Atomic Structure ©2011 University of Illinois Board of Trustees .

Atomic StructureAtomic Structure

http://gallery.mudpuddle.co.nz/d/2344-2/orbitals.png

©2011 University of Illinois Board of Trustees •

http://islcs.ncsa.illinois.edu/copyright

DemocritusDemocritus

• Greek philosopher ~ 460-370 B.C.

• Described the world as composed of “atomos”– “a” not, unable– “tomos” divisible

• World made of invisible, indivisible units

• Forgotten theory for many years

• No evidence, only idea©2011 University of Illinois Board

of Trustees • http://islcs.ncsa.illinois.edu/copyright

Dalton’s Atomic TheoryDalton’s Atomic Theory

1. Elements are made of tiny, indivisible particles called atoms.

2. All atoms of a given element are identical.

3. Atoms of a given element are different from those of any other element.



Dalton’s Atomic TheoryDalton’s Atomic Theory

4. Atoms of one element combine with atoms of other elements to form compounds in definite proportions.

5. Atoms are joined, separated, or rearranged during chemical reactions.



Observing an AtomObserving an Atom

Atoms can be seen with a scanning tunneling microscope.

Iron atom viewed by

STM



Discovery of the ElectronDiscovery of the ElectronIn 1897, J.J. Thomson used a cathode ray tube to deduce the presence of a negatively charged particle.

Cathode ray tubes pass electricity through a gas that is contained at a very low pressure.

Cathode Ray Demo



http://www.its-about-time.com/htmls/ac/cathode-ray_large.wmv

Conclusions from the Study Conclusions from the Study of the Electronof the Electron

Cathode rays have identical properties regardless of the element used to produce them. All elements must contain identically charged electrons.

Atoms are neutral, so there must be positive particles in the atom to balance the negative charge of the electrons

Electrons have so little mass that atoms must contain other particles that account for most of the mass (Millikan)



Thomson’s Atomic Thomson’s Atomic ModelModel

Thomson believed that the electrons were like plums embedded in a positively charged “pudding,” thus it was called the “plum pudding” model.



Rutherford’s Gold Foil Rutherford’s Gold Foil ExperimentExperiment

Alpha particles are helium nuclei Particles were fired at a thin sheet of gold foil Particle hits on the detecting screen (film) are recorded

Gold Foil Experiment©2011 University of Illinois Board of Trustees •


http://www.shsu.edu/~chm_tgc/sounds/pushmovies/l2ruther.gif

Try it Yourself!Try it Yourself!In the following pictures, there is a target hidden by a cloud. To figure out the shape of the target, we shot some beams into the cloud and recorded where the beams came out. Can you figure out the shape of the target?



The AnswersThe Answers

Target #1 Target #2



Rutherford’s FindingsRutherford’s Findings

The nucleus is small The nucleus is dense The nucleus is positively charged Atom is mostly empty space

Gold Foil – Microscopic View

Most of the particles passed right through A few particles were deflected VERY FEW were greatly deflected

“Like howitzer shells bouncing off of tissue paper!”

Conclusions:



http://micro.magnet.fsu.edu/electromag/java/rutherford/

NeutronNeutron

• Discovered by James Chadwick in 1932

Neutron- subatomic particle found in the nucleus- no charge, mass nearly equal to a proton



Properties of Subatomic Properties of Subatomic ParticlesParticles

Particle Symbol

Charge

Mass(Compare

d to proton)

Actual Mass(g)

Electron

e- 1- 1/1840 9.11 x 10-28

Proton p+ 1+ 1 1.67 x 10-24

Neutron

n0 0 1 1.67 x 10-24©2011 University of Illinois Board


Atomic NumberAtomic NumberAtomic number (Z) of an element is the number of protons in the nucleus of each atom of that element.

*Elements are different because they contain different numbers of protons.

Element # of protons Atomic # (Z)

Carbon 6 6

Phosphorus 15 15

Gold 79 79



Mass NumberMass NumberMass number is the number of protons and neutrons in the nucleus of an isotope.Mass # = p+ + n0

Nuclide p+ n0 e- Mass #

Oxygen - 10

- 33 42

- 31 15

8 8 1818

Arsenic 75 33 75

Phosphorus 15 3116

How many neutrons are in:

1. Fluorine – 19

2. Chromium – 52

3. Gold - 197

$ Some problemswith 18, 75, 31.



IsotopesIsotopesIsotopes are atoms of the same element having different masses due to varying numbers of neutrons.Isotope Proto

nsElectron

sNeutron

sNucleus

Hydrogen–1

(protium)

1 1 0

Hydrogen-2

(deuterium)

1 1 1

Hydrogen-3

(tritium)

1 1 2



Atomic Atomic MassesMasses

Isotope Symbol Composition of the nucleus

% in nature

Carbon-12

12C 6 p6 n

98.89%

Carbon-13

13C 6 p7 n

1.11%

Carbon-14

14C 6 p8 n

<0.01%

Atomic mass is the average of all the naturally occurring isotopes of that element.

Carbon = 12.011



Niels BohrNiels Bohr• Student of Rutherford

• Wanted to explain:– Location of

electrons– Line spectra

Spectra of Gases

• Began investigating Hydrogen atoms in terms of energy

“Those who are not shocked when they first come across quantum mechanics cannot possibly have understood it.”

~ Niels Bohr©2011 University of Illinois Board of Trustees •


http://astro.u-strasbg.fr/~koppen/discharge/discharge.html

Electromagnetic RadiationElectromagnetic Radiation



Bohr’s Planetary ModelBohr’s Planetary Model

• Electrons orbit nucleus like planets do the sun.

• Electrons absorb energy (photon) and move to higher energy levels– ground excited

• Electrons release energy and move to lower orbitals– excited ground– light is emitted

creating a line spectrum

Energy and Orbital Applet

*Only explained line spectrum of hydrogen!!!

http://ths.sps.lane.edu/chemweb/unit8/problems/bohr/H%20Atom.gif



The Bohr Model of the AtomThe Bohr Model of the Atom

Neils Bohr

I pictured electrons orbiting the nucleus much like planets orbiting the sun.But I was wrong! They’re more like bees around a hive.

WRONG!!!



Electron: Particle, Wave, or Electron: Particle, Wave, or Both???Both???

Quantum Mechanics Video

• What were the two main theories about the electron and who were the scientists?

• What evidence supported each theory?• How was the differences between the two

schools of thought resolved?• So which is it? Particle? Wave? Both?

Neither?©2011 University of Illinois Board


Quantum TheoryQuantum Theory• Electrons exist in

orbitals– a.k.a. density

clouds– Probable regions

where electrons exist

– Different shapes (s, p, d, f)

– Electrons behave like waves and particles

– Impossible to know both the location and momentum of an electron

– “Beehive” model$ simultaneously!



Heisenberg’s Uncertainty Heisenberg’s Uncertainty PrinciplePrinciple



Heisenberg Uncertainty Heisenberg Uncertainty PrinciplePrinciple

• It is impossible to know both the exact momentum of an object and its location simultaneously

• Why?– To observe

electron light must interact with it.

– Light alters electron’s position



Quantum ModelQuantum Model



Modern Beliefs about the Modern Beliefs about the AtomAtom

Atoms of all elements on Earth have a characteristic average mass which is unique to that element.

Atoms cannot be subdivided, created, or destroyed in ordinary chemical reactions. However, these changes CAN occur in nuclear reactions!

All matter is composed of atoms

Atoms of any one element differ in properties from atoms of another element

Atom Video©2011 University of Illinois Board


ContinuedContinued

Most of the mass of the atom is in the nucleus (protons and neutrons) Electrons are found outside of the nucleus (the electron cloud) Most of the volume of the atom is empty space

Constituents Video“q” is a particle called a “quark”



Quantum NumbersQuantum Numbers

Quantum numbers– are used to describe the “address of an

electron.”– Specify properties of atomic orbitals and

the electrons inside orbitals

State – principal quantum numberTown – angular momentum quantum number,

azimuthal, designated as lStreet – magnetic quantum numberHouse number – spin quantum number



1. Principal Quantum 1. Principal Quantum Number (n)Number (n)• Probable distance

from nucleus• Represents main

energy level or shell• Positive integers

(i.e. 1, 2, 3, 4…)• Smaller the number

the closer to the nucleus (ex: n=1, closest to nucleus)

• As “n” increases, energy increases

• Known elements utilize n=1 to n=7

• Number electrons in shell = 2n2

How many electrons can fit into the 1st energy level? 3rd?



2. Angular Momentum Quantum 2. Angular Momentum Quantum

Number, Azimuthal ( Number, Azimuthal ( l l ))• Indicates the shape of the orbital

– s, p, d, f• # of possible orbitals = principal

quantum number (n)



3. Magnetic Quantum Number 3. Magnetic Quantum Number (m(mll))

Orbital - An orbital is a region within an energy level where there is a probability of finding an electron. Only 2 electrons can occupy one subshell.

• Describes the orientation of a subshell around the nucleus

• Think 3-D with x, y, z axis– s orbital = 1 orientation– p orbital = 3 orientations– d orbital = 5 orientations– f orbital = 7 orientations©2011 University of Illinois Board


The s-orbital has a spherical shape centered aroundthe origin of the three axes in space.

s orbital



There are 3-p orbitals beginning at the 2nd energy level.

p orbitalp orbital

$ Need the xyz axes to agree with other sketches.©2011 University of Illinois Board


5 d-orbitals beginning at the 3rd energy level

d orbital



f orbitalf orbital

There are 7 f-orbitals beginning with the 4th energy level.



4. Spin Quantum Number 4. Spin Quantum Number (m(mss))

• Electron spin describes the behavior (direction of spin) of an electron within a magnetic field.

• Only 2 electrons can occupy an orbital with opposite spins.

Possibilities for electron spin:1

2

1

2

clockwise or counterclockwise©2011 University of Illinois Board


Energy

Level (n)

Sublevels inmain energylevel

(n sublevels

)

Number oforbitals per

sublevel

Number ofElectrons

per subleve

l

Number ofelectrons

permain energy

level (2n2)

1 s 1 2 2

2 sp

13

26

8

3 spd

135

2610

18

4 spdf

1357

261014

32

Energy Levels, Sublevels, Energy Levels, Sublevels, ElectronsElectrons



Electron ConfigurationsElectron Configurations

Electron configurations– arrangement of electrons within an

atom around a nucleus– each element has a unique electron

configuration



Rules for Writing Electron Rules for Writing Electron ConfigurationsConfigurations

1. Aufbau Principle– Electrons occupy the orbitals of

lowest energy first.– Not the expected order– 1s2s2p3s3p4s3d4p5s4d5p6s4f5d6p7s

5f6d

Why?- Beginning with 3rd energy level,

orbitals begin to overlap.- 4s is occupied before 3d because it is

at a lower energy©2011 University of Illinois Board


Aufbau DiagramAufbau Diagram




2. Pauli Exclusion Principle– No two electrons in the same atom can

have the same set of 4 quantum numbers.

– An orbital can hold 2 electrons with opposite spins.




3. Hund’s Rule– Orbitals of equal energy are occupied

by one electron before any orbital is occupied by a second electron.

– All electrons in singly occupied orbitals have the same spin.



Types of Electron Types of Electron ConfigurationsConfigurations

1. Orbital notation– Lines, boxes, or circles represent

orbitals– Energy level and sublevel go under

the line.– Up and down arrows represent

electrons and their spin

Orbital Notation of

Elements 3-11©2011 University of Illinois Board


Practice…Practice…

Ex: Draw the electron configurations of the following atoms using orbital notation.

1. Helium2. Boron3. Iron4. Rubidium




2. Electron Configuration– Eliminates lines and arrows.– Number of electrons is designated by

a superscript next to the subshell letter

Electron Configurations for First 3 Periods



Practice…Practice…

Ex: Write the electron configurations for the following elements:

1. Helium2. Boron3. Iron4. Rubidium




3. Noble gas notation– Same format as electron configuration– Uses the preceding noble gas (Group

18) as a starting point– Encloses noble gas symbol in brackets

Ex: Draw the electron configurations of the following atoms using noble gas notation.

1. Chlorine2. Iron3. Cadmium4. Tungsten©2011 University of Illinois Board


Types of ElectronsTypes of ElectronsValence shell electrons

– Electrons in the outermost principal energy level

– Are always in “s” or “s” and “p” orbitals– Maximum number is 8!

Ex: How many valence electrons are in:1. Carbon2. Magnesium3. Bromine4. Radon

Octet – filled (contain 8 e-) “s” and “p” orbitals in the highest energy level; very stable.©2011 University of Illinois Board


Types of ElectronsTypes of ElectronsInner shell electrons

– Electrons that are not in the highest principal energy level.

– All electrons but valence electrons.

# inner shell e- = (total e-) - (valence e-)

Ex: How many inner shell electrons are in:1. Sulfur2. Mercury3. Lead



Types of ElectronsTypes of ElectronsUnpaired electrons

– Only one electron in an orbital

Ex: How many unpaired electrons are in:1. Nitrogen2. Oxygen3. Sulfur4. Sodium5. Chlorine



ResourcesResources• PowerPoint was adapted from Mr. Allan of El Diamante High

School at http://www.sciencegeek.net/Chemistry/Powerpoint/Unit1/Unit1_files/frame.htm

• Cathode Ray Video http://www.its-about-time.com/htmls/ac/cathode-ray_large.wmv

• Gold Foil Experiment Video http://www.shsu.edu/~chm_tgc/sounds/pushmovies/l2ruther.gif

• Gold Foil Experiment Applet http://micro.magnet.fsu.edu/electromag/java/rutherford/

• Gas Spectra Applet http://astro.u-strasbg.fr/~koppen/discharge/discharge.html

• Energy and Orbitals Applet http://www.colorado.edu/physics/2000/quantumzone/bohr.html

• Quantum Mechanics Video http://www.youtube.com/watch?v=Y8IQbL_DnGk

• Atom Video http://www.youtube.com/watch?v=vUzTQWn-wfE

• Constituents Video http://www-personal.umich.edu/~jach/constituents/



Atomic Structure ©2011 University of Illinois Board of Trustees .

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Transcript of Atomic Structure ©2011 University of Illinois Board of Trustees .