AtomsHistory of Atomic Discovery—Models over Time

Democritus vs. Aristotleor round one of atomic theoryAristotle, a Greek

philosopher living from 384-322 BC, believed that matter was continuously divisible. ◦ This theory held until nearly

two millennia had passedDemocritus (460-370 BC)Democritus came up with

the idea that matter could be divided in half only until it was cutting apart extremely tiny indivisible pieces: atomos.

Dalton (1766-1844)Dalton rejected the

idea that matter is continuous, and he revived the idea of atoms around 1803.

Atoms are solid balls that are capable of being ‘hooked’ together to form bonds.

Molecules have atoms of different of elements in fixed specific ratios.

Dalton’s Atomic TheoryDalton's atomic theory had five main points:

1. All elements consist of minuscule particles called atoms.

2. All atoms of a given element are identical to each other.

3. All atoms of a given element are different than those of other elements.

4. Atoms of one element combine with other elements to create compounds. They always combine in whole number ratios.

5. Atoms cannot be created, divided, nor destroyed.

Thomson (1856-1940)

The plum pudding theory The atom is not a solid ball, but rather

something that could be embedded with the negatively charged electrons.

Thomson deduced the charge and character of the electron with his cathode ray experiments.

He discovered isotopes and invented the mass spectrometer.

http://www.youtube.com/watch?v=IdTxGJjA4Jw 3 minuteshttp://www.youtube.com/watch?v=7YHwMWcxeX8

Rutherford (1871-1937)or shall we have plum pudding?Rutherford’s gold foil experiment

demonstrated that much of the atom is empty space. Electrons are low mass and unable to deflect the much heavier alpha particle.

Father of nuclear physics http://www.youtube.com/watch?v=XBqHkraf8iE&feature=related&s

afety_mode=true&persist_safety_mode=1&safe=active The experiment 4minutes

http://www.youtube.com/watch?v=wzALbzTdnc8 Rutherford discovery 3 minutes

Rutherford Atomic ModelBased on his experimental evidence:The atom is mostly empty spaceAll the positive charge, and almost all

the mass is concentrated in a small area in the center. He called this a “nucleus”

The nucleus is composed of protons and neutrons (they make the nucleus!)

The electrons distributed around the nucleus, and occupy most of the volume

His model was called a “nuclear model”

Schrodinger (1887-1961)His thought experiment, gedanken, about the

cat in the box changed how we think about quantum events and full scale events.

He discovered the wave properties of electrons which became part of quantum mechanics.

http://www.youtube.com/watch?v=JNalMWLnt0o The cat 3:07 minuteshttp://www.youtube.com/watch?v=HCOE__N6v4o Big Bang version 2:19 minuteshttp://www.youtube.com/watch?v=CrxqTtiWxs4 Sixty Symbols UK 7:57 minutes

Atom—the basicsBasic particles

are proton (+), neutron (no charge), and electron (-).

Large mass particles are in the nucleus

Low mass particles, electrons, are in the cloud around the nucleus.

Atomic NumberAtoms are composed of identical

protons, neutrons, and electrons◦How then are atoms of one element different

from another element?Elements are different because they

contain different numbers of PROTONSThe “atomic number” of an element is

the number of protons in the nucleus# protons in an atom = # electronsWhen the atom is neutral, net 0 charge.

Complete SymbolsContain the symbol of the

element, the mass number and the atomic number.

Xmass number(superscript)

atomic number(subscript)

Frederick Soddy—Isotopes Frederick Soddy (1877-1956) proposed the idea of isotopes in 1912

Isotopes are atoms of the same element having different masses, due to varying numbers of neutrons.

Soddy won the Nobel Prize in Chemistry in 1921 for his work with isotopes and radioactive materials

IsotopesDalton was wrong about all atoms of an element are of the same type, being identical.

Atoms of the same element can have different numbers of neutrons.

Thus, different mass numbers.These are called isotopes.Periodic table reports average atomic mass units

Naming Isotopes

We can also put the mass number after the name of the element:◦carbon-12◦carbon-14◦uranium-235

The particlesProton pNeutron nElectron e-

Alpha particle α or α2+,

Beta particlePositron +1e+ or

β+

This diagram demonstrates the constitution of different kinds of ionizing radiation and their ability to penetrate matter. Alpha particles are stopped by a sheet of paper whilst beta particles halt to an aluminium plate. Gamma radiation is dampened when it penetrates matter. Gamma rays can be stopped from 4 meters of lead. Tungsten and tungsten alloys can stop Gamma radiation with much less mass than lead

Alpha particleDecay—particles must add upTotal protons equals protons in

alpha plus protons in daughter nucleus.

Same for neutrons.Alpha radiation is the most

dangerous of the three types.◦Paper can shield us.◦Low penetration.

Alpha decayThe nucleus of an atom splits into two parts.One of these parts (the alpha particle) goes

zooming off into space.Usually occurs in atomic number, Z > 83The nucleus left behind has its atomic number

reduced by 2 and its mass number reduced by 4 (that is, by 2 protons and 2 neutrons).

Note:1) The atom on the left side is the one that splits into two pieces. 2) One of the two atoms on the right is ALWAYS an alpha particle.3) The other atom on the right ALWAYS goes down by two in the atomic number and four in the mass number.

Example alpha decayWhat element is produced by

alpha decay of Americium with atomic number of 95 and atomic mass of 241?

241Am95 ZXA + 4He2

241 = z + 495 = A + 2The element is Neptunium, 237Np93

These alpha decay examples

Beta minus particleIn beta decay, a neutron breaks

into a proton, an electron, and an anti-neutrino.

The electron and the anti-neutrino are emitted.

In the reaction, Ac has one more proton than Ra. Ac has 89 protons compared to Ra’s 88. 228 Ra had one more neutron than 228Ac.

Beta minusA neutron inside the nucleus of an atom

breaks down, changing into a proton.It emits an electron and an anti-neutrino

(more on this later) which go zooming off into space.

The atomic number goes UP by one and mass number remains unchanged.

Note1) The nuclide that decays is the one on the left-hand side of the equation.2) The order of the nuclides on the right-hand side can be in any order.3) The way it is written above is the usual way.4) The mass number and atomic number of the antineutrino are zero and the bar above the symbol indicates it is an anti-particle.5) The neutrino symbol is the Greek letter "nu."

ExampleThe decay by beta minus of

carbon-14 to nitrogen-14 is used in carbon dating.

The electron and anti-neutrino are lost while the new proton is retained. The atomic weight is nearly the same.

Write an equation for carbon-1414C6 14N7 + e- + antineutrino

Try these beta minus examples60Co27

◦60Co27 60Ni28 + e- + antineutrino

137Cs55 ◦137Cs55 137Ba56 + e- + antineutrino

Try these beta minus

Positron or Beta plus In beta plus decay, a

proton decays into a neutron, a positron (the antiparticle of the electron) and a neutrino.

The positron and the neutrino are emitted.

The radioactive particle is the positron.

Electron and positron collision results in annihilation.

Positron or Beta PlusSomething inside the nucleus of an atom

breaks down, which causes a proton to become a neutron.

It emits a positron and a neutrino which go zooming off into space.

The atomic number goes DOWN by one and mass number remains unchanged.

Same notes as for beta minus. This is like a mirror image to beta minus

Beta plus examples10C6

◦10C6 10B5 + e+ + neutrino

22Na11 ◦22Na11 22Ne10 + e+ + neutrino

Try these

Gamma emissionsGamma radiation is part of the

electromagnetic spectrum just as light, UV, and radio waves are.

Gamma readily penetrates most materials including many centimeters of human tissue.

X-rays are like gamma radiation.Dense elements such as lead or tungsten are

needed for shielding.

Gamma ExposuresThe sun emits gamma radiation. This is part

of the background radiation a person is continually exposed to.

Radon gas in homes is a source of gamma.Doses are measured in rem, a unit that is

based on both the radiation received and the biological effect for that radiation. Common form of the unit is millirem, mrem.

~damage from one rad of gamma radiation = rem.

Background annual dose averages 360mrem total. 300 from natural sources and 60 from man-made sources.

Median Radon Exposure in Houses, DOE

Your Home and RadonThe danger of radon exposure in dwellings was

discovered in 1984 by an employee at the Limerick Nuclear Power Plant in Pennsylvania. The employee set off the radiation alarms on his way into work for two weeks straight. While authorities searched for the source of the contamination. They were shocked to find that the source was astonishingly high levels of Radon in his basement, and it was not related to the nuclear plant. The risks associated with living in his house were estimated to be equivalent to smoking 135 packs of cigarettes every day.

Sumps equipped with ventilation fans pull the gas from under foundations as required by current code.

Common Man-made Sources Gastrointestinal series (upper & lower): 1400 millirem CT Scan (head & body): 1100 millirem

Radon in average household: 200 millirem/year Plutonium-powered pacemaker: 100 millirem/year Natural radioactivity in your body: 40 millirem/year

Cosmic radiation: 31 millirem/year Mammogram: 30 millirem Smoking Cigarettes (1 pack/day): 15-20 millirem/year *Maximum possible from normal operations on the Oak Ridge

Reservation: 12 millirem/year Consumer products: 11 millirem/year Chest X-ray: 10 millirem Dental X-ray: 10 millirem Using natural gas in the home: 9 millirem/year Road construction material: 4 millirem/year Living near a nuclear power station: 1 millirem/year Air travel (every 2006 miles): 1 millirem *Source, 2004 DOE Annual Site Environmental Report Summary

Electron captureThere are two ways in which neutron-

deficient / proton-rich nuclei can decay. When the mass change Δm < 0 yet is insufficient to cause spontaneous positron emission, a neutron can form by an alternate process known as electron capture. An outside electron is pulled inside the nucleus and combined with a proton to make a neutron, emitting only a neutrino.

11p + 0

-1e → 10n + ν

Examples of Electron Capture81

36Kr + 0-1e- →

◦8136Kr + 0

-1e- → 8135Br + ν

23192U + 0

-1e- →◦231

92U + 0-1e- → 231

91Pa + ν

Electron Capture ProblemsWrite an equation for electron

capture in 207Bi.◦207

83Bi + 0-1e- → 207

82Pb + ν

Isotope StabilityWhy are some isotopes unstable? Isotopes all have the same number of protons, but

vary in the number of neutrons. Some combinations are unstable.

A nuclide is an atom with a specific number of protons and neutrons in the nucleus.

Unstable nuclides are radioactive. They decay to reach a balance between the neutrons and protons that is stable.

Stable is when an isotope does not decay in an observable manner. The half-life can be over 80 million years.

The nucleus is bound together by the residual strong force.

Comparison of Atomic Number vs. Mass Number

The stable line of isotopes are bracketed with others that have more or fewer neutrons and less stability. As stability goes down, the half-life becomes longer.Note that above 80 million is functionally stable.

Pattern of Radioactive Decay

Decay Sequence

Half-lifeThe time it takes for one half of

the radioactive atoms of an element present in a sample to decay is the half-life.

The time for a specific radioactive isotope to decay half of its atoms is a constant.◦Decay rate constant = λ (units time-

1)◦Half-life = t1/2 = ln2/λ (units time)

Graph of Radioactive Decay

Half Life, 1st Order Reaction

This experimental data graphed to form a non-linear first order reaction line.

Half-Life CalculationsRemaining radioactive atoms are

equal to the starting amount multiplied by the number ½ raised to the number of half-lives that have elapsed.

(starting amount) x 1/2number half-lives = (remainder amount)

Number of half-lives = time elapsed

length of 1 half-life

ExampleThe half-life of Zn-71 is 2.4

minutes. If one had 100.0 g at the beginning, how many grams would be left after 7.2 minutes has elapsed?

7.2 / 2.4 = 3 half-lives(1/2)3 = 0.125 (the amount remaining after 3 half-lives) 100.0 g x 0.125 = 12.5 g

remaining

Try ThisOs-182 has a half-life of 21.5

hours. How many grams of a 10.0 gram sample would have decayed after exactly three half-lives?

(1/2)3 = 0.125 (the amount remaining after 3 half-lives) 10.0 g x 0.125 = 1.25 g remain

10.0 g - 1.25 g = 8.75 g have decayed Note that the length of the half-life

played no role in this calculation.

ExampleAfter 24.0 days, 2.00 milligrams of an

original 128.0 milligram sample remain. What is the half-life of the sample?

Time elapsed = 24.0 daysInitial amount = 128.0 mgRemainder amount = 2.0 mg128.0 mgm x (½)n =2 mg(1/2) n = 2/128 = 1/64 = (1/2)6

n = 6 half-lives and 24.0 days/6 = 4 days t1/2

Characteristics of Particles

Nuclear Emissions

Particle Emitted Particles ΔZ ΔA Occurrence

Alpha 42He2+ -2 -4 Z>83

Beta Minus energetic e-, γ 1 0A/Z > (A/Z)stable

Positron energetic e+, γ -1 0A/Z < (A/Z)stable light nuclei

Electron Capture ν -1 0A/Z < (A/Z)stable heavy nuclei

Gamma photon 0 0Any excited nucleus

Resourceshttp://orise.orau.gov/reacts/guide

/gamma.htm REAC/TS Oakridge

http://www.oakridge.doe.gov/external/PublicActivities/EmergencyPublicInformation/AboutRadiation/tabid/319/Default.aspx

http://www.chemteam.info/Radioactivity/Radioactivity.html