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Modern Atomic Theory &

Atomic StructureBy

Prof. Dr. Muhammad.Attique.Kh

an Shahid

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In The Name Of ALLAHIn The Name Of ALLAH

The Most Merciful & Benevolent The Most Merciful & Benevolent

Beyond ReckoningBeyond Reckoning

The Development of Atomic Theory

How did different scientists

contribute to atomic theory?

The Greeks

• Start of the idea of the atom

• In 400 B.C the Greeks tried to understand matter and broke everything down into earth, wind, fire, and air.

Fire Water Earth Air~~

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The Hellenic Market

Fire Water Earth Air~~

Blend these “elements” in different proportions to get all substanceshttp://www.unit5.org/christjs/site%20map.htm

Origin of “Mother Earth” statement?

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Greek Model

• Greek philosopher• “All matter is made up of tiny

particles, invisible to the naked eye, that cannot be divided.”

• He called the particles ‘atomos’• No experiments to support his idea,

which was, soon after, lost for about 2000 years.

Democritus

“To understand the very large, we must understand the very small.”

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Democritus’s model of atom

(no protons, electrons, or neutrons)

Solid and INDESTRUCTABLE

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A ‘Model’… • is not a real thing, but is used to explain,

mimic or simulate reality,• is used as a tool,• is used to predict what happens in the

real world,• is changed or modified until it best fits

new information,• may have some limitations or be valid

only under certain conditions.Examples: globes, computer simulations, product prototypes

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Early Atomic Theories

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Dalton’s Atomic Theory - 1808• All matter is composed of atoms which

cannot be subdivided• Atoms of same element are identical

(size, mass, reactivity)• Atoms combine to form compounds in

simple, whole # ratios• Chemical reactions involve the

separation, combination, or rearrangement of atoms; it does not result in their creation or destruction

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Dalton’s Theory• He deduced that all

elements are composed of atoms. Atoms are indivisible and indestructible particles.

• Atoms of the same element are exactly alike.

• Atoms of different elements are different.

• Compounds are formed by the joining of atoms of two or more elements.

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Thomson Model• He proposed a

model of the atom that is sometimes called the “Plum Pudding” model.

• Atoms were made from a positively charged substance with negatively charged electrons scattered about, like raisins in a pudding.

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Thompson• Discovered the electron• Determined the charge-to-mass

ratio of an electron

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Cathode Ray Experiments• Any metal worked

for anode• Negative electric

field repelled beam• Object placed in

path of glow blocked beam

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Evidence & Conclusions• cathode rays consisted of

subatomic particles from atoms of anode

• cathode rays are negatively charged

• must also be positive charge• Millikan calculated electron’s mass

to be 9.11 x 10-31 kg

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Thompson’s Atomic Model

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RUTHERFORD ATOMIC MODEL– Most of the positively

charged “bullets” passed right through the gold atoms in the sheet of gold foil without changing course at all.

– Some of the positively charged “bullets,” however, did bounce away from the gold sheet as if they had hit something solid. He knew that positive charges repel positive charges.

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The Gold Foil Experiment: Setup

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The Gold Foil Experiment: Hypothesis

• The α-particles will pass straight through the atoms

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The Gold Foil Experiment: Outcome

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What’s happening?

Rutherford’s Model• Rutherford’s model describes an

atom as mostly empty space, with atom as mostly empty space, with a center a center nucleusnucleus that contains nearly all the mass– Like the pit in a peachpit in a peach

• The nucleus is tiny compared to the atom as a whole.

• Image a marble is the nucleus. Place it on the 50 yard line in a stadium. Now place several electrons in random seats. The rest of the stadium/atom is empty!

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Rutherford’s Problems• How is nucleus held together?• Why don’t electrons collapse into

nucleus?• H atom has 1 proton & He atom has

2 protons, mass ratio should be 2:1; instead the ratio is 4:1…there must be another particle

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The Neutron• Discovered by James Chadwick in

1932.• Neutron is electrically neutral &

has slightly greater mass than a proton

Mystery solved.

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The Bohr Model

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Bohr Model• According to

Bohr’s atomic model, electrons move in definite orbits around the nucleus, much like planets circle the sun. These orbits, or energy levels, are located at certain distances from the nucleus.

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Emission Spectrums• When electricity is run through a

sample of hydrogen gas, hydrogen atoms gain energy

• H atoms loose that energy by emitting photons

• Resulting spectrum is discontinuouscontinuous

discontinuous

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What’s happening?

Energy Levels The energy that an

electron has is based on its location around the nucleus. (Electrons that are closer to the nucleus have less energy than those that are farther away from the nucleus)

How can bookshelves help you understand the movement of electrons?

•Each shelf represents an energy level

•Each book represents an electron

•You can move a book to a higher or lower shelf with the correct amount of energy.

•A book cannot be between shelves

(An electron can move by gaining or losing energy but can never be between energy levels)

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Bohr Model• Electrons move in

circular orbits around the nucleus

• Only certain energy levels are “permitted” (this explains the discrete lines for the emission spectrum of hydrogen)

Bohr’s Problems• Theory only explained behavior of

Hydrogen’s electron• Violates Heisenberg’s Uncertainty

principle- claimed to know exactly where an electron is and what it was doing.

Electron Orbital Notation• e- don’t actually zoom

around the nucleus on little electron highways as seen in the Bohr Model of the Atom.

• THEREFORE– true electron

configuration isn’t that easy!

What are Quantum Numbers?

Quantum number are a set of four values that define theenergy state of an electron in an atom.

Quantum number values are designated as n, l, m and s(s is often written as ms )

n is called the principal quantum number and rangesfrom 1, 2, 3, etc. (also refers to the energy level or shell

l represents the orbital type and depends on n. It rangesfrom 0 through n – 1. It often called the azimuthal

quantum number

m depends on l. It ranges from – l thru 0 to + l. It definesthe orbital orientation in space and is call the magnetic

quantum number.

S is the spin number and is either + ½ or – ½

Quantum numbers may be view as an electrons address.Just like your address, each has its own distinct set of values.

For example in order to receive a letter, the address must containstate and zip, city, street and name. No other person has

the exact same set of information. It is similar for electrons.They each have their own address, n, l, m, and s.

NO TWO ELECTRON IN AN ATOM CAN HAVE THEEXACT SAME SET OF QUANTUM NUMBERS.

QUANTUM NUMBERS ARE ASSIGNED TO EACHEACH ELECTRON USING THE RULES PREVIOUSLYSTATED, STARTING FROM THE LOWEST VALUES.

Assigning Quantum Numbers

Orbital types defined by the azimuthal quantum number

l = 0 s type orbital

l = 1 p type orbital

l = 2 d type orbital

One orientation

Three orientations

Five orientations

l = 3f type orbital Seven orientations (not shown)

Assigning Quantum Numbers to Atoms

n l m satom

H (1 e-) 1

Lowest possible n value

0

Lowest possible l value (n – 1)

0

Lowest possible m value (-l > 0 > +l)

-½

Lowest possible m value (- ½ or + ½ )

Assigning Quantum Numbers to Atoms

n l m satom

He (2 e-) 1 0 0 -½

1 0 0 +½

This time we can use the same n, l and m values as the firstelectron and still get a different set of values by changing

s to = + ½ Energy level 1 is now complete. We are at the end of

period (row) 1 on the Periodic Table.

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Wave Model

This model is based upon Bohr’s model, except that electrons orbit the nucleus in random patterns. The region where these particles are found is referred to as the electron cloud.

Electron Clouds

NucleusNext SlideNext Slide

Current Model

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The Wave Model• Today’s atomic

model is based on the principles of wave mechanics.

• According to the theory of wave mechanics, electrons do not move about an atom in a definite path, like the planets around the sun.

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The Wave Model• In fact, it is impossible to determine the exact

location of an electron. The probable location of an electron is based on how much energy the electron has.

• According to the modern atomic model, at atom has a small positively charged nucleus surrounded by a large region in which there are enough electrons to make an atom neutral.

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Electron Cloud:• A space in which

electrons are likely to be found.

• Electrons whirl about the nucleus billions of times in one second

• They are not moving around in random patterns.

• Location of electrons depends upon how much energy the electron has.

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Electron Cloud:• Depending on their energy they are

locked into a certain area in the cloud.• Electrons with the lowest energy are found in

the energy level closest to the nucleus• Electrons with the highest energy are found

in the outermost energy levels, farther from the nucleus.

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Indivisible

Electron

Nucleus

Orbit Electron Cloud

Greek XDalton XThomson XRutherford

X X

Bohr X X XWave X X X

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Modern Atomic Theories

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Modern View of Dalton’s Atomic Theory

3 major differences between modern atomic theory & Dalton’s atomic theory:

• Atoms are NOT indivisible – they are made up of protons, neutrons, and electrons

• Atoms of the same element are NOT exactly alike – they can have different masses (isotopes)

• Atoms CAN be changed from one element to another, but not by chemical reactions (nuclear reactions)

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Atomic Structure & Isotopes

Atoms of the same element are not

necessarily alike . . .

Same atomic number

different mass number

Isotopes…Are atoms that have the same number of protons, but a different number of neutrons.

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Isotopes• All atoms in an element have the

same atomic number• However, 2 atoms of the same

element can have different mass numbers – called isotopes

• Isotopes have:– Same # of p+

– Different # of no

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Some Common Isotopes

H

H

H

11

21

31

C

C

C

126

136

146

U

U

23592

23892

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Relative Abundance

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Mass Spectrometry• Technique used to determine

atomic mass

e-

Atom bombarded by stream of high energy electrons

e-

e- collides with atom, “bounces” off, but transfers some energy to it

e-

+Atom dissipates excess energy by expelling an electron

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Mass Spectrometry, cont.• Ions are accelerated through a magnetic field• Amount of deflection depends on the ion’s mass• Highest mass deflected least• Lowest mass deflected most

N

S++ +

+

++++

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Mass Spectrometry, cont.

Mass (amu)

Sample mass spec for chlorine

Relative abundance of each isotope can be determined from relative peak heights

35 37

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Relative Abundance & Atomic Mass

• Relative isotopic abundance is then used to calculate atomic mass

• Atomic mass is the weighted average of the mixture of isotopes

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Example

average atomic mass = (atomic mass 35Cl)(fraction 35Cl) + (atomic mass 37Cl)(fraction 37Cl)= (34.968 amu)(0.7577) + (36.965 amu)(0.2423)= 35.45 amu

Calculate the atomic mass of Cl given the relative abundances of its isotopes:35Cl – 75.77%37Cl – 24.23%

Einstein• 1879-1955• The End

Do Theories in Science Stay the Same?• Ideas and theories in Science change as

new iinformations gathered.

Our theory about the atom has changed over time as new studies are done. Even though no one has ever seen an atom up close we are still able to make new discoveries – just like we have made new discoveries about dinosaurs.

The FUTURE Since atomic theory is just a theory, we will continue to challenge it and hopefully find more new and exciting things in our atom

The FUTURE Since atomic theory is just a theory, we will continue to challenge it and hopefully find more new and exciting things in our atom

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