introduction to atomic structure - pbworksyooschem1314.pbworks.com/w/file/fetch/69988747/atoimc...

atomicstructurepresentationStudent Notes Blank 2012.notebook

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August 17, 2012

Introduction to Atomic Structure

Chemistry

Chemistry is the study of

The type of matter that is changing and what types of changes it undergoes determines the field of chemistry that is being studied.

Branches of Chemistry

Physical Chemistry: the study of matter and the physics behind its changes

Biochemistry: the study of the makeup and changes undergone by living species

Analytical Chemistry: The study of the composition (or analysis) of substances

Organic Chemistry: The study of compounds which primarily contain Carbon

Inorganic Chemistry: The study of compounds which do NOT contain Carbon

MatterYou will recall that we define matter as anything

that has mass and takes up space.

Atoms

You will also recall that all matter is made of very small particles called atoms

Atoms are so small that it is only with recent technological and imaging break throughs that we are able to see even the most general shape of an atom.

Light and SoundIn 1905 Einstein derived an equation relating mass and energy. You should be familiar with this equation:

E = mc2

This equation has been changed a bit since, but a relationship has now, for the first time in history, been established between matter and energy, and between physics and chemistry.


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The nature of light has been debated for thousands of years.

In the 1600's, Newton argued that light was a stream of particles. Huygens countered that it was a wave.

Both had good arguments, but neither could prove their case.

The Nature of Light: Wave or Particle?

wave!particle!

If two waves are perfectly in sync with one another or, if the extra distance one sound has to travel is exactly one wavelength longer, the interference will be constructive, causing the waves to feed off one another.

Sound Waves: ___________ Interference

λListener

λ

But they will destructively interfere if one sound travels half a wavelength longer than the other.

λ/2Listener

λ

Sound Waves: __________ Interference

quiet

loud

loud

loud

quiet

d

B

ASo for sounds waves, we expect to get a pattern of maxima (light bands) and minima (dark bands) like this.

Sound waves from 2 sources

But because waves are waves, this would be the case for all waves, not just sound waves.

We can see what is called an "interference" pattern when we look at how 2 waves interact with one another. This pattern is clearly seen in the dark and light bands noticed as the end of the tank.

Ripple Tank Waves

As you can see in the picture, 2 sources interacting with one another creates a pattern of black bands and light bands

source 1 source 2

Interference Patterns: Maxima

Light Band(Maxima)

Light Band(Maxima)

Light Band(Maxima)

Light Band(Maxima)

Light Band(Maxima)


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`

Dark Band(Minima)

Interference Patterns: Minima

Dark Band(Minima)

Dark Band(Minima)

Dark Band(Minima)

Diffraction

As we can see this is true if we have 2 waves forming from 2 different sources, but it is the same when 1 wave from 1 source enters a double slit.

But why?

When a wave, including light, meets an obstacle it bends around it to some extent. When it meets a small opening, the opening generates a new wave on the other side.

1 What principle is responsible for alternating light and dark bands when light passes through two or more narrow slits?

A refractionB polarizationC dispersionD interference

2 What principle is responsible for light spreading as it passes through a narrow slit?

A refraction

B polarization

C diffraction

D interference

Young's Double Slit Experiment

This photo is of light (of one color) striking a distant screen after passing through 2 slits. This only makes sense if light is a wave.

d

L

slitscreen

measurementscreen

x

lightsource

Diffraction and Interference

The double slit experiment relies on two properties of waves:

Each slit generates a new wave due to diffraction. Those waves then either constructively or destructively interfere on a far away screen.

S1

S2

viewing screen

diffraction and interference


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August 17, 2012

DoubleSlit Maxima and Minima

Interference occurs because each point on the screen is not the same distance from both slits. Depending on the path length difference, the wave can interfere constructively (bright spot) or destructively (dark spot).

The electric and magnetic waves are perpendicular to each other, and to the direction of propagation.

Electromagnetic Waves

Young showed that light is a wave.

Maxwell showed that electromagnetic waves exist and travel at the speed of light.

Light was shown to be an electromagnetic wave.

The frequency of an electromagnetic wave is related to its wavelength. For electromagnetic waves (including light), in a vacuum:

Light is an Electromagnetic Wave

For all waves: velocity = wavelength x frequency

Therefore for light:

Electromagnetic Radiation

All electromagnetic radiation travels at the same velocity: the speed of light (c)

c = ____________

3 All electromagnetic waves travel through a vacuum at A the same speed.

B speeds that are proportional to their frequency.

C speeds that are inversely proportional to their frequency.

D none of the given answers

4 In a vacuum, the velocity of all electromagnetic waves

A is zero.B is 3.0 × 108 m/s.C depends on the frequency.D depends on their amplitude.


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5 For a wave, the frequency times the wavelength is the wave's

A speed.B amplitude.C intensity.D power.

6 Electromagnetic radiation travels through vacuum at a speed of

A 186,000 m/sB 125 m/sC 3.00 x 108 m/s

D It depends on wavelength

7 The wavelength of light that has a frequency of 1.20 x 1013 Hz is

A 25 m

B 2.5 x 105 m

C 0.040 mD 2.5 m

c = λf

c = 3.00 x 108 m/s

8 What is the frequency of light whose wavelength is 600 nm?

A 5.0 x 1014 Hz

B 1.0 x 1015 Hz

C 1.5 x 1015 Hz

D 2.0 x 1015 Hz

c = λf

c = 3.00 x 108 m/s

Blackbody Radiation

All objects emit electromagnetic radiation which depends on their temperature: thermal radiation.

A blackbody absorbs all electromagnetic radiation (light) that falls on it.

Because no light is reflected or transmitted, the object appears black when it is cold. However, black bodies emit a temperaturedependent spectrum termed blackbody radiation. For example, the temperature of the above Pāhoehoe lava flow can be estimated by observing its color.

The wave nature of light could not explain the way an object glows depending on its temperature: its spectrum.

In 1900, Max Planck explained it by assuming that atoms only emit radiation in quantum amounts.

Planck’s Quantum Hypothesis

These days, this assumption is regarded as the birth of quantum physics and the greatest intellectual accomplishment of Planck's career.


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_________

where h is Planck’s constant (6.63 x 1034 Js) and f is the frequency of the light

Planck’s PostulateWhen light strikes a metal, electrons sometimes fly off causing an electric current.

The Photoelectric Effect

Classical physics couldn't explain some specific features about how the effect works. So Einstein used Planck's idea to solve it.

If atoms can only emit light in packets of specific sizes; maybe light itself travels as packets of energy given by Planck's formula.

He called these tiny packets of energy or light photons.

The Photon

E = hf

where h is Planck’s constant(6.63 x 1034 Js)

voltagesource

Currentindicator

Radiant energy metal

surface

e

evacuated chamber

WaveParticle Duality

Earlier we proved that light is a wave.

Now we've proven that light is a particle.

So which is it?

9 The ratio of energy to frequency for a given photon gives

A its amplitude.

B its velocity.C Planck's constant.D its work function.

E = hf

c = 3.00 x 108 m/sh = 6.63 x 1034 Jsc = λf

10 What is a photon? A an electron in an excited state

B a small packet of electromagnetic energy that has particlelike properties

C one form of a nucleon, one of the particles that makes up the nucleus

D an electron that has been made electrically neutral


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11 The energy of a photon depends on

A its amplitude.B its velocity.C its frequency.D none of the given answers

12 The photoelectric effect can be explained assuming A that light has a wave nature.B that light has a particle nature.

C that light has a wave nature and a particle nature.

D none of the above

13 The energy of a photon that has a frequency 110 GHz is

A 1.1 × 1020 JB 1.4 × 1022 JC 7.3 × 1023 JD 1.3 × 1025 J

E = hf

c = 3.00 x 108 m/sh = 6.63 x 1034 Jsc = λf

14 The frequency of a photon that has an energy of 3.7 x 1018 J is

A 5.6 × 1015 HzB 1.8 × 1016 HzC 2.5 × 1015 JD 5.4 × 108 JE 2.5 × 1015 J

E = hf

c = 3.00 x 108 m/sh = 6.63 x 1034 Jsc = λf

15 The energy of a photon that has a wavelength of 12.3 nm is

A 1.51 × 1017 JB 4.42 × 1023 JC 1.99 × 1025 JD 2.72 × 1050 JE 1.61 × 1017 J

E = hf

c = 3.00 x 108 m/sh = 6.63 x 1034 Jsc = λf

16 If the wavelength of a photon is halved, by what factor does its energy change?

A 4

B 2

C 1/4D 1/2

E = hf

c = 3.00 x 108 m/sh = 6.63 x 1034 Jsc = λf


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August 17, 2012

Dalton's Postulates

Building on Democritus' idea, in the early 1800s, English chemist John Dalton was the first scientist to observe the physical world and matter and via these observations, this draw some conclusions about atoms.

Dalton's Four Postulates

(1) Matter is made of

(2) All atoms of one element are

Various atoms and molecules as depicted in John Dalton's 1808 book:

A New System of Chemical Philosophy

Dalton’s Four Postulates

(4) ____________ are formed when atoms of more than one element combine; a given compound always has the same relative number and kind of atoms.

Hydrogen Oxygen Water

(3) Atoms of an element are ____ _________ into atoms of a different element by ______________; atoms are _____________________________ in chemical reactions.

Law of Conservation of Mass

The total mass of substances present at the end of a chemical process is the same as the mass of substances present before the process took place.

click here for an explanation of conservation of mass

17 Which one of the following is not one of the postulates of Dalton's atomic theory?

A Atoms are made of protons, neutrons, and electrons.

B All atoms of a given element are identical

C Atoms are neither created nor destroyed in chemical reactions.

D Compounds are formed when atoms of more than one element combine

E Each element is composed of extremely small particles called atoms.

[∗]Discovery of The Electron

Streams of negatively charged particles were found to emanate from cathode tubes.

J. J. Thomson is credited with their discovery (1897).

Electric field plateMagnetic coils

High voltage


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August 17, 2012

Plum pudding model

The prevailing theory was that of the “plum pudding” model, put forward by J. J. Thomson around 1900.

It featured a positive sphere of matter with negative electrons imbedded in it.

So we know that charge is a continuous variable. This was thought in much the same way that many of the properties of light can be explained by treating it as a continuous wave rather than as a stream of photons.

The e is one of the fundamental physical constants and its accurate value is of great importance. In 1923, Millikan won the Nobel Prize in physics in part because of his experiment involving the electron.

Robert Millikan

The currently accepted value of e is:

Knowing e allows the electron mass to be calculated:

Discovery and Properties of the Electron

electrons x integers = charge

Radioactivity

Radioactivity is the spontaneous emission of radiation by an atom.It was first observed by Henri Becquerel.

Marie and Pierre Curie also studied it.

The 3 shared the 1903 Nobel prize in physics for their work.Marie Curie also went on to win the 1911 Nobel Prize for Chemistry.

Radioactivity

Three types of radiation were discovered by Ernest Rutherford:

alpha particlesbeta particlesgamma rays

18 Of the three types of radioactivity characterized by Rutherford, which are particles?

A βrays

B αrays, βrays, and γrays

C γrays

D αrays and γrays

E αrays and βrays

[∗]


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August 17, 2012

GeigerMarsden Experiment(Gold Foil Experiment)

Ernest Rutherford shot alpha particles at a thin sheet of gold foil and observed the pattern of scatter of the particles.

Discovery of the Nucleus

According to the Plum Pudding Model of the atom, if we fire a particle at a wall made of solid particles we would expect most of the particles to bounce right back.

A few of the particles would fly through the "cracks" between the atoms but these gaps would be so small compared to the atoms that most of the particles would bounce off.

Discovery of the Nucleus

What actually happened was very surprising.

Most of the particles flew right through the foil!

19 The gold foil experiment performed in Rutherford's lab __________.

A confirmed the plumpudding model of the atom

B led to the discovery of the atomic nucleus

C was the basis for Thomson's model of the atom

D utilized the deflection of beta particles by gold foil

E proved the law of multiple proportions

[∗]

20 In the Rutherford nuclearatom model:

A the heavy subatomic particles reside in the nucleus

B the principal subatomic particles all have essentially the same mass

C the light subatomic particles reside in the nucleus

D mass is spread essentially uniformly throughout the atom

Subatomic ParticlesProtons were discovered by Rutherford in 1919.Neutrons were discovered by James Chadwick in 1932.

Protons and electrons are the only particles that have a charge.Protons and neutrons have essentially the same mass.

The mass of an electron is so small we ignore it.

Particle Charge Mass (amu)

Proton

Neutron

Electron


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August 17, 2012

Atomic SpectraAn atomic spectrum is a line spectrum – only certain frequencies appear. If white light passes through such a gas, it absorbs at those same frequencies.

Atomic Spectra

Why don't atoms radiate, or absorb, all frequencies of light?

Why do they radiate light at only very specific frequencies, and not at others?

Bohr's Modeln represents the energy level where n=1

is the lowest naturally occupied level or "ground state"

+

n=1

n=2

n=3

The Bohr Atom

These possible energy states for atomic electrons were quantized – only certain values were possible. The spectrum could be explained as transitions from one level to another.

upper

lower

e

Electrons would only radiate when they moved between orbits, not when they stayed in one orbit.

upper

lowere

Absorption

Absorption of electromagnetic radiation is the way by which the energy of a photon is taken up by matter, typically the electrons of an atom.

The electromagnetic energy is transformed to other forms of energy for example, to heat.

Emission

Emission is the process by which a higher energy quantum mechanical state of a particle becomes converted to a lower one through the emission of a photon, resulting in the production of light.

When the electrons in the atom are excited, for example by being heated, the additional energy pushes the electrons to higher energy orbitals. When the electrons fall back down and leave the excited state, energy is reemitted in the form of a photon.


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August 17, 2012

The wavelength of the photon is determined by the difference in energy between the two states. These emitted photons form the element's emission spectrum.

Emission spectrum of Hydrogen

Emission spectrum of Iron

Emission Spectrum Flame Test

Light consists of electromagnetic radiation of different wavelengths.

Therefore, when the elements or their compounds are heated either on a flame they emit energy in form of light.

A flame test is a procedure used in chemistry to detect the presence of certain metal ions, based on each element's characteristic emission spectrum.

The Bohr Atom

Using the Coulomb force, he calculated the energy of each orbit. For hydrogen he arrived at this result:

n = 1, 2, 3, 4, ....

E = 13.6 eVn2

Notice that the energy levels are all negative, otherwise the electron would be free of the atom.

The levels get closer together, and closer to zero, as n increases.1

2

345

n

The Bohr Atom

The lowest energy level is called the ground state; the others are excited states.

Bohr's ModelAccording to Bohr's model, first an electron is excited

from its ground state.

+

n=1

n=2

n=3

In this case we will consider an electron being excited from it's ground state (n=1) to its second excited state

(n=3, since n=2 would be its first excited state)

Bohr's Model

In order to make that jump, we would need to calculate

the amount of energy required for this electron to increase from ground state (n=1) to the second excited

state (n=3). +

n=1

n=2

n=3


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August 17, 2012

Bohr's ModelTo do that we take the Final Energy (Ef) the initial Energy (E0).

+

n=1

n=2

n=3

Bohr's ModelHere we see 2 separate emissions coming from the same electron. The electron can either go from n=3 right to n=1 or it can go from n=3 to n=2 to n=1.

+

n=1

n=2

n=3

+

n=1

n=2

n=3

Both are acceptable and both will occur.

+

n=1

n=2

n=3

Now you can solve for the frequency of light, but be sure to use the correct value for h!

Also, since frequency and wavelength cannot be negative, you must use the absolute value of energy. Since energy is still energy no matter how you look at it, the negative or positive sign simply tells us if its coming (absorption) or going (emission). That's why we can do this and still be doing legitimate calculations.

Bohr's Model

+

n=1

n=2

n=3

We can calcuate wavelength one of two ways, we can use our Lymen, Balmer, or Paschen equations we learned about earlier, or we can use the frequency we just found.

Bohr's Model

de Broglie’s Hypothesis Applied to Atoms

These are circular standing waves for n = 2, 3, and 5.

Quantum Physics

While a big step forward, Bohr's model only worked for atoms that had one electron, like hydrogen or certain ionized atoms.

It failed for all atoms other than hydrogen.

The idea that the electron was a particle in orbit around the nucleus, but with wavelike properties that only allowed certain orbits, worked only for hydrogen.

Semiclassical explanations failed except for hydrogen. It turned out that only a lucky chance let it work even in that case.


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Quantum Mechanics

Our goal was to explain why electrons in an atom don't fall into the nucleus. An electron, as a charged particle, would fall in because of Newton's Second Law.

ΣF = ma

But electrons, in atoms, aren't particles, they're waves. Waves don't follow Newton's Second Law. Schrodinger had to invent a new equation for wave mechanics.

Hψ = Eψslide to reveal new equation

…to be continued in chapter 2…….

introduction to atomic structure - pbworksyooschem1314.pbworks.com/w/file/fetch/69988747/atoimc...

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