Materials Science and Engineering

Subrata B Ghosh

ATOMIC STRUCTURE & BONDING

Properties

Structure Processing

Materials Science Materials Engineering

Electronic level (subatomic)

Atomic (molecular level, chemical composition)

Crystal (arrangement of atoms or ions wrt one another)

Microstructure (can study with microscopes)

Macrostructure (can see with naked eye)

Objectives

To describe the underlying physical concepts related to the structure of matter.

To examine the relationships between structure of atoms-bonds-properties of

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structure of atoms-bonds-properties of engineering materials.

Learn about different levels of structure i.e. atomic structure, nanostructure, microstructure, and macrostructure.

Nanotechnology

Micro-electro-mechanical (MEMS) systems-Airbag

The Structure of Materials: Technological Relevance

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systems-Airbag sensors

Nanostructures

Level of Structure Example of Technologies

Atomic Structure Diamond edge of (10-10 m) cutting tools

Atomic Arrangements: Amorphous silica - fiberShort-Range Order optical communications(SRO) industry

Levels of Structure

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(SRO) industry(1-10 A)

Atomic Arrangements: Lead-zirconium-titanate Long-Range Order [Pb(Zrx Ti1-x )O3] or PZT (LRO) gas igniters, Ultrasound(10 nm-cm)

Level of Structure Example of Technologies

Nanostructure Nano-sized particles of

(1-500 nm) iron oxide ferrofluids

(Continued)

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Microstructure Mechanical strength of

(0.1-100m) metals and alloys

Macrostructure Paints for automobiles

(10-4 m or more) for corrosion resistance

Importance of Atomic Structure?

Properties of materials depend on geometrical atomic arrangements and interactions among the atoms, which eventually are controlled by the subatomic structure of the materials.

For example: Carbon (pure) can exist as graphite For example: Carbon (pure) can exist as graphite and diamond.

Graphite is soft and greasy feel to it, Diamond is the hardest known material. This difference is because of the type of interatomic bonding in graphite and diamond.

The Structure of the Atom

The atomic number of an element is equal to the number of electrons or protons in each atom.

The atomic mass of an element is equal to the average number of protons and neutrons in

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average number of protons and neutrons in the atom.

The Avogadro number of an element is the number of atoms or molecules in a mole.

The atomic mass unit of an element is the mass of an atom expressed as 1/12 the mass of a carbon atom.

Calculate the number of atoms in 100 g of silver.

SOLUTION

The number of silver atoms is = )10023.6)(100(23

molatomsg

Example 1

Calculate the Number of Atoms in Silver

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The number of silver atoms is =

)868.107(

)10023.6)(100(

molg

molatomsg

=5.58 1023

Scientists are considering using nano-particles of such magnetic materials as iron-platinum (Fe-Pt) as a medium for ultrahigh density data storage. Arrays of such particles potentially can lead to storage of trillions of bits of data per square incha capacity that will be 10 to 100 times higher than any other devices such as computer hard

Nano-Sized Iron-Platinum Particles For Information Storage

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higher than any other devices such as computer hard disks. If these scientists considered iron (Fe) particles that are 3 nm in diameter, what will be the number of atoms in one such particle?

SOLUTION

The radius of a particle is 1.5 nm.

Volume of each iron magnetic nano-particle

= (4/3)(1.5 10-7 cm)3

= 1.4137 10-20 cm3

Density of iron = 7.8 g/cm3. Atomic mass of iron is 56 g/mol.

Mass of each iron nano-particle

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Mass of each iron nano-particle

= 7.8 g/cm3 1.4137 10-20 cm3

= 1.102 10-19 g.

One mole or 56 g of Fe contains 6.023 1023

atoms, therefore, the number of atoms in one Fe nano-particle will be 1186.

Bohr atomic model describe the electrons in terms of their positions (orbitals) and energy (quantized energy levels).

Atomic Model

Adapted from Fig. 2.1, Callister 6e.

electrons:

n = principal

quantum number

n=3 21

Nucleus: Z = # protons

= 1 for hydrogen to 94 for plutonium

N = # neutrons

Atomic mass A Z + N

n labels shells; shells are composed of sub-shells: s, p, d, f,

An s orbital has a sphere of electron density and is lower in energy than the other orbitals of the same shell.

A p orbital has a dumbbell shape and contains a node of electron density at the nucleus. It is higher in energy than an s orbital. There are three p orbitals in the same shell.

Atomic Orbitals:

Atomic Structure

have discrete energy states

tend to occupy lowest available energy state.

Electrons...

Electron Energy States

Maximum electronsMaximum electrons

In orbitals/sub-shells

s = 2

p = 6

d = 10

f = 14

Maximum electrons in

nth shell = 2n2

Limitations of Bohr model was resolved by wave-mechanical model of the atom. In this model, electrons exhibit wavelike and particle like characteristics and position is considered as the probability of an electrons being at various locations or electron cloud.

Atomic Model

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NOTE: Atomic spectra: When an electric discharge (spark) passes through a gas (H2), it excites or energizes the atoms of the gas. More specifically it excites the electrons of the atoms. The atoms then emit the absorbed energy in the form of light as the electrons return to a lower energy state.

Aufbau principle: Orbitals fill in order of increasing

energy, lowest to highest.

Pauli exclusion principle: Only two electrons are

allowed to occupy each orbital and the spins must be

Atomic Structure

Rules for sequentially adding electrons:

allowed to occupy each orbital and the spins must be

paired.

Hunds Rule: When filling orbitals of equal energy

(degenerate), one electron is added to each orbital of

equal energy until all have one electron. The three 2p

orbitals are degenerate.

Since there is only one orbital in the first shell, and each orbital can hold a maximum of two electrons, there are two elements in the first row, H and He. Rows represent the major energy levels.

Each element in the second row of the periodic table has four orbitals available to accept electrons: one 2s orbital, and three 2p

Atomic Structure

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orbitals available to accept electrons: one 2s orbital, and three 2p orbitals, therefore the second row elements are limited to 8 electrons.

Atomic orbitals of Carbon

Quantum numbers are the numbers that assign electrons in an atom to discrete energy levels.

A quantum shell is a set of fixed energy levels to which electrons belong.

The Electronic Structure of the Atom

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to which electrons belong.

The valence of an atom is the number of electrons in an atom that participate in bonding or chemical reactions.

Electronegativity describes the tendency of an atom to gain an electron.

have complete s and p sub-shells 8 electrons (octet)

tend to be unreactive.

Stable electron configurations...

Stable Electron Configurations

Z Element Configuration

2 He 1s2

10 Ne 1s22s22p6

18 Ar 1s22s22p63s23p6

36 Kr 1s22s22p63s23p63d104s24p6

Element Hydrogen Helium Lithium Beryllium Boron Carbon ... Neon

Atomic # 1 2 3 4 5 6

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Electron configuration 1s1

1s2 (stable) 1s22s1 1s22s2 1s22s22p1 1s22s22p2 ...

1s22s22p6 (stable)

Adapted from Callister 6e.

Survey of Elements

Valence (outer) shell usually is not an octet.

Most elements: Electron configuration not stable. Why?

Neon Sodium Magnesium Aluminum ... Argon ... Krypton

10 11 12 13

18 ... 36

1s 2s 2p (stable) 1s22s22p63s1 1s22s22p63s2 1s22s22p63s23p1 ...

1s22s22p63s23p6 (stable) ...

1s22s22p63s23p63d104s246 (stable)

Columns: Similar Valence Structure

THE PERIODIC TABLE

He

Ne

i

n

e

r

t

g

a

s

e

s

a

c

c

e

p

t

1

e

a

c

c

e

p

t

2

e

g

i

v

e

u

p

1

e

g

i

v

e

u

p

2

e

g

i

v

e

u

p

3

e

F Li Be

Metal

Nonmetal

Intermediate

H

O

Electropositive elements:

Readily give up electrons

to become + ions.

Electronegative elements:

Readily acquire electrons

to become - ions.

Adapted from

Callister 6e.

Ne

Ar

Kr

Xe

Rn

g

i

v

e

u

p

F Li Be

Na Cl

Br

I

At

O

S Mg

Ca

Sr

Ba

Ra

K

Rb

Cs

Fr

Sc

Y

Se

Te

Po

Second Row Elements: filling the 2nd major energy level.

Since each of the four orbitals available in the second shell can

hold two electrons, there is a maximum capacity of eight

electrons for elements in the second row. Each atom has an

increasing number of electrons.

The second row of the periodic chart consists of eight

Atomic Structure

The second row of the periodic chart consists of eight

elements, obtained by adding electrons to the 2s and three 2p

orbitals. (The electronic configuration of C = 1s22s22p2 )

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Figure 2.8 The atomic structure of sodium, atomic number 11, showing the electrons in the K, L, and M quantum shells

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The complete set of quantum numbers for each of the 11 electrons in sodium

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Figure 2.10 The electronegativities of selected elements relative to the position of the elements in the periodic table

Using the electronic structures, compare the electronegativities of calcium and bromine.

SOLUTION

The electronic structures, obtained from Appendix C, are:

Ca: 1s22s22p63s23p6 4s2

Comparing Electronegativities

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Br: 1s22s22p63s23p63d10 4s24p5

Calcium has two electrons in its outer 4s orbital and bromine has seven electrons in its outer 4s4p orbital. Calcium, with an electronegativity of 1.0, tends to give up electrons and has low electronegativity, but bromine, with an electronegativity of 2.8, tends to accept electrons and is strongly electronegative. This difference in electronegativity values suggests that these elements may react readily to form a compound.

III-V semiconductor is a semiconductor that is based on group 3A and 5B elements (e.g. GaAs).

II-VI semiconductor is a semiconductor that is based on group 2B and 6B elements (e.g. CdSe).

Transition elements are the elements whose electronic

The Periodic Table

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Transition elements are the elements whose electronic configurations are such that their inner d and f levels begin to fill up.

Electropositive element is an element whose atoms want to participate in chemical interactions by donating electrons and are therefore highly reactive.

2003 Brooks/C

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ublish

ing / T

homson Learn

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