high melting temperatures•very hard solids (like diamond)• insoluble in nearly all solvents•Non-ductile (or non malleable)

•Exhibit brittle fracture under a strong forceExhibit brittle fracture under a strong force•Since all electrons are locked in the bonds between the

atoms, the electrons are not free to drift in an electric field: Poor ConductorsPoor Conductors

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Metallic bonding

Consider Ag Electronic configuration: [Kr] 4d10 5s1

In metallic bonding the valence electrons from the metal atoms form a “cloud of electrons” which fills the space between the metal ions and

“ l ” h i h h h h l bi i b h “glues” the ions together through the coulombic attraction between the electron gas and the positive metal ions.

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Properties of metallic-bonded materials

• ionic cores tend to pack closely, like stacked oranges, i.e. hexagonal close-packed, face-centered cubichexagonal close packed, face centered cubic

• bond is non-directional under an applied force, metal ions can move with respect to each otherp

• as a result, metals are ductile

• electrons can drift freely with an applied electric field e ect o s ca d t ee y t a app ed e ect c e d high conductivity

• with temperature gradients, electrons can contribute to energy transfer Good thermal conductivity

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Ionic bonds

• Bond between a positively charged ion (the cation) and a negatively charged ion (the anion)negatively charged ion (the anion)

•frequently found between metal atoms and non-metals

•i.e., NaCl

N h l l l t th t b il Na has only one valence electron that can be easily removed (1s22s22p63s1)

Cl has 5 electrons in its 3p subshell and can readily Cl has 5 electrons in its 3p subshell, and can readily accept one more electron to close this subshell

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Ionic bonds in NaCl

The formation of ionic bond between Na and Cl atoms in NaCl. The attractionis due to coulombic forces.

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Potential energy per ion-pair in solid NaCl

Ionization energy: +1 5eV (energy to Ionization energy: +1.5eV (energy to transfer the electron from Na to Cl)

Cohesive energy: -6.3 eV (energy gy ( gyrequired to take solid NaCl apart into

individual Na and Cl atoms)

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A schematic illustration of a cross section from solid NaCl. NaCl is made of Cl-

and Na+ ions arranged alternatingly so that the oppositely charged ions are closest to each other and attract each other. There are also repulsive forces

between the like ions. In equilibrium the net force acting on any ion is zero.

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Properties of ionically-bonded materials

• Strong, brittle materials

•High melting temperatures compared to metalsg g p p

•Soluble in polar liquids

•No free electrons are all fairly rigidly positioned No free electrons are all fairly rigidly positioned within the ions

• electrically insulatingy g

• poor thermal conductivity

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Are there bonds between atoms that have full shells, and therefore cannot share electrons?

Yes! Yes!

•Liquid He (~4K)

•Solid Ar (below 189oC)•Solid Ar (below -189oC)

• Water: although each H2O molecule is neutral, these molecules attract to form a liquid state below 100oC and molecules attract to form a liquid state below 100 C and

the solid state below 0oC.

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Van der Waals Forces

• Electrostatic attractions between the electron distribution of one atom and the positive nucleus of the other

Dipole: negative and Polar molecules (i.e., exhibiting

a dipole) can attract or repel positive charge of equal

magnitude

p ) peach other depending on their

relative orientations.

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Water & Van der Waals Forces

The H2O molecule is polar and has a net permanent dipole moment

Attractions between the various dipole moments in water gives rise to p gvan der Waals bonding

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Van der Waals bonding

• can also occur between neutral atoms based on random motions of electrons around the nucleus

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Van der Waals bonding

• can also occur between neutral atoms based on random motions of electrons around the nucleus

induced synchronization of electronic motions can lead to attractions solid Ne, Ar, liquid He

also responsible for the attractive interactions between C-chains in polymers

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Van der Waals bonding

• can also occur between neutral atoms based on random motions of electrons around the nucleus

induced synchronization of electronic motions can lead to attractions

solid Ne, Ar, liquid Healso responsible for the attractive interactions between C-

chains in polymerschains in polymers

• poor thermal conductivity • poor thermal conductivity

• electrically insulating

l l ti d li• low elastic moduli

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Mixed bonding

• Bonding in silicon is totally covalent, because the shared electrons in the bonds are equally attracted by the q y y

neighboring positive ion cores and therefore equally shared

• However, where there is a covalent-type bond between different atoms, the electrons become unequally-shared

GaAs III V compounds GaAs, III-V compounds

“polar bonds”

i G A h l d li h l i d in GaAs, the electrons spend slightly more time around the As5+ ion than the Ga3+ ion

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Group Activity

Rank the following materials according to their melting points, from lowest to highest:p g

NaClNaCl

Al

SiSi

He

H OH2O

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Group Activity

Rank the following materials according to their melting points, from lowest to highest:p g

Lowest Tm

(weakest bonds)

Van der Waals: He (-272oC)

H-bonding: H2O (100oC)g 2 ( )

metallic: Al (660oC)

Highest Tm

ionic: NaCl (801oC)

l t Si (1414oC)(strongest bonds) covalent: Si (1414oC)

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Group ActivityWhat periodic table element do you think has the highest

melting point, at ambient pressure?

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Group ActivityWhat periodic table element do you think has the highest

melting point, at ambient pressure?

Tm=3683K (3410oC)

Note: Carbon has no melting point at atmospheric pressure, but will sublime around 4000K

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Kinetic Molecular Theory

Understanding the relationship between energy of atoms and temperature. p

Can be used to explain seemingly diverse topics as the Can be used to explain seemingly diverse topics as the heat capacity of metals, the average speed of electrons in a

semiconductor, and electrical noise

We’ll start with the kinetic molecular theory of gases.

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