Atom Lec Ppt

8/6/2019 Atom Lec Ppt

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ATOMIC STUCTURE


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Objectives:

y Learn about the brief historical background on theconcepts of the atomic theory

y Review on the atomic structure, atomic number, massnumber and the electronic arrangements of atoms.

y Learn on the structure of the atomic nucleus


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Internal Structure of an AtomInternal Structure of an Atom

The Atomic Model

Is the atom really fundamental?

The atom is composed oftiny, dense positive nucleus andclouds of negative electrons


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AtomicTheories

Democritus- gave the name atomos-indivisible

Antoine Lavoisier- made Chemistry a science; proposedLaw of Conservation of Matter

John Dalton- proponent of Daltons AtomicTheories

1.Elements consist of indivisible particles called atoms.

2.Atoms of the same elements are identical.

3.Atoms can neither be created nor destroyed in a chemical reaction.

4.Atoms of different elements are different and have different masses.

5. Atoms chemically combine in definite whole number proportions

to form compounds.


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y Neils Bohr- proposed quantum theory

1. Electrons could only reside in certain energy level

2. Primary shells, labeled as 1,2,3,4

3. Subshells as s(1), p(3), d(5), f(7)


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Modern AtomicTheory

y Atoms are complex and not the simplest particles.

y Not all atoms of the same elements are alike becausesome elements called Isotopes do not have the same

mass.


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Subatomic Particle

y A very small particle that is a building block for atoms.

ThreeTypes of Subatomic Particles:

y The atom contains:

Electrons found outside the nucleus; negativeelectrical charge; smallest mass.

Protons found in the nucleus; positive charge equal

in magnitude to the electrons negative charge

Neutrons found in the nucleus; no charge; virtuallysame mass as a proton


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QuarksTheoryQuarksTheory

What is the world made of?

In the ancient times, people believed and

organized everything with the fourfundamental elements: Fire,Air, Earth

and Water.

By the word "fundamental, we mean

objects that are simple and

structureless -- not made of anything

smaller.


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IstheNucleusFundamental?

Are protons and

neutrons fundamental?


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Are protons and neutrons fundamental?

Physicists have discovered that

protons and neutrons arecomposed of even smaller particlescalled quarks.


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TheModern AtomicModel

Electrons are in constant motionaround the nucleus, protons and

neutrons jiggle within the nucleus,and quarks jiggle within the

protons and neutrons.


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How do wescalethe atom?

The nucleus is 10,000 timessmaller than the atom

The electrons and quarks are 100million times smaller than the atom


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The Standard Model

This model explains what the world is and what holds ittogether

Atom is composed of only:

-6 quarks

-6 leptons

-Force carrier particles


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Trivia Question

y

Forhow manyyears have physicistsknownthatthere were morethan just protons,

neutrons,electrons, and photons? 5? 25?

60? 100?

Answer: 60 years! In the 1930's physicists found muons,

but hundreds more were found with high energyaccelerators in the 1960's and 1970's.


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Quarks and Leptons

All these are made up of quarksand leptons


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Matterand Antimatter

For every type of matter particle we've found, therealso exists a corresponding antimatterparticle, orantiparticle.

This picture of the bubblechamber shows the evidence of

antimatter.

The magnetic field in this chambermakes negative particles curl leftand positive particles curl right.

One such electron-positron pair ishighlighted

1953-1970s


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Quarks

There are six quarks in threepairs: up/down, charm/strange, andtop/bottom

Quarks have the unusual

characteristic of having afractional electric charge

Quarks also carry another type ofcharge called colorcharge


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Hadrons, Baryons, and Mesons

y

Quarks only exist in groups called Hadronsy Hadrons are classified into :

1. Baryons- made up of 3 quarks

2. Mesons- 1 quark and 1 antiquark

With the hadrons given above, which one is thebaryon?The meson?


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Baryons

y Protons are baryons made up of two up quarksand one down quark (uud)

y Neutrons are also baryons made up of one upquark and two down quarks (udd)


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Mesons

y These particles are very unstable because they

are composed of a particle and an antiparticle


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Leptons

y There are six leptons, three of which have electricalcharge and three of which do not.

y The best known lepton is the electron (e-).The othertwo charged leptons are the muon() and the tau()

The other leptons are thethree types ofneutrinos ( ).

They have no electricalcharge, very little mass, andthey are very hard to find.


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Neutrinos

y a type of lepton

y no electrical or strong charge

y

are produced in a variety of interactions,especially in particle decays

y rarely interact with matter

y Their tiny mass but huge numbers may

contribute to total mass of the universe andaffect its expansion


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QuarksTheory Summary

y The most fundamental matter particles are thesix quarks and the six leptons.

y We never find isolated quarks, we only observethem in composite particles called hadrons.

y

And there is an antimatterparticle for everymatter particle.


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Charge and Mass Characteristics


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y The nucleus is:

Small compared with the overall size of theatom.

Extremely dense; accounts for almost all ofthe atoms mass.

Positively charged center of an atom


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ChargeNeutrality of an Atom

y An atom as a whole is electrically neutral

(no net electrical charge)

# of Protons = # of Electrons


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AtomicNumber& MassNumber

y Atomic Number (Z) # of protons in thenucleus of an atom

y Mass Number (A) sum of the # of protons andthe # of neutrons in the nucleus of an atom


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CompleteChemical Symbol Notation

23

11Na


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Exercises

1. What is the mass number of an atom containing:

a) 58 protons, 58 electrons and 78 neutrons

b) 17 protons, 17 electrons and 20 neutrons

c) 15 protons, 15 electrons and 16 neutrons

d) 86 protons, 86 electrons and 136 neutrons

2. Name the element given in problem 1a and 1b and writethe symbols for their atomic nuclei.


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Element

y A pure substance in which all atoms present havethe same atomic number.

y All atoms with the same atomic number have thesame chemical properties and are atoms of thesame element.


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Isotopes

y Atoms of an element that have the same numberof protons and the same number of electronsbut different numbers of neutrons.

y Show almost identical chemical properties;chemistry of atom is due to its electrons.

y Physical properties are often slightly differentbecause they have different masses.

y In nature most elements contain mixtures ofisotopes.


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Two Isotopes of Sodium

23

11Na

24

11Na

Numberof Protons = 11

Mass number= 23

NumberofNeutrons = 12

Numberof Protons = 11

Mass number= 24

NumberofNeutrons = 13


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Exercise

A certain isotope X contains 23 protons and 28neutrons.

y What is the mass number of this isotope?


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Exercises

y How many neutrons are in each isotope of Oxygen?Write the symbol of each isotope.

a) Oxygen-16

b) Oxygen-17

c) Oxygen-18

y Two iodine isotopes are used in medical treatments:

iodine-125 and iodine-131. How many neutrons are ineach isotope?Write the symbol for each isotope.


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AtomicMasses

y Elements occur in nature as mixtures of isotopes

y Carbon = 98.89% 12C

1.11% 13C


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Exercise

1. An element consists of62.60% of an isotope withmass 186.956 amu and 37.40% of an isotope withmass 184.953 amu.

y Calculate the average atomic mass and identifythe element.

Average Atomic Mass = 186.207 amu

The element is rhenium (Re).


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2. Magnesium occurs in nature in 3 isotopic forms: Mg-24,(78.70% abundance), Mg-25 (10.13% abundance) andMg-26 (11.17% abundance).Their relative masses are23.985, 24.986 and 25.983 amu, respectively. Calculate

the atomic mass of Magnesium.


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Atomic Subshells

ssubshell


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p subshell


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d subshell


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fsubshell

y Seven sets of four pear-shaped lobes around the different

planes of the atom. This is extremely difficult to draw.


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Quantum Mechanical Model ofthe Atom


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Arrangement oftheOrbitalsin the Atom


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Electron Configuration & Orbital Diagram

Rules for Assigning Electrons to Various Shells, Subshells and

Orbitals:

1. Electron subshells are filled in order of increasing energy.

2. Electrons occupy the orbitals of a subshell such that eachorbital acquires one electron before any orbital acquires

a second electron. All electrons in such singly occupiedorbitals must have the same spin.

3. No more than two electrons may exist in a given orbitaland then only if they have opposite spins.


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Subshell Energy

Order


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Electron

Configurations


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Electron Configurations

y A statement of how many electrons an atom hasin each of its electron subshells.

y An oxygen atom has an electron arrangement oftwo electrons in the 1s subshell, two electrons inthe 2s subshell, and four electrons in the 2psubshell.

Oxygen: 1s22s22p4


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Orbital Diagrams

y A notation that shows how many electrons anatom has in each of its occupied electron orbitals.

Oxygen: 1s22s22p4

Oxygen: 1s 2s 2p


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Exercise

y Determine the expected electron configurations for eachof the following.

a) S

1s22s22p63s23p4

b) Ba

1s22s22p63s23p64s23d104p65s24d105p66s2


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y

Determine the electron configuration and orbital diagramof the following elements:

a) Carbon

b) Neonc) Sodium

d) Phosphorus


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y The electron arrangement in the outermost shell is thesame for elements in the same group.

y This is why elements in the same group have similarchemical properties.

Group 1A very reactive

Li: 1s22s1

Na: 1s2

2s2

2p6

3s1

K: 1s22s22p63s23p64s1


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Electron Configurations and the Periodic Table


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Distinguishing Electron

y Last electron added to the electron configuration for anelement when electron subshells are filled in order ofincreasing energy.

y This last electron is the one that causes an elementselectron configuration to differ from that of an elementimmediately preceding it in the periodic table.


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Radioactivity


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Discovery of Radioactivity

y 1896 by French Physicist Henri Becquerel

y Exposure of uranium salt to sunlight until theyphosphoresced

y Phosphorescing salts emit some type of radiation evenafter the phosphorescence has ceased

y Awarded in 1903 Nobel Prize for Physics


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Radioactivity

4 major types:

y Alpha particle (helium nuclei); has 2 protons & 2

neutrons; each has an atomic number of 2 and charge of+2

y Beta particle (electron); charge of-1

y Gamma rays (high-energy photons); no mass or charge

y Positrons (positively charged electrons)

*Each type behaves differently when it passes between

electrically charged plates

*radioactivity comes from the atomic nuclei and not

from the electron cloud


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Wavelength

y Greek letter lambda ()

y Distance from one wave crest to the next

Frequency

y Greek letter nu(v)

y

Number of crests that pass a given point in one second

*the lowerthe frequency,the longerthe wavelength;theshorter

the frequency,the longerthe wavelength

*thehigherthe frequency,thehighertheenergy


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Long (), low (v), low (E)

Short (), high (v), high (E)


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What happens when a nucleusemitsradioactivity?

A. Radioactive & stable nuclei

y >300 naturally occurring isotopes; 264 of these are

stable (nucleidoesnotgive offradioactivity); rest areradioactive isotopes

y >1000 artificial isotopes all of which are radioactive

y Stable nuclei are isotopes in which there are equal

number of protons and neutrons

*in lighterelements- # ofprotons & neutrons are approximately

equal(C-12, O-16)*inheavierelements- stabilityrequires moreneutronsthan

protons(Pb-206)


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y If there is imbalance between the proton-to-neutron ratio(either too few or too many neutrons), a nucleus willundergo nuclear reaction to make the ratio favorable orthe nucleus more stable

B. Beta Emission

y Process by which a nucleus emits an electron or a betaparticle after it converted its excess neutrons intoprotons and electrons to become stable

y Ex: P-32 S-32 + electron

y Transmutation is the changing of one element intoanother every time an element gives off a beta particle


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How to balance a NuclearEquation?

2 rules for balancing nuclear equation:

1) The sum of the mass numbers (superscripts) on bothsides of the equation must be equal.

2) The sum of the atomic number (subscripts) on bothsides of the equation must be equal. For the purposes ofdetermining atomic numbers in a nuclear equation, anelectron emitted from the nucleus has an atomic number

of-

1.


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C. Alpha Emission

y

Loss of alpha particle in heavy elements is an importantstabilization process

y Ex: U-238 Th- 234 + Helium

*as a general rule: thenewnucleus alwayshas a massnumberfourunits lowerandan atomicnumbertwo units lowerthan

the originalnucleus

y Ex: Polonium-218 is an alpha emitter.Write anequation for this nuclear reaction, and identify the

product.


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D. Positron Emission

y particle that has the same mass as an electron but acharge of +1 rather than -1

y no appreciable mass; rarer than alpha and beta emission

*nucleusistransmutedinto anothernucleus withthesame massnumberbut an atomicnumberthatis oneunit less

y Ex: C-11 B-11 + electron(+1)

y Ex: N-13 is a positron emitter.Write an equation

for this nuclear reaction and identify the product.

y Ex: As-74 is a positron emitter used in locating

brain tumors.Write an equation for this nuclearreaction and identify the product.


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E. Gamma Emission

y often accompanies alpha and beta emissions

y

Ex: B*-11 B

-11 + y

F. Electron Capture

y extranuclear electron is captured by the nucleus and

inside reacts with a proton to form a neutron*atomic number of the element is reduced by 1, but the

mass number is unchanged

y Ex: Be-7 + electron (-1) Li-7

y

Ex: Chromium-51 decays by electron capture &gamma emission.Write an equation for this nuclear

decay and identify the product.

Wh t i l h lf lif ?


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Whatis nuclearhalf-life?

y time it takes for one-half of any sample of radioactivematerial to decay

y radioactive material decay one at a time and at a fixed rate

y written as t

y independent of temperature and pressure; a property of aparticular isotope

y no known way to speed up radioactive decay or to slow itdown

y Usefulness and inherent danger in radioactive isotopes isrelated to their half-lives


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y Ex: If 10 mg of Iodine-131 is administered to a patient,much is left in the body after 32 days? (t of I-131 is 8days)

y Ex: Barium-122 has a half -life of 2 minutes. Suppose you

obtain a sample weighing 10 g. It takes 10 minutes to setup the experiment in which barium-122 will be used. How

many grams of barium-122 will be left when you begin theexperiment?

D t ti & M t f N l R di ti


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Detection & Measurement ofNuclearRadiation

Interaction of rays with matter

Electrons are knocked out of the electron cloudssurrounding the nucleus

Resulted to formation of positively charged ions from aneutral atom

y Ionizingradiation- characterized by 2 physical

measurements:1. Intensity- # of particles or photons emerging per unit

time

2. Energy- of each particle or photon emitted


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y Alpha particles- most massive & most highly charged; leastpenetrating; can be stopped by several sheets of paper,ordinary clothing and skin

y Beta particles- have less mass & lower charge than alphaparticles; greater penetrating power; can penetrate

several millimeters of bone

y Gamma particles- no mass nor charge; most penetrating ofthe 3 types of radiation; can pass completely through the

body


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NuclearFusion

y combining of 2 lighter nuclei to form a heavier nucleus

y Ex: fusion of 2 hydrogen nuclei to form a helium nucleuswith the release of a very large amount of energy

H-2 + H-3 He + n + energy

(Deuterium) (Tritium)


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NuclearFission

y splitting of a heavier nucleus into 2 or more smaller nuclei

y releases large amount of energy which can be controlled(nuclear reactors) or uncontrolled (nuclear weapons)

y first observed in the 1930s when scientists in Germany

tried to produce transuranium element by bombardinguranium-235 with neutrons; however had Ba, Kr, neutron,

gamma particle and atomic energy as its products

y used in nuclear power plant

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Atom Lec Ppt