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Lecture 2

Atomic Structure and Bonding

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Learning objectives

Understand basic structures of atoms

Understand basic bonding types of matter and their role in forming different types of materials

Able to estimate the bonding types from the position of constituent elements in periodical table

Gain basic knowledge on the relation between bonding types and corresponding materials properties (mechanical, electrical, thermal).

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macroscopic

microscopic atomic

subatomic

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Structure of Atoms

ATOMBasic Unit of an Element

Diameter: 10–10 m.Neutrally Charged

NucleusPositive Charge

Electron CloudNegative Charge

ProtonPositive Charge

NeutronNeutral Charge

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Atomic massAtomic mass of an element is the mass in grams of 6.023×1023 atoms (Avogadro’s number NA) of that element.

The atomic numberatomic number of an atom indicates the number of protons (positively charged particles) in nucleus. It equals to the number of electrons in a neutral atom. The The atomic number identifies the element.atomic number identifies the element.

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Example:Example:

1. What is the mass in grams of one Cu atom?2. How many atoms are in 1 g of Cu?

Solution:Solution:

1. The atomic mass of Cu is 63.55 g/mol. Thus, the mass of one Cu atom is

atom/g.mol/atoms.

mol/g.x 2223

100551100236

5563 −×=×

=

2. The number of atoms in 1 g of Cu is

!atoms.mol/g.

mol/atoms.x 21

2310489

5563100236

×=×

=

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Electron Configuration lists the arrangement of electrons in orbitals.

Maximum number of electrons in each atomic shell is given by 2n2.

Atomic size (radius) increases with addition of shells.

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The atomic structure of sodium, atomic number 11, showing the electrons in the K, L, and M quantum shells and filled energy states

Valence e-

Valence e-

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Electron Structure of MultiElectron Structure of Multi--electron Atomelectron Atom

Principal Quantum Numbers

Orbital letters Number of Electrons

Example: Fe, (Z=26), Electron configuration is

1s22s22p6 3s2 3p6 3d6 4s2

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Electron Structure and Chemical ActivityElectron Structure and Chemical Activity

ElectropositiveElectropositive elements give electrons during chemical reactions to form cationscations.

Cations are indicated by positive oxidation numbers.

For example:

Fe2+, Fe3+

positivepositiveoxidation number

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Electron Structure and Chemical Activity (Cont..)Electron Structure and Chemical Activity (Cont..)

ElectronegativeElectronegative elements accept electrons during chemical reaction to form anionsanions. Anions are indicated by negative oxidation numbers.

For example:

O2-, Cl-

Some elements behave as both electronegative and electropositive.

negativenegativeoxidation number

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Oxidation numbers of the elements

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Electron Structure and Chemical Activity (Cont..)Electron Structure and Chemical Activity (Cont..)

ElectronegativityElectronegativity is the degree to which the atom attracts attracts electrons to itself

Measured on a scale of 0 to 4.1

Example: Electronegativity of Fluorine is 4.1

Electronegativity of Sodium is 1.

0 1 2 3 4K

Na N O Fl

W

Te

SeH

Electro-positive

Electro-negative

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ElectronegativitiesElectronegativities of elementsof elements

Atomic bond between different elements is related to their electronegativities!

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r0r0

(a) The dependence of repulsive, attractive, and net forces on interatomicseparation for two isolated atoms.

(b) The dependence of repulsive, attractive, and net potential energies on interatomic separation for two isolated atoms.

FN = FA + FR

∫∞=r

NN drFE

(Review)

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The equilibrium distance between atoms is caused by a balance between repulsive and attractive forces.

Equilibrium separation occurs when no net force acts to either attract or separate the atoms or the total energy of the pair of atoms is at a minimum.

for a solid metal the interatomic spacing is equal to the atomic diameter or 2r.

for ionically bonded materials, the spacing is the sum of the two different ionic radii.

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Atomic and Molecular BondsAtomic and Molecular Bonds

Primary bondsPrimary bonds

Metallic bonds: Non-directional bonds formed by sharing of free electronsIonic bonds: Strong atomic bonds due to transfer of electronsCovalent bonds: Large interactive force due to sharing of electrons

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Metallic BondingMetallic Bonding

Atoms in metals are closely packed in crystal structure.

The metallic bond forms when atoms give up their valence electrons, which then form an electron sea.

The positively charged atom cores are bonded by mutual attraction to the negatively charged electrons

These free electrons are

reason for electric

conductivity and ductility

Since outer electrons are

shared by many atoms,

metallic bonds are

Non-directional

Positive Ion

Valence electron cloud

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Ionic BondingIonic Bonding

An ionic bond is created between two unlike atoms with differentelectronegativities. It can form between metallic and nonmetallic elements. Electrons are transferred from electropositive to electronegative atomsIonic bonding is due to electrostatic forceelectrostatic force between cations and anions that holds the ions together.

ElectropositiveElement

ElectronegativeAtom

Electron Transfer

Cation+ve charge

Anion-ve charge

IONIC BOND

ElectrostaticAttraction

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Ionic Bonding Ionic Bonding -- ExampleExample

Ionic bonding in NaCl

3s13p6

Sodium AtomAtomNa (Z=11)

Chlorine AtomAtomCl (Z=17)

Sodium IonIonNa++

Chlorine IonIonCl−−

IONIC

BOND

1s22s22p63s1 1s22s22p63s23p5

1s22s22p6 1s22s22p63s23p6

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When sodium donates its valence electron to chlorine, each becomes an ion; attraction occurs, and the ionic bond is formed

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Covalent bonding requires that electrons be shared between atoms in such a way that each atom has its outer sp orbital filled.

Example: In Hydrogen (Z=1), a bond is formed between 2 atoms by sharing their 1s1 electrons

Covalent BondingCovalent Bonding

H + H H H

1s1

Electrons

ElectronPair

HydrogenMolecule HOverlapping Electron Clouds

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Example

Si

The outmost shell needs 8electrons (3s23p6) to be saturated – achieved by covalent bonding

Si – atomic number 14covalent bond1s22s22p63s23p2

Thus, the valence band is completely filled

In Si, with a valence of four, four covalent bonds must be formed

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Mixed bondingMixed bonding

Ionic-Covalent bonding

( ) ( )%% . 10012250 ⎟⎠⎞⎜

⎝⎛ −= −− BA XXecharacterionic

XA, XB - electronegativitieselectronegativities of the elements A and B in a compound AB

Few compounds exhibit pure ionic or pure covalent bonding.

The bond type degree depends on their position in the Periodic Table:

the greater the difference in electronegativity, the more ionicthe bond. Conversely, the smaller the difference larger degree of covalency.

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Covalent bonding tends to take place between elements with smalldifferences in electronegativity and close by in periodic table. Ionic bonding tends to take place between elements with large differences in electronegativity and more separated in periodic table.

( ) %echaracterionic% BA XX. 10012250 ×⎟⎠⎞⎜

⎝⎛ −= −−

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Mixed bondingMixed bonding

Metallic-Covalent bonding

Some transition metals

Metallic-Ionic bonding

Some intermetallic compounds (large difference in electronegativities)

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Carbon Containing MoleculesCarbon Containing Molecules

In Methane, Carbon (Z=6) forms four covalent bonds with Hydrogen.

Molecules are very weekly bonded together resulting in low melting temperature (-183oC).

Carbon also forms bonds with itself.

Molecules with multiple carbon bonds are more reactive.

Examples: C CH

H

H

HEthylene

C CH HAcetylene

Methanemolecule

C: 1s22s22p2

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Covalent Bonding in BenzeneCovalent Bonding in Benzene

Chemical composition of Benzene is C6H6.The Carbon atoms are arranged in hexagonal ring.Single and double bonds alternate between the atoms.

Structure of Benzene Simplified Notations

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Illustration of London forces, a type of a van der Waals force, between atoms

Secondary bondsSecondary bonds

weak bonds due to attraction of the electric dipoles contained weak bonds due to attraction of the electric dipoles contained in atoms or moleculesin atoms or molecules

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The Keesom interactions are formed as a result of polarization of moleculespolarization of molecules or groups of atoms. In water, electrons in the oxygen tend to concentrate away from the hydrogen. The resulting charge difference permits the molecule to be weakly bonded to other water molecules (hydrogen bondhydrogen bond).

Secondary bondsSecondary bonds

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Atomic bond and material propertiesAtomic bond and material properties

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When voltage is applied to a metal, the electrons in the electron sea can easily move and carry a current

Good electric conductor

Good thermal conductor

Good ductility

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When voltage is applied to an ionic material, entire ions must move to cause a current to flow. Ion movement is difficult and slow and the electrical conductivity is poor

Poor electric conductor

Poor thermal conductor

Poor ductility

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The force-distance curve for two materials, showing the relationship between atomic bonding and the modulus of elasticity, a steep dFldaslope gives a high modulus

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(a) In polyvinyl chloride (PVC), the chlorine atoms attached to the polymer chain have a negative charge and the hydrogen atoms are positively charged. The chains are weakly bonded by van der Waals bonds. This additional bonding makes PVC stiffer, (b) When a force is applied to the polymer, the van der Waals bonds are broken and the chains slide past one another

© 2003 Brooks/Cole Publishing / Thomson Learning™ © 2003 Brooks/Cole Publishing / Thomson Learning™

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Example: Design of a Space Shuttle ArmExample: Design of a Space Shuttle Arm

NASA’s space shuttles have a long manipulator robot arm, also known as the Shuttle Remote Manipulator System or SRMS, that permits astronauts to launch and retrieve satellites. It is also used to view and monitor the outside of the space shuttle using a mounted video camera. Select a suitable material for this device.

NASA’s Shuttle Remote Manipulator System: SRMS (Courtesy of Getty Images)

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Example Solution

Let’s look at two of the many material choices.

First, the material should be stiffstiff so that little bending occurs when a load is applied; this feature helps the operator maneuver the manipulator arm precisely. Generally, materials with strong strong bondingbonding and high melting pointshigh melting points also have a high modulus of elasticity, or stiffness.

Second, the material should be lightlight to permit maximum payloads to be carried into orbit; a low density is thus desired. It is estimated that it costs about US $100,000 to take the weight of a beverage can into space! Thus, the density must be as low as possible.

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Example SOLUTION (Continued)

Good stiffness is obtained from high-melting-point metals (such as beryllium and tungsten), ceramics, and certain fibers (such as carbon).

W, however, has a very high density, while ceramics are very brittle.

Be, which has a modulus of elasticity that is greater than that of steel and a density that is less than that of aluminum, might be an excellent candidate. However, toxicity of Be and its compounds must be considered.

The preferred material is a compositeThe preferred material is a composite consisting of carbon fibers embedded in an epoxy matrix. The carbon fibers have an exceptionally high modulushigh modulus of elasticity, while the combination of carbon and epoxy provides a very lowlow--densitydensity material. Other factors such as exposure to low and high temperatures in space and on earth must also be considered. The current shuttle robot arm is about 45 feet long, 15 inches in diameter and weighs about 900 pounds. When in space it can manipulate weights up to 260 tons.

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Example ProblemExample ProblemA 100 gram alloy of nickel and copper consists of 75 wt% Cu and 25

wt% Ni. What are percentage of Cu and Ni Atoms in this alloy?

Given: 75g Cu Atomic Weight 63.54 g/mol25g Ni Atomic Weight 58.69 g/mol

Number of Cu atoms =

Number of Ni atoms =

Atomic Percentage of Cu =

Atomic Percentage of Ni =

AA N.Ng.g

×=× 180315463

75

AA N.Ng.g

×=× 426006958

25

( ) %.N..

N.

A

A 5731004260018031

18031=×

+

( ) %.N..

N.

A

A 5251004260018031

42600=×

+