Atomic Structure-Eng

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Atomic Structure

Prof. Dr. Hatem AKBULUT



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Objectives of ChapterThe goal of this chapter is to describe theunderlying physical concepts related to thestructure of matter.

To examine the relationships betweenstructure of atoms-bonds-properties ofengineering materials.Learn about different levels of structure i.e.atomic structure, nanostructure,microstructure, and macrostructure.




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Chapter Outline

2.1 The Structure of Materials:Technological Relevance2.2 The Structure of the Atom2.3 The Electronic Structure of theAtom2.4 The Periodic Table




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NanotechnologyMicro-electro-mechanical (MEMS)systems-AirbagsensorsNanostructures

Figure 2.1

Section 2.1The Structure of Materials:

Technological Relevance




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Level of Structure Example of Technologies

Atomic Structure Diamond – edge ofcutting tools

Atomic Arrangements: Lead-zirconium-titanateLong-Range Order [ Pb ( Zr x Ti 1-x )] or PZT – (LRO) gas igniters

Atomic Arrangements: Amorphous silica - fiber

Short-Range Order optical communications(SRO) industry

Figures 2.2 – 2.4

Table 2.1 Levels of Structure




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Level of Structure Example of Technologies

Nanostructure Nano-sized particles ofiron oxide – ferrofluids

Microstructure Mechanical strength ofmetals and alloys

Macrostructure Paints for automobilesfor corrosion resistance

Figures 2.5 – 2.7

Table 2.1 (Continued)




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Section 2.2The Structure of the Atom

The atomic number of an element is equal to thenumber of electrons or protons in each atom.The atomic mass of an element is equal to the averagenumber of protons + neutrons in the atom.The Avogadro number of an element is the number ofatoms or molecules in a mole.The atomic mass unit of an element is the mass of anatom expressed as 1/12 the mass of a carbon atom.




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Atom= Nucleus (neutron+proton) + electronThe effect hold the atoms together nuclear force

The effect hold atoms around atoms electrostatic force

ForcesElectrons negativeProtons positive

Neutrons neutralThe electrical charge that electrons and protons have,is equal= 1,6 10 -19 C (Coulombs)

MassesProtons and neutrons have equal mass= 1,6 10 -27 kgMass of Electrons = 9,11 10 -31 kg

Mass of electron is omitted in atomic mass calculationElectron charge cloud constitutes approximately all

volume of atom. However, contitutes extremly smallamount of mass

8Prof. Dr. Hatem AKBULUT



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Atomic number (Z) = Number of Protons in nucleusIn a neutral atom the number of protons equal tonumber of electrons

Each of the element has specific atomic number.Atomic number introduce the element

Mass of Atom (M) = no. of protons + no. of neutrons= atomic number(Z)+no. of neutrons

Isotopes are the atoms that the neutron numbers arediferent in the nucleus.




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Atomic Mass: Equal to mass of the atom interms of grams of the atoms that equal to

Avogadro number (N A) Unit= gr/mol.

NA (Avogadro number)= 6,02 10 23 atom/mol

Alternative unit is the atomic mass unit (a.m.s.)(It is 1/12 th of the carbon atom that atomicmass is 12)

Example: One mole iron (Fe) contains6,02 10 23 atoms and its mass is 55,847 gror 55,847 a.m.s.




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Calculate the number of atoms in 100 g of silver.

Example 2.1 SOLUTION

The number of silver atoms is =)868.107(

)10023.6)(100( 23

mol g

mol atoms g

=5.58 10 23

Example 2.1Calculate the Number of Atoms in Silver




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Scientists are considering using nano-particles of suchmagnetic materials as iron-platinum (Fe-Pt) as amedium for ultrahigh density data storage. Arrays ofsuch particles potentially can lead to storage oftrillions of bits of data per square inch —a capacity thatwill be 10 to 100 times higher than any other devicessuch as computer hard disks. If these scientistsconsidered iron (Fe) particles that are 3 nm indiameter, what will be the number of atoms in onesuch particle?

Example 2.2Nano-Sized Iron-Platinum Particles

For Information Storage




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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 cm 3

Density of iron = 7.8 g/cm 3 . Atomic mass of iron

is 56 g/mol.Mass of each iron nano-particle

= 7.8 g/cm 3 1.4137 10 -20 cm 3

= 1.102 10 -19 g.

One mole or 56 g of Fe contains 6.023 10 23 atoms, therefore, the number of atoms in oneFe nano-particle will be 1186.




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Example 2.3Dopant Concentration In Silicon Crystals

Silicon single crystals are used extensively to makecomputer chips. Calculate the concentration of silicon atomsin silicon, or the number of silicon atoms per unit volume ofsilicon. During the growth of silicon single crystals it is oftendesirable to deliberately introduce atoms of other elements(known as dopants) to control and change the electricalconductivity and other electrical properties of silicon.Phosphorus (P) is one such dopant that is added to makesilicon crystals n -type semiconductors. Assume that theconcentration of P atoms required in a silicon crystal is 10 17 atoms/cm 3 . Compare the concentrations of atoms in siliconand the concentration of P atoms. What is the significance ofthese numbers from a technological viewpoint? Assume thatdensity of silicon is 2.33 g/cm 3 .




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Atomic mass of silicon = 28.09 g/mol.

So, 28.09 g of silicon contain 6.023 10 23 atoms.

Therefore, 2.33 g of silicon will contain

(2.33 6.023 1023

/28.09) atoms = 4.99 10 22 atoms. Mass of one cm 3 of Si is 2.33 g.

Therefore, the concentration of silicon atoms inpure silicon is 5 10 22 atoms/cm 3 .




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Example 2.3 SOLUTION (Continued)

Significance of comparing dopant and Si atomconcentrations: If we were to add phosphorus (P)into this crystal, such that the concentration of P is10 17 atoms/cm 3 , the ratio of concentration ofatoms in silicon to that of P will be

(5 1022

)/(1017

)= 5 105

. This says that only 1out of 500,000 atoms of the doped crystal will bethat of phosphorus (P)! This is equivalent to oneapple in 500,000 oranges! This explains why thesingle crystals of silicon must have exceptionalpurity and at the same time very small anduniform levels of dopants.




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Quantum numbers are the numbers that assign electronsin an atom to discrete energy levels.A quantum shell is a set of fixed energy levels to whichelectrons belong.

Pauli exclusion principle specifies that no more than twoelectrons in a material can have the same energy. Thetwo electrons have opposite magnetic spins.The valence of an atom is the number of electrons in anatom that participate in bonding or chemical reactions.

Electronegativity describes the tendency of an atom togain an electron.

Section 2.3 The Electronic Structureof the Atom




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If sufficient energy is given toan electron on a specific

energy level, electron can jump to a upper energy level.

e.i. For shifting of an electron

to an energy level of E 2 , thenecessary energy for theelectron which is stable at theenergy level of E 1, receivedenergy is:

ΔE = E 2 -E 1

Absorbed energy

Photon




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However, since theelectron is unstable in theE2 level it can not behosted forever in thisenergyElectron returns in the E 1level and it emits receivedΔE energy in the form ofelectromagnetic radiationto the environment.

Yayılan enerji

Photon

Emitted energy




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During the returning to the lower energy level,the frequency of emitted specific amount ofenergy wave ( ) in the form of radiation(photon) is proportional with ΔE energy:

ΔE = h

h: Planck constant (6,63x10 -34 Js)

The wavelength of the emitted radiation with thespeed of light (c=3x10 8 m/s) wave ( );

c= Then the energy becomes

λ

chEΔ




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A hydrogen atom model,representing that a single

electron is rotatingaround a proton.Model was developed in1913 by Niels Bohr.

Bohr equation explainingthe model givesapproximate energy at

the permitted energylevel e: Electron chargem: Electron massn: Main (primary) quantum number

eV n

E 222

42 6,13

hn

em2π

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Orbital radius

R= 0.05 nmElectron:Charge –eMass mVelocity v Proton

Charge +e

Electronorbital


Continiuty



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According to Bohr’s equation the energy at the base condition is –13,6 eV.If the hydrogen atom excited to higher energy levels its energyraised and numerical value decreased.For completely separating an electron from hydrogen atom therequired energy is 13.6 eV and this is the ionization energy ofhydrogen atom..

Continiuty

Base condition




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Nucleus: Z = # protons

= 1 for hydrogen to 94 for plutonium

N = # neutronsAtomic mass A ≈ Z + N

Adapted from Fig. 2.1,Callister 6e.

BOHR ATOMelectrons:n = principalquantum number

n=3 2 1

n labels shells; shells are composed of subshells: s, p, d, f, …

Prof. Dr. Hatem AKBULUT23



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An electron configuration describes the distribution ofelectrons among the various orbitals in the atom.Electron configuration is represented in two ways.

Electron Configurations

The spdf notation usesnumbers to designate aprincipal shell and letters ( s,

p, d, f ) to identify asubshell; a superscriptindicates the number ofelectrons in a designatedsubshell.




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1s 2 2s 2 2p 63s 23p 63d 64s 2 .




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In an orbital (box) diagram a box represents eachorbital within subshells, and arrows representelectrons. The arrows’ directions represent electronspins; opposing spins are paired.

N:26




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ELECTRONıC C ONFıGURATıON the arrangement of the electron in the atom.Electrons are arranged in Energy Levels or

Shells around the nucleus of an atom.

nucleus

1 3 4

n l x

sd f

sp

sp d

p

s

Atomic orbital

f = 7

d = 5

p = 3s = 1

1 Atomic or bit al = 2 e -

x 2 = 2x 2 = 6

x 2 = 10

x 2 = 14

2e - 8e - 32e -

Mainenergy level

Subenergy level

no.ofelectrons

18e -




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• have discrete energy states • tend to occupy lowest available energy state.

Electrons...ELECTRON ENERGY STATES

Maximum electronsIn sub-shells

s = 2p = 6

d = 10f = 14

Maximum electrons inn th shell = 2n 2




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• have complete s and p sub -shells 8 electrons (octet)• tend to be unreactive .

Stable electron configurations...

Adapted from Table 2.2,Callister 6e.

STABLE ELECTRONCONFIGURATIONS


S O S



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• Valence (outer) shell usually is not an octet.

• Most elements: Electron configuration not stable . Why?

Electron configuration1s 1 1s 2 (stable)

1s 2 2s 1 1s 2 2s 2 1s 2 2s 2 2p 1 1s 2 2s 2 2p 2 ...1s 2 2s 2 2p 6 (stable)

1s 2 2s 2 2p 6 3s 1 1s 2 2s 2 2p 6 3s 2 1s 2 2s 2 2p 6 3s 2 3p 1 ...1s 2 2s 2 2p 6 3s 2 3p 6 (stable)...

1s 2 2s 2 2p 6 3s 2 3p 6 3d 10 4s 2 4 6 (stable)

Adapted from Table 2.2,Callister 6e.

SURVEY OF ELEMENTS




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Quantum Numbers of Electrons

● Electrons have different energy levels in theatomsEach of electron has a specific energy and therecan not be more than two electrons that havesimilar energy level in an atom.

● Each of the energy levels belong to electronsdetermined by four quantum numbers. Theseare:

Primary (principal) quantum numberSecondary (Azimuthal) quantum numberMagnetic quantum numberSpin quantum number




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1. Basic (primary) quantum number:Shows the basic energy level of theelectron. In an energy shell that has

n quantum number, there can bemaximum 2n 2 electrons.

Additionally, basic quantum numberscan be shown with K, L, M, N,... symbols.

n=1= K shell, 2 en=2= L shell, total 8 en=3= M shell, total 18 en=4= N shell, total 32 e

Basic quantumnumbers

1 2 3 4 5 ….

Symbols (Letters) K L M N O ….




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©2 0 0 3 Br o ok s / C

ol e P u b l i s h i n g

/ T h om s onL e a r ni n g™

Figure 2.8 The atomic structure of sodium, atomic number11, showing the electrons in the K, L, and M quantum shells


2 S d (A i h l) b ( l)



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2. Secondary (Azimuthal) quantum number( l ): Define secondary energy levels inside the basic energy levelsand there can be as much as (n-1).

l number can takes the values of 0,1,2,3,...,n-1 .

These are shown by s, p, d, f letters instead of numbers.l= 0 for sl= 1 for pl= 2 for dl= 3 for f

Specific numbers of electrons can be existed in each s, p, d, forbitals.

In s orbital 2 e,In p orbital 6 e,In d orbital 10 eIn f orbital 14 e

SecondaryQuantum numbers

0 1 2 3 4 ….

Letters s p d f g …. 35Prof. Dr. Hatem AKBULUT



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3. Magnetic quantum number (m l): Electrons move at different directions on atom’s around and have different angular momentum.

If the movement is in positive direction at secondaryquantum shell (+), in negative (-), the movementdirection is indefinite (0).

The value that the magnetic quantum number cantake is 2l+1.

l=0 for m l =1 number, m l = 0

l=1 for m l =3 number m l = -1, 0, +1l=2 i for m l =5 number m l = +2, -1, 0, +1, +2




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4. Spin quantum number, m s In an orbital and energy level there can not be

more than two elctrons that move oppositedirections (Pauli exclusion principle)

Electrons have two different rotating directions

one is at the clockwise and the other is anti-clockwise.

Rotating can takes +1/2 and -1/2 values for

determining different spins.




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Figure 2.9 The complete set of quantum numbers for eachof the 11 electrons in sodium




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©2 0 0 3 Br o ok s / C

ol e P u

b l i s h i n g / T h om s onL e a r ni n g™

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




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s orbitals




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p orbitals




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2s orbital

1s orbital

2porbitals

3s orbital

The quantum model of hydrogen

Nucleus




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Using the electronic structures, compare the electronegativitiesof calcium and bromine.


The electronic structures, obtained from Appendix C, are:

Ca: 1 s 2 2 s 22 p 63 s 2 3 p 6 4 s 2

Br: 1 s 22 s 2 2 p 63 s 23 p 6 3 d 10 4 s 24 p 5

Calcium has two electrons in its outer 4 s orbital and brominehas seven electrons in its outer 4 s 4 p orbital. Calcium, with anelectronegativity of 1.0, tends to give up electrons and has lowelectronegativity, but bromine, with an electronegativity of 2.8,tends to accept electrons and is strongly electronegative. Thisdifference in electronegativity values suggests that theseelements may react readily to form a compound.

Example 16.9Comparing Electronegativities




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III-V semiconductor is a semiconductor that is based ongroup 3A and 5B elements (e.g. GaAs).II-VI semiconductor is a semiconductor that is based ongroup 2B and 6B elements (e.g. CdSe).

Transition elements are the elements whose electronicconfigurations are such that their inner “d” and “f” levelsbegin to fill up.Electropositive element is an element whose atoms wantto participate in chemical interactions by donating

electrons and are therefore highly reactive.

Section 2.4 The Periodic Table




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Figure 2.11 (a) and (b) Periodic Table of Elements




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Valance: It is related with chemical activity of anatom with another element and generally is

determined by outer the electron number ofcombined “sp” level.Mg: 1s 2 2s 2 2p 6 [3s 2 ] Valance: 2Al: 1s 2 2s 2 2p 6 [3s 2 3p 1 ] Valance:3

Ge: 1s2

2s2

2p6

3s2

3p6

3d10

[4s2

4p2

] Valance:4

Electronegativity: Defines electron receivingtendency of an atom and varies between 0-4,1

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Electronegativity of some elements




In this table the elements that their valance electronstructures are similar are arranged one under theother.

The equal quantum shell number elements are listed ina same row.

Columns are ranged in the horizontal direction interms of electrons numbers and take group numbers.


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