Nuclear Forces, Bohr, and Light

1

• Rutherford’s gold foil experiment tells us that all atoms have a nucleus– Contains nucleons, protons and neutrons,

and always has a + charge as a whole– Extremely dense – Electrons are outside of it

Background Information

2

There’s More?

• Actually over 200 different particles in atoms, not just the 3

• The theory of Quantum Chromodynamics (QCD) says that some of these particles contribute to holding the nucleus together

3

Some Examples• Quark - a single proton is made up of

3 quarks• Pi meson (pion) - exchanged between

proton and neutron• Nu meson• Neutrino• Leptons, etc.

4

• Smashing atoms using particle accelerator, a device that uses:• 1. electric fields to

propel electrically charged particles to high speeds

• 2. magnetic fields to contain them

How Do We Know?

5

• Use of a bubble chamber (superheated liquid container) with accelerator – Charged particles pass

through and leave trails of bubbles

– Form helical shapes, due to a magnetic field applied to the chamber

– Analyze shapes to find the mass and charge of the particle

How do we know?

6

CERN

• On the Franco-Swiss border near Geneva

• "Conseil Européen pour la Recherche Nucléaire“ (European Council for Nuclear Research)

• 1952 • Established a world-class fundamental

physics research organization in Europe with a large Hadron Collider

• CERN Video 7

Atoms Have a Nucleus • 2 questions associated with an atom that

has a nucleus– 1) How does the nucleus stay together when

the protons repel each other? – 2) How do the negative electrons stay out of

the nucleus if they are attracted to the + protons in the nucleus?

8

Electromagnetic Force• Results from the

repulsion of like charges and the attraction of opposites

• Holds the electrons around the nucleus

-

+

+

+

--

Particles with the same charge move

apart and the particles with

different charges move together.

Why are neutrons not pictured above?

9

Strong Force• Force that holds the

nucleus together• Counteracts the

electromagnetic force’s repulsion (+-+)

• Only strong if the protons and neutrons are close!

• Large nuclei have lots of repulsion compared to strong forces

++

++

Notice how the electromagnetic force

causes the protons to repel each other but, the strong force holds them together.

Would an atom have a nucleus if the strong force did not exist?

10

Nuclear Forces

• Nuclear forces like the strong force that holds the nucleus TOGETHER are NOT

- gravitational in nature- electrical in nature

11

Isotopes• Isotopes are atoms of the same element

that have different numbers of neutrons (therefore, different mass #s)

• Hydrogen-1 Hydrogen-2 Hydrogen-3 1 2 3 H H H 1 1 1• Some isotopes are stable, some are not• Those that aren’t stable are radioactive

12

Stable vs. Unstable Isotopes-Reference

Unstable &Radioactive

13

Importance of Neutrons-Reference

• All atoms > 1 proton have neutrons • As proton # increases, so does neutron

#NUCLEUS # OF PROTONS # OF NEUTRONS

12 C 6

6 6

56 Fe 36

36 26

207 Pb 82

82 125

14

Importance of Neutrons-Reference

• Neutron to proton ratio is related to the stability of the nucleus

• The greater the proton #, the larger the repulsion and higher ratio of neutrons needed STABLE NUCLEUS # OF NEUTRONS # OF PROTONS N:P RATIO

12 C 6

6 6 6÷6 = 11N:1P

56 Fe26

30 26 30÷26 = 1.151.15N:1P

207 Pb 82

126 82 126÷82 =1.541.54N:1P

15

General Rules of Stable Atom Size-Reference

Atom Size Example Stable Ratio of N:P

< 20 protons(Small)

He-4 2P: 2N1:1 ratio

21-55 protons(Medium)

Br-79 44N:35P1.3:1 ratio

> 55 protons(Large)

Au-197 118N:79P1.5:1 ratio

16

• This graph shows stable and unstable isotopes• Stable atoms have neutrons:proton ratios

that place them in the belt in the graph • In general, the more protons an atom has,

the more neutrons it needs to be stable• Note axes!

Importance of Neutrons

17

• Note most stable isotopes are blue dots• Neon? End of blue dots?

Importance of Neutrons-Reference

18

• There’s a limit on the number of protons in a stable nucleus no matter what the n:p ratio is:– The maximum number of protons in a stable

nucleus is 83!– More than 83 protons in the nucleus = unstable

and radioactive no matter what• Sea of Instability video 14 min

Importance of Neutrons

19

Review • Number your paper from 1-5 and answer

the following questions. Two will be cumulative review!

• 1. Which of these is the correct symbol for Chlorine-35?

• a. 35 Cl 17

• b. 17 Cl 37

• c. 17 Cl 35

20

Review

• A• 2. A typical nitrogen atom has 7

protons and 7 neutrons. What’s the actual mass of a typical nitrogen atom?

• a. 1.67 x10-24 g• b. 1.17 x10-23 g • c. 2.24 x10-23 g• d. 6.02x1023 g

21

Review

• A• 3. Which of these is true

• a. any atom can be stable if it has enough protons

• b. atoms below atomic number 83 are all unstable

• c. atoms with enough protons to balance the repulsive force of the electrons are stable

• d. the larger the atoms is, the more neutrons it needs to be stable

22

Review

• D• 4. Strong forces

• a. help overcome the negative charges of the electrons

• b. help overcome the repulsion of the neutrons

• c. help overcome the repulsion of the protons

• d. help overcome the attraction of the protons and electrons

23

Review

• C• 5. Which of these is false?

• a. protons repel protons• b. protons attract neutrons• c. protons attract electrons• d. neutrons don’t repel anything

because they have no charge

24

Review

• B

25

Nuclear Symbols-Reference• Nuclear symbols are a way to write

atoms using the mass number and atomic number

• Format:

• Which of these numbers can change and still have the element be the same?

• Note! The element symbol tells you the atomic #, so sometimes this nuclear symbol is written without it: 12 C

 

26

Let’s Do It!!!

•  Read over the left side of the Nuclear Symbols Review

• Get with a partner and complete the right side 10 min

27

• Radiation (radioactive decay)- release of particles and/or energy from an unstable nucleus

• Two kinds: natural or artificial – Natural radiation - nuclear changes that occur

in nature, not caused by humans

Radioactivity

28

3 Main Types of Natural Radioactive Decay-

ReferenceTYPE SYMBOL COMPOSITION STOPPED BYALPHA α or 4

He 2

2 p+

2 nOPAPER

BETA β or 0 e -1

1e- ALUMINUM

GAMMA ϒ Energy (no mass) LEAD

29

Alpha Radiation (Decay)

• Alpha particle consists of 2 protons and 2 neutrons • It’s essentially a He-4 nucleus (no

electrons)• 4 He 2

• Released when too many protons in the nucleus

• Atom changes in identity and mass • atomic # decreases by 2 (lose 2 p)• mass # decreases by 4 (2 p + 2 n)

• Alpha decay and Plutonium 30

31

Alpha Radiation-Reference

241 4 237 Am He + Np 95 2 93 alpha 4 particle symbol also α

2

32

33

Beta Radiation (Decay)

• Beta particle has the same mass and charge as an electron BUT released from nucleus – Released when n:p ratio is too high for size

(too many neutrons)

– O e -1

34

35

-n

Beta Radiation• Neutron can

change into a proton(+) and an electron(-)

• n p + e-• The electron is the

beta particle

+

Notice how the original particle changes to

something new.

36

• n p + e- • Atom changes in identity because new

proton increases atomic # by 1 • -Mass # stays same• More penetrating (higher velocity) than

alpha radiation so more dangerous

Beta Radiation

37

Beta Radiation-Reference 3 0 3 H e + He 1 -1 2 beta particle symbol is also β

38

Beta Radiation-Reference

39

Gamma Radiation

• Just energy so no mass and most penetrating and dangerous of the 3 types

• Form of electromagnetic radiation like X-rays or light

• Causes no change in atom’s identity or mass

• Animation

40

Gamma Radiation-Reference

3 0 3

He g + He 2 0 2 gamma ray

41

Gamma Radiation-Reference

42

Positron Emission

• Two other, more complicated forms of radioactive decay– positron – electron capture

• Positrons have same mass as an electron, but opposite charge - released from nucleus

• Proton changes to neutron when positron is released - not sure how

• Atom changes in identity only (atomic number decreases by 1 due to loss or proton) - mass stays the same

43

Positron Emission-Reference

11 0 11 C e + B 6 1 5 positron

44

Positron Emission-Reference

45

Electron Capture

• Sometimes the nucleus will capture an electron that gets too close to it

• This process is more rare, but it can occur - gamma radiation is always released

• Atom changes in identity only (atomic number decreases by 1) - mass stays the same

46

 Electron Capture-

Reference 7 0 0 7

Be + e g + Li 4 -1 0 3 capture e- gamma

47

 Electron Capture-

Reference

48

Nuclear Radiation Equations-Reference

• Write a chemical equation so on the left of the arrow is the starting atom in nuclear notation– Think of the arrow as an equal sign– The arrow means “produces”

• On the right is the particle + resulting atom

• Simple math makes it easy to solve• 238 = ? + 4•

92 = ? + 2

?49

Nuclear Radiation Equations-Reference

• Write a chemical equation so on the left of the arrow is the starting atom in nuclear notation– Think of the arrow as an equal sign– The arrow means “produces”

• On the right is the particle + resulting atom

• Simple math makes it easy to solve• 238 = 234 + 4•

92 = 90 + 2

50

Let’s Do It!!!Nuclear Equations video 7

min    214 0

82Pb ---------> __?__+ -1 e

51

Let’s Do It!!!

   214 214 0

82Pb ---------> 83Bi + -1 e

52

Let’s Do It!!!

   230 226

90Th ---------> 88Ra + __?__

53

Let’s Do It!!!

   230 226 4

90Th ---------> 88Ra + 2He

54

• Number your paper from 1-5 and answer the following questions. Two will be cumulative review!

• 1. Atoms with the same atomic number but different mass numbers are

• a. different elements• b. ions• c. isotopes of the same element• d. isotopes of different elements

ReviewRadioactive Decay Video

8min

55

Review

• C• 2. Which of these has a 1:1 ratio of

neutrons to protons?• a. 34 Cl 17

• b. 17 Cl 37

• c. 35 Cl 17 56

Review

• A• 3. Naturally radioactive elements

• a. have unstable nuclei• b. give off alpha, beta, or gamma rays• c. can decay into another element• d. all of the above

57

Review

• D• 4.An alpha particle

• a. has a negative charge• b. can penetrate a sheet of aluminum• c. is identical to a helium nucleus• d. all of the above

58

Review• C• 5. Finish the equation: 222 4

86Rn ---------> __?__ + 2He

• a. 226 Cl 88

• b. 218 Po 84

• c. 218 Ra 88

59

Review

• B 222 218 4

86Rn ---------> 84Po + 2He

60

• If the nucleus is STILL unstable, it will continue to decay further– This is one reason why radioactive material

can remain dangerous for long times– The other is that radioactive material doesn’t

all decay at once• Each radioactive element decays at

different rates, some very quickly, and some very slowly

 Stability

61

Half-Life• Half-life (t1/2)- the time it takes for half the

mass of a radioactive sample to decay– Different for each radioactive isotope, not

affected by external conditions (T, P, etc.)– Range from fractions of seconds to billions of

years

62

• If you start with 10.0 g of Uranium, after 4.5 x 109 years (4.5 billion), you will have 5.00 g left

Half-life-Reference

𝐔❑𝟐𝟑𝟖𝟗𝟐 𝟒 .𝟓𝒙 𝟏𝟎𝟗𝒚𝒓𝒔 .

→

𝐓𝐡+ 𝜶❑

𝟒𝟐

❑

𝟐𝟑𝟒𝟗𝟎

63

64

65

Half-Life-Reference

• The half life of polonium-210 is 138 days• How many days are required for 7/8 of a

given amount of Po-210 to decay?• 3 half lives = 414 days where 1/8 is left

and 7/8 is decayed

138 days

66

• The half life of polonium-210 is 138 days

• What was the original mass of a sample of Po-210 if 0.250 g remains after 414 days?

• 414 days = 3 half-lives• The original mass was 2.000 g

Half-Life-Reference

0.250 g2.000 g 1.000 g 0.500 g

67

• The half life of polonium-210 is 138 days

• How much of a given amount of Po-210 remains after 552 days?

Let’s Do It!!!

68

• The half life of polonium-210 is 138 days

• How much of a given amount of Po-210 remains after 552 days?

• 552 days = 4 half-lives• 1/16 remains

Let’s Do It!!!

1/1669

Radioactive Isotopes in Medicine

• Some radioactive isotopes have extremely important uses in medicine

• Can be used to detect certain types of tumors

• Technetium-99m is an ideal isotope for scanning organs because it has a short half-life and is a pure gamma emitter

70

𝐓𝐜𝟔.𝟎𝟎𝒉𝒐𝒖𝒓𝒔→

𝐓𝐜+ 𝜸❑

𝟎𝟎

❑

𝟗𝟗𝟒𝟑

❑

𝟗𝟗𝒎𝟒𝟑 71

• Radioisotopes used in medicine have pros and cons

• Positives– Generate extremely accurate maps of

internal organs– Most have short half-lives and decay quickly

• Negatives:– Exposes patient to high levels of gamma

radiation (ypical dose = 500 chest X-rays)– Family members of patient also at risk of

exposure

Pros vs. Cons

72

• Suppose 80.0 mg of Tc-99m were prepared this morning and shipped to St. Anthony’s hospital. How many milligrams would remain after 24 hours?– 1 half-life = 6 hours– 24 hours = 4 half lives

6hrs 12hrs 18hrs 24hrs80.0mg 40.0mg 20.0mg 10.0mg 5.0mg

It would obviously be cost effective to use this isotope quickly

Sample problem

73

ReviewHalf-Life video 7min

• Number your paper from 1-5 and answer the following questions. Two will be cumulative review!

• 1. How many atoms of the isotope 14C are in a mole?

• a. 1• b. 14• c. 6• d. 6.02 X1023

74

Review • D• 2. Which of these is an alpha particle?

• a. 34 Cl 17

• b. 37 He 17

• c. 4 He 2

• d. 2 A -1

75

Review• C• 3. If you had 10 grams of each of the

following isotopes, which would still be highly radioactive after one million years?

• a. 90Sr b. 238U• c. 222Rn d. 218Po

76

Review

• B• 4. An isotope’s half-life

• a. is measured in grams • b. can be changed by chemical reactions• c. is usually greater than 1000 years• d. is the time required for half the mass

to decay into another element

77

Review• D• 5. If I have 8 grams of Rn-222 and 24

grams of Po-18, which sample will have more radioative material left in 12 days?

• a. Rn-222 b. Po-18 c. Both the same

78

Review

• A• Rn-222 will have 1 gram• 8421 with a half-life every 4 days,

or 3 half-lives in 12 days• Po-218 will have 1.5 grams• 2412631.5 with a half-life everyt 3

days, or 4 half-lives in 12 days

79

Natural Decay

• Uranium spontaneously decays on it’s own because it’s unstable

• 238 234 4

92 U 90 Th + 2He

80

Artificial Decay• Bombardment of a stable isotope to

force it to decay:• 14 4 1

• 7N + 2H 1p + ?

nuclear “bullet”• This is an example of the first ever done

– “Bullets” are + charged, and repelled by the huge + nucleus they are bombarding

– They must be accelerated to very high speeds by a particle accelerator to overcome repulsion and collide

81

Artificial Decay• Bombardment of a stable isotope to

force it to decay:• 14 4 1 17

• 7N + 2H 1p + 8 O

nuclear Hydrogen “bullet” atom• First ever done by Rutherford

– “Bullets” are + charged, and repelled by the huge + nucleus they are bombarding

– Accelerated to very high speeds by a particle accelerator like CERN has to force collision

82

Fission

• Nuclear fission - heavy nuclei are bombarded with neutrons, become highly unstable, and split– The gain in stability

releases lots of energy mostly heat

– Only certain isotopes of uranium and plutonium will undergo fission

83

Fission-Reference

1 235 142 91 1

0 n + 92U 56Ba + 36Kr + 3 0n + E• Mass of particles produced (234amu) is

slightly less than the mass of the reactants (236amu)—mass defect!

• This mass is converted into huge amounts of energy

• E = mc2

84

Fission-Reference

85

E = mc2

• Albert Einstein proposed that mass and energy were related in his special theory of relativity – If a substance gains mass, it gains energy– If a substance loses mass it loses energy

• Ex one gram of mass is converted to about 100 trillion joules of energy

•

86

Fission• Note that there are 3 neutrons produced

in the reaction• Atoms are mostly empty space, so

fission won’t continue naturally • Critical mass- minimum mass of

fissionable material required for a chain reaction

87

Fission-Reference• Controlled fission chain reactions used

to produce energy– Used in atomic bombs & nuclear reactors• Nuclear Fission

•

88

Fossil FuelsEasy AccessEasy to work

withAir PollutionOil SpillsNon-renewable

Nuclear FissionMore

energy/reactant (cheaper)

RadioactiveAccidents can be

catastrophic (Chernobyl)

89

• Same Substance Decays Different Ways-Reference

235 1 93 140 1 U + n Kr + Ba + 3 n +

ENERGY 92 0 36 56 0______________________________________ 235 1 144 90 1 U + n Xe + Sr + 2 n +

ENERGY 92 0 54 38 0______________________________________ 235 1 95 138 1 U + n Kr + Ba + 3 n +

ENERGY 92 0 36 56 0

90

Fusion

• 2 small nuclei fuse together to form one larger, more stable nucleus of intermediate size

• The increase in stability releases a large quantity of energy 91

• Essentially, hydrogen atoms are fused together to make helium atoms

• Used in hydrogen bombs and provides energy in stars like our sun

• Mass is converted into energy by the Einstein equation - mass of products is less than the mass of the reactants

Fusion

92

Our Sun

• As the Sun ages, the hydrogen nuclei are used up as they are fused into helium

• It’s a giant fusion reactor • It is estimated that the Sun has enough

hydrogen to keep this reaction going for another 5 billion years

93

FusionMore energy/reactantMore reactantsDifficult to controlDifficult to initiate without

super high temps (cold fusion not possible)

Less Radioactive

FissionRadioactiveAccidentsMore technologyEasy to initiate

with just neutrons

Fission vs. Fusion

94

Review Fission vs. Fusion video

7min• Number your paper from 1-5 and

answer the following questions. Two will be cumulative review!

• 1. Which of these is the correct specific heat for water ?• a. 1 cal/g °C • b. 0.897 J/g °C • c. 1 J/g °C • d. 4.184 cal/g °C

95

Review

• A• 2. An isotope’s half-life

• a. is measured in time • b. can be changed by chemical reactions• c. is usually greater than 1000 years• d. is half the time required for the mass

to decay into another element

96

Review

• C• 3. Where is this picture showing?

• a. Fission• b. Electron capture• c. Fusion• d. The Billiard Ball model of atomic

structure

97

Review

• A• 4. Where would you find this nuclear

reaction occurring? • a. the sun• b. a hydrogen bomb• c. your body• d. a nuclear reactor

98

Review

• D• 5. The minimum mass of fissionable

material required for a chain reaction is called

• a. nuclear mission • b. critical mass• c. half-life• d. theory of relativity

99

Review

• B

100

Still want to know more about radiation?

• Radioactive Teddybear Zombies

101

Atoms Have a Nucleus • Recall these 2 questions associated with

an atom that has a nucleus– 1) How does the nucleus stay together when

the protons repel each other? – 2) How do the negative electrons stay out of

the nucleus if they are attracted to the + protons there?

102

Why Doesn’t the Atom Collapse?

• It should because of the electrical attraction between the positive nucleus and the negative electrons, but it doesn’t

103

Outdated Explanation

• Solar System Model: electrons travel around the nucleus like planets around the sun, relying on gravity, so the electrons stay away from the nucleus like the planets stay away from the sun

• It was known this model had flaws and will not work with matter that has electric charges (planets are neutral)

104

Niels Bohr (1885 – 1962)

In 1913, proposed the Bohr Model of the atom- electrons travel at definite speeds around the nucleus of an atom in definite paths called orbits Electrons can jump from a path in

one level to a path in another level (depending on their energy)

How did he find this? Well, we need to talk about light

105

LIGHT

• Properties of light: – Electromagnetic radiation

spectrum– Can be produced in chemical

reactions– Can change matter– Constant velocity (symbol is c)

c = 3.00 X 108 m/sec – Can be reflected and refracted

(bent)– Responsible for color

106

• It takes approximately 8.3 minutes for light from our sun to reach Earth. How far away is the sun in miles?

? miles = 

8.3 min in

ft

12

1*

min1

sec60*

m

cm

1

100*

ft

mile

5280

1*

sec1

10*00.3*

8m

= 93,000,000 miles

LIGHT-Reference

cm

inch

54.2

1*

107

Light Models

• There are 2 models that can explain light– 1. Wave model– 2. Particle model

• Neither theory is better than the other

108

• Light is a wave in which each has a– Wavelength (l)-

distance per cycle (m/cycle) between corresponding points on adjacent waves

– Frequency (v)- the number of waves passing a given point in a given time in cycles per second= cycles/sec (or Hz)

1. Wave Description of Light

109

Wave Description of Light• We can relate frequency and wavelength

of a wave to the speed of light as c = l v– c = 3.0 x 108 m/s : speed of light in a vacuum– Rearrange equation so that l = c/v – When frequency goes up, wavelength goes

down and vice versa (inversely proportional)

110

Sample Problem #1-Reference

• What is the frequency of light if the wavelength is 6.0 x 10 -7m/cycle?

l = 6.0 x 10 -7m/cycle• c = 3.0 x 108 m/s • c = l v so divide each side by l• v = c/ l • So v = 3.0 x 108 m x 1 cycle = sec 6.0 x 10 -7m v= 5.0 X 1014 cycles or Hz sec

111

Sample Problem #2-Reference

• Look at light wave (b). How many total waves are there? 3

• If the wavelength is 5 x10-7 m/cycle, what’s the frequency of this wave?

• v = c/ l so 3.0 x 08 meters x 1 cycle =

sec 5 x10-7 m

v = 6 x 1014 Hz = 6 x 1014 cycles sec

112

Let’s Do It!!!

• What is the wavelength of light if its frequency is 5.0 x 10 14Hz (cycles/sec)?

113

Let’s Do It!!!

• What is the wavelength of light if its frequency is 5.0 x 10 14 cycles/sec (Hz)?

• v = 5.0 x 10 14 cycles/ sec or Hz• c= 3.0 x 108 m/s • c = l v so divide each side by v = l c/v = l 3.0 x 108 m x sec = sec 5.0 x 10 14 cycles =l 6.0 x 10-7 m/cycle

114

2. Particle Description of Light

• Energy also exists as particles, called quanta or photons, which are bundles of energy with no measurable mass

115

The Modern View of Light

• Put forth by Einstein– Light has a dual nature– Light may behave as a wave– Light may behave as a stream of particles

called quanta or photons• Most scientists consider light to be

waves made up of particles

116

• An equation ties these 2 theories together:

• Photon E = (Planck’s constant) (frequency)

• E = h • n – h = Plank’s constant = 6.6 X 10-34 J. sec/cycle

or 6.6 X 10-27 erg . sec/cycle (1 erg = 1 x 10-7 J)– and n = frequency of wave

The Modern View of Light

117

• E = h • n – E and n are directly related– large n, then large E or small n, then small

E– And if v = c/ l then E and l are inversely

related– small l , large E or large l , small E

The Modern View of Light-Reference

118

Given a wave with wavelength of 5.6 X 10-

3 m, calculate the (a) frequency and (b) energy of the wave.

a) c = l n so v = c/ ln = 3.00 X 108 m x cycle = sec 5.6 X 10-3m n = 5.4 X 1014 cyc/secb) E = h n E =6.6 X 10-34 J. sec x 5.4 X 1014 cycles cycle secE = 3.6 X 10-19 Joules

The Modern View of Light-Reference

119

Given a wave with frequency 6.2 X 1014

cyc/sec, calculate the (a) wavelength and (b) energy of this wave

Let’s Do It!!!

120

Given a wave with frequency 6.2 X 1014

cyc/sec,calculate the (a) wavelength and (b)

energy ofthis wave(a) c = l n so l = c/v = l 3.00 X 108 m x sec = sec 6.2 X 1014 cycle l = 4.8 X 10-7 m/cycle(b) E = h n E = 6.6 X 10-34 J . sec x 6.2 X 1014 cycle cycle sec E = 4.1 X 10-19 Joules

Let’s Do It!!!

121

Matter and Energy Interact

• These things were known by the ancients:– Heat a lump of metal and it glows– Burn wood and it gives off light– Place an object in the sun and it can get

hotter• Today we understand that matter can

give off energy and it creates a spectrum

122

The EM Spectrum• EM spectrum- the full range of

frequencies of EM radiation • EM Spectrum includes:

– Radio waves– Infrared rays– Visible light– UV rays– X-rays– Gamma Rays

• Electromagnetic Spectrum Video on science 360

123

EM Spectrum-Reference Color!

124

• Color is related to wavelength– White light- not a single color; made up of a

mixture of the seven colors of the rainbow and is all wavelengths mixed together

– A prism can separate white light into colors by different wavelength

– This is how rainbows are formed: sunlight is “split up” by raindrops

Visible Light in the EM Spectrum

125

Seeing Color

• The color an object looks depends on the colors of light it reflects

• For example, a red book only reflects red light:

Only red light is

reflected126

Purple light

White

light

A pair of purple trousers would reflect purple light (and red and blue, as purple is made up

of red and blue):

A white hat would reflect all seven colors:

127

Review

• Number your paper from 1-5 and answer the following questions. Two will be cumulative review!

• 1. What is the mass of 50mL of water?– a. 50cm– b. 5mL– c. 50g– d. 50mg

128

Review

• C• 2. Why don’t the protons in the nucleus

repel each other and fly apart?• a. Repulsive force• b. Neutrino force• c. Positron emission• d. Strong force

129

Review

• D• 3. Which of these relationships, based

on the equation c = l n is true?• a. frequency goes up, the speed of light goes

down• b. wavelength goes up, frequency goes up• c. frequency goes up, wavelength goes down• d. frequency goes down, the speed of light

goes down

130

Review

• C• c = l n so v = c/ l or = l c/v and c

doesn’t change!• 4. If a shirt is black, what colors of the

visible spectrum are being reflected into your eyes from the shirt?• a. black• b. all• c. none• d. red, blue, and purple

131

Review

• C• 5. If a wave have a wavelength of 500

nm (500 x 10-9 m or 5.0 x 10-7 m) what color is it?• a. blue• b. yellow• c. red• d. purple

132

Review

• A

133

Bohr

• Examined the light released by atoms of certain elements– Worked mostly

with H since it has only 1 electron

– Used spectroscope to analyze light released from a vacuum tube

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Bohr’s Energy Levels• Electrons reside in certain energy levels, or orbits• Are at least 7 levels and each represents a

different amount of energy• As long as an e- remains in one level, it

maintains the same amount of energy• Rungs of a ladder or stairs

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Bohr’s Energy Levels• Lower energy levels

are closer to nucleus• Higher energy levels are farther• Ground state- electron in lowest energy level possible

– Atom can stay in the ground state indefinitely

– Electrons move only when energy is involved

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Excited Atoms• Electrons can absorb

energy (called quanta or photons) and become excited – Greater the energy

absorbed, further from the nucleus they move

– Excited state is unstable so atom soon emits same amount of energy absorbed

– This can be seen as visible light!

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Spectroscopy• When electrons move

from a higher energy level (further from the nucleus) to a lower energy level (closer to the nucleus), there is energy emitted– This energy is emitted

as light– These bands of light are

called the Emission Spectrum

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• Only certain wavelengths or photon energies of light are present in the emission spectrum- Emission Video

• An element’s emission spectrum is unique to that element (like a fingerprint!)

Emission Spectrum-Reference

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Absorption Spectrum-Reference

• Only certain wavelengths or photon energies of light are missing in the absorption spectrum

• An element’s absorption spectrum is unique to that element

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For the same element’s atoms, the emission and absorption spectra will match each other so that if combined, they would yield continuous spectrum

This means that the same atoms release and absorb the same energies due to their electrons

Spectroscopy-Reference

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Energy level diagram-Reference

A B C D

H emission spectrum

4 x 10-7m λ 7 x 10-7 m400nm 700nmsmaller λ larger λ

A C D

• 21 possible emission lines

U V Visible I R

violet red

• Absorption lines (arrows) would point UP

larger E smaller E

ENERGY

1

2

3

4

5

67

B

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Bohr Conclusions:

1. The electron is responsible for the emission and absorption spectra of light energy

2. The electron orbits the nucleus with a certain energy

3. When the electron absorbs energy, it moves to an orbit further from the nucleus, which causes the absorption spectrum

4. When the electron releases energy, it moves to an orbit closer to the nucleus, which causes the emission spectrum

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• Bohr tried to:– Apply his ideas about

electron behavior of hydrogen atoms to other elements’ atoms

– Explain the arrangement of elements on the periodic table by showing that elements in a column of the periodic table have similar chemical properties because they have similar electron structures

– Interactive Periodic Table

More Bohr…

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Summary of Bohr…

Bohr was correct about:1. The electron does have certain energies2. The electron is responsible for the emission

and absorption spectra of atoms3. There is a minimal (ground state) for the

electron which keeps the electron out of the nucleus

4. Chemical properties of elements are related to electron arrangement

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Summary of Bohr

Problems with Bohr’s ideas:1. Do not explain the behavior of atoms with

more than 20 electrons2. Violate the Heisenberg Uncertainty Principle

- both the path and velocity of a particle with the size and mass of the electron can not be determined at the same instant in time - in other words, electrons cannot be in orbits!

THEN WHERE ARE THEY?

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Erwin Schrodinger (1887-1961)

• In 1926, he further explained the nature of electrons in an atom:– The exact location of an

electron cannot be stated– It is more accurate to

view the electrons in regions called electron clouds-places where the electrons are likely to be found

• Won a Nobel Prize

Image taken from: nobelprize.org/.../1933/schrodinger-bio.html

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149

ReviewBohr and Atomic Spectra video

7min• Number your paper from 1-5 and

answer the following questions• 1. Which of these is a metric unit?

– a. pounds– b. cups– c. liters– d. inches

150

Review

• C• 2. An isotope’s half-life

• a. can be changed by chemical reactions• b. is usually greater than 1000 years• c. is measured in time• d. is half the time required for the mass

to decay into another element

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Review

• B• 3. Which of these energy levels of an

atom is the level with the lowest energy?• a. n=1• b. n=2• c. n=4• d. n=6

152

Review

• A• 4. What kind of spectrum is this?• a. super• b. absorption• c. electronic• d. emission

153

Review

• B• 5. Which color of light represents the

largest release of energy, and therefore, the biggest jump between energy levels in Bohr’s model?• a. red • b. yellow• c. blue• d. purple

154

Review

• D

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