History of the Atom

Leading up to the current model

History of the Atom

Leading up to the current model

  Assessment statement Obj Teacher’s notes

2.1.1 State the position of protons, neutrons and electrons in the atom.

1 TOK: None of these particles can be (or will be) directly observed. Which ways of knowing do we use to interpret indirect evidence gained through the use of technology? Do we believe or know of their existence?

2.1.2 State the relative masses and relative charges of protons, neutrons and electrons.

1 The accepted values are:

2.1.3 Define the terms mass number (A), atomic number (Z) and isotopes of an element.

1  

2.1.4 Deduce the symbol for an isotope given its mass number and atomic number.

3 The following notation should be used: , for example,

2.1.5 Calculate the number of protons, neutrons and electrons in atoms and ions from the mass number, atomic number and charge.

2  

2.1.6 Compare the properties of the isotopes of an element. 3  

2.1.7 Discuss the uses of radioisotopes 3 Examples should include 14C in radiocarbon dating, 60Co in radiotherapy, and 131I and 125I as medical tracers.Aim 8: Students should be aware of the dangers to living things of radioisotopes but also justify their usefulness with the examples above.

2.1 The atom 1 hourTOK: What is the significance of the model of the atom in the different areas of knowledge? Are the models and theories that scientists create accurate descriptions of the natural world, or are they primarily useful interpretations for prediction, explanation and control of the natural world?

Models of the Atom

Earliest Models:Dalton (and contemporaries)

“Billiard Ball” Solid sphereCannot be divided up into smaller particles or pieces.Atom neutral and no chargeAtoms of same element are made of the same types of atoms.

FaradaySuggested that structure of atom is somehow

related to electricity. Long series of experiments:

Atoms contain particles that have electrical charge.

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Models of the Atom (cont.)Faraday (cont.)

Greeks knew that if you rubbed amber with a cloth, it attracted dust or other particles.

Static electricity

FranklinStudied Static Electricity

Famous Kite Flying experimentFindings:

Object could have one of two electrical charges

Called them positive and negative (+, -)

Alike charges repel (+ +) and (- -)

Lightning was static electricity on a larger scale.

Models of the Atom (cont.)Others (mid-1800s)

Scientists investigated electric currents

Cathode Ray Tube (CRT)

J.J. Thompson (1896)Systematic studies on cathode rays Movieconcluded that:

Through experiments, cathode rays were composed of negative particles and these negative particles could be manipulated with magnet & electric currents.

Atoms were not indivisible, solid sphere. But had substructure(s).

Eventually, addition of “pinwheel” to the tube showed that particles in beam had mass.

Uses of this Technology

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Models of the Atom (cont.)J.J. Thompson (cont.)

called negative particles “Electrons”Was able to determine ratio of electron’s electrical

charge to mass (1.76 x 108 coulombs per gram)

Millikan (1909)Oil Drop Experiment (Movie)

Measured the charge of an electron charged droplets with x-rays (negative charge)Varied rate of falling by changing charge in two charged plates. Calculated that the charge on each oil drop was a multiple of 1.60 x 10-19 C (coulombs)

Figured the charge for an electron must be 1.60 x 10-19

From this and Thompson’s ratio, calculated the mass of e-.

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Models of the Atom (cont.)What did this do to the idea of the Model?

This new research/data showed that the atom was NOT a solid sphere. But had “parts” that had a negative charge.However, the overall atom was neutral in charge.Therefore, there must be positive “parts” to balance the negative “parts”.Gave rise to the “Plum Pudding” Model.

Electrons (-) spread randomly throughout the atom. And surrounded by randomly spread out positive (+) parts.

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Models of the Atom (cont.)

RadioactivityBecquerel (1896)

Accidentally discovered uranium sample was radioactive (placed on photography film).

Radioactivity: Spontaneous emission from atom

Curie (Marie and hubby Pierre)Becquerel’s colleagues

Isolated 2 other radioactive materialsRadium and Polonium

Models of the Atom (cont.)Radioactivity

Scientists soon after discovered several things:Radioactivity accompanies fundamental changes in

an atom.A chemical change happens as radiation is given off!!!

Rutherford (early 1900s)Studied Radioactivity

Used 2 electrically charged plates (Movie)Found that some radiation deflected towards negative

plate. Called it an ALPHA RADIATION.Other radiation deflected towards positive plate. Called

it BETA RADIATION.Later scientists found another radiation which was

undeflected by electrical charge. GAMMA RADIATION.Both Alpha and Beta radiations were shown to be particles.

Models of the Atom (cont.)Rutherford (cont.)

Concluded that atoms contain electrons, but were electronically neutral

Alpha-scattering experiment (Gold Foil Experiment)

Movie

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Models of the Atom (cont.)Gold Foil Experiment (results)

A few particles were deflected off their path.Some particles “bounced back”

Think of playing pool. Glancing hits vs. more direct hits.Findings.

Most of the mass was concentrated in the core. The positive charge was concentrated in the core. Most of the area around an atom was “empty”.Nucleus is small compared to the atom, but very

large compared to electrons.Rutherford called this core, the nucleus. Lead to new model:

Nuclear Model

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MoseleyStudent of Rutherford’s

Found that atoms of each element contained unique positive charge in their nucleus.

Helped solve the mystery of what makes atoms of one element different from another element.

Atomic Number = # of PROTONSProton is the positive charge within an atom’s nucleus.

Atoms are electronically neutral.# of Protons (atomic #) = # of Electrons

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Bohr (1913):Niels Bohr came to work in Rutherford’s laboratory

Bohr asked to work on model because there were some problems with the nuclear model

According to Bohr's model only certain orbits were allowedVisualized the atom like the solar system. (Electrons in distinct “orbits” around the atom’s nucleus).

Bohr's model of the atom is important introduced the concept of the quantum (levels) Started to explain atomic properties. However, Bohr's model needed revision

failed to explain the nature of atoms more complicated than hydrogen.

It took roughly another decade before a new more complete atomic theory was developed

the modern atomic theory.

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Louis de Broglie introduces the wave/particle duality of matter (1921)  Traditional (classical) physics had assumed that particles were particles

and waves were waves

However, de Broglie suggested that particles could sometimes behave as waves and waves could sometimes behave as particles - the wave/particle duality of nature.

He suggested a simple equation that would relate the two: Particles have momentum (p), waves have wavelengths (l) and the two are related by the equation

l=h/p

h=Planck's constant = 6.634x10-34 Js

p=(mass)x(velocity)

This wave/particle duality of nature turned out to be a key to the new atomic theory.

Werner Heisenberg elucidated the Uncertainty Principle (1923)

Classical physics had always assumed that precise location and velocity of objects was always possible.

Heisenberg, however discovered that this was not necessarily the case at the atomic level.

In particular, he stated that the act of observation interfered with the location and velocity of small particles such as electrons.

This is the case because observation requires light and light has momentum.

When light bounces off an electron, momentum exchange can occur between light and the electron which means the electrons location and velocity have been altered by the act of measurement.

This scenario has important implications to what we can measure at the atomic level.

Erwin Schrodinger took the ideas developed by de Broglie, Heisenberg and others and put them together in a single equation that is named after him.

Schrodinger Equation

Hamiltonian represents the total energy of the system.

Solving this equation can, in principle, predict the properties and reactivities of all atoms and molecules.

Unfortunately, it is extremely difficult to solve for any but the most simple atoms and molecules. some essential conclusions:

I) Energies are quantized: Atoms and molecules cannot have any energy but only certain energies. This means that energies are "quantized".  

II) The orbitals, associated with each energy, determine where the electrons are located.

•These orbitals can be seen as the "rooms" in which the electrons in an atom "live". •The quantum energies together with the orbitals can be used to explain chemical properties and reactivities.

Electron Cloud Model

Electrons are located within

certain 3-D regions around the nucleus called clouds.

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Atoms

The practical “stuff”

Atoms Made up of:Electrons

Negative charge

Around the nucleus

VERY small massProtons

Positive charge

In the nucleus

Accounts for large part of mass of an atomNeutrons

No charge

In the nucleus

Accounts for large part of mass of an atom.

“Glue” of an atomA force called the strong force opposes and overcomes

the force of repulsion between the protons and holds the nucleus together. (Binding Energy.)

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Atoms (cont.)Ions Atoms can gain or lose electrons Become Ions (more or less electrons than the # of

protons) Can be either POSITIVE or NEGATIVE Charge of ion = # of Protons - # of Electrons e.g. Magnesium (Mg) atomic # 12. Loses 2 e-

# of Protons 12

-# of electrons -10

+2

Mg2+

Try a few:

a) Ca loses 2 e- b) F gains 1 e- c) As gains 3 e-

Ca2+ F1- As3-

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Atoms (cont.)Isotopes All about Neutrons

“Glue”

Mass is 1 a.m.u. (Atomic Mass Unit)

(Mass of Proton are each 1 a.m.u.)

Think about the charges in the nucleus

(Repelling + charges)

More “glue” is needed as the # of protons climbs Same # of protons (why?) and different # of neutrons e.g. Hydrogen

Hydrogen (1H) has 1 proton, 0 neutrons. Mass is 1 a.m.u.

Deuterium (2H) has 1 proton, 1 neutron. Mass is 2 a.m.u.

Tritium (3H) has 1 proton, 2 neutron. Mass is 3 a.m.u. Mass Number:

Sum of isotope’s protons and neutrons.

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Isotopes (cont.)

Mass # 37Cl

Atomic # 17

What is that number (decimal) at the bottom under the symbol?

WHY?

Average of the isotope’s mass

e.g.12C 98.90% (of mass # 12) 0.9890 x 12 = 11.868 13C 1.10 % (of mass # 13) 0.0110 x 13 = 0.143

Average atomic mass: 12.011 amu

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Radioactivity

Changes in the NucleusRadioactivity

Nuclear stability (instability)

Recall:Protons (? Charge)

Therefore: REPEL each otherNeutron (? Charge)

“Glue” to hold Protons togetherStrong Nuclear Force (the glue)

Nuclear stability (cont.)From 1-20, approximate 1:1 Protons: Neutrons.

Beyond 20, more Neutrons.

83 and beyond, spontaneous emissions Can’t hold together indefinitelyFalls apart

Called “Decay”

As a radioisotope tries to stabilize, it may transform into a new element in a process called transmutation.

Not only too little “glue”, but also too much.

As a general rule, lighter & heavier isotopes (vs. common isotope) are likely radioactive.

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Nuclear stability (cont.)The basic unit of measure for radioactivity is the curie,

named after Marie Curie.

A quantity of 1 curie (or 1 C) is 37 billion atoms decay (disintegrate) in one second.

1C = 3.7 X 1010 disintegrations/sec. If the rate of decay is greater than 37 billion atoms in one second, then the source would have an activity greater than one curie if that source had fewer than 37 billion atoms decaying in one second, its activity would be less than one curie.

Types of Radioactive Decay (basics)AlphaBetaGammaSpecials

Positron EmissionElectron Capture

Alpha Consists of 2 protons & 2 neutrons

What is that? 4

2He or 42a

Helium Nucleus (no electrons)Penetration power:

Stopped by paperCharge/Mass:

2+ / 4 amuWith Alpha, think LOSS

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Types of Radioactive Decay (cont.)Beta

Consists of high speed electronsWhat is that?

e- or 0-1b

Where do they come from????

Neutron changing into a proton (flip of a quark), ejects e-

Penetration power:

Stopped by heavy clothing

Charge/Mass:

1- / ~0 amu

With Beta, think CHANGE

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Types of Radioactive Decay (cont.)Gamma

Consists of high energy photons

What is that? gSimilar to X-rays

Penetration power:

Stopped by lead, concrete

Charge/Mass:

0 / 0

With Gamma,

Think ENERGY•See Radiation Movie

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Types of Radioactive Decay (cont.)Specials

Positron Emission (also called Beta positive decay)

A positron is exactly like an electron in mass and charge force except with a positive charge.

Charge/Mass: 1+ / 0 amu

It is formed when a proton breaks into a neutron with mass and no charge

= positron (no mass and the positive charge)

Positron emission is most common in lighter elements with a low neutron to proton ratio.

Specials (cont)Electron Capture

A captured electron joins with a proton in the nucleus to form (change to) a neutron.

= one less proton, turned into a neutron.

Charge/Mass: changes from + 0 / same

Electron capture is common in larger elements with a low neutron to proton ratio.

Decay of Uranium

Uranium

Reminder:

Nuclear Equations Equation that keeps track of the reaction’s components.

Alpha decay of Gold185

79Au 18177Ir + 4

2a

Decay of Iodine131

53I 13154Xe + 0

-1b

Try a few (solve):238

92U 23490Th + ______

2411Na ______ + 0

-1b

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Uranium:

Enriched Uranium: U-235Low: 3-4% U-235 (remaining is U-238)

Reactor GradeHigh: 90% U-235 (remaining is U-238)

Weapons GradeSlightly (0.9%-2%) (replaces natural U in some reactorsRecovered: less U-235 than in natural occurring Uranium

Depleted Uranium: U-238Remnants after enrichment

Less radioactive then natural Uranium

VERY Dense

-Useful for armor and penetrating weapons

NOTE: Depleted U-238 is still radioactive. Just LESS.

Uses for Radioactive materials:

Weapons:Nuclear weapons

“Little Boy”

-Uranium, gun-type

-City of Hiroshima on

August 6, 1945

“Fat Man”

-Plutonium, Implosion

-City of Nagasaki on

August 9, 1945

Uses for Radioactive materials:

Smoke Detectors: Am-241Gives off a particles

Ionizing energy (makes Ions)Ionize smoke particles

Allows completion of a circuit (allows electricity to flow)With a complete circuit, alarm sounds.

Cancer Treatment (BNCT)Boron Neutron Capture TherapyPatient is given Boron-10 (10B)Using a neutron beam, doctors create thermal neutrons which changes Boron-10 into excited Boron-11Boron-11 decays, given off an a particle

a particle penetrated one-two cells deep,

Kills the cell(s)

Uses for Radioactive materials:

Carbon-14 datingThe radioactive C-14 method of dating is used to determine the age of organic matter that is several hundred years to approximately 50,000 yrs old.

C-14 is continually formed in nature by the interaction of neutrons with N-14 in the Earth’s atmosphere.

The neutrons required for this reaction are produced by cosmic rays interacting with the atmosphere.

C-14, along with non-radioactive C-13 and C-12, is converted into CO2 and assimilated by plants and organisms.

When plant or animal dies, it no longer acquires carbon.

C-14 begins to decay.

Nucleosynthesis:

How were/are elements formed?

Nucleosynthesis (cont.):

How were/are heavier elements formed?

Fusion with increasingly larger and larger elements4

2He + 42He 8

4Be + g

42He + 8

4Be 126C + g

Elements present in stars (depends on its size)H, He, C, O, Ne, Mg + other heavier elements

Larger stars (greater gravitational energies), heavier elements

Each “layer” acts to fuel the next “layer”.H He; He C; C O; O Ne; Ne Mg; Mg Si; Si Fe

Heavier elements are created in supernovas (exploding stars).