Wonders of the Atom

Within the atom

Hang out with the protons, same mass (more or less) not always same numbers of neutrons as protons.

A negative charge. Very far away from the protons, weigh only .

A positive particle with the same charge as an electron (), but nearly 2,000 times the mass (). An element has the same number of protons as electrons.

An Atom’s two regions

1. Nucleus very small region located near the center of an atom.

a) In the nucleus there is at least one positively charged particle called the proton.

b) Usually at least one neutral particle called the neutron.

2. Surrounding the nucleus is a region occupied by negatively charged particles called electrons.

Nucleons

Atoms are different sizes, but they are on the scale of the nucleus (all those protons and neutrons) packed into a pea….Picture that pea sitting in the middle of a stadium The electrons would be whizzing away somewhere in the stands.

How Big?

Car… still much bigger than a pea!

The atom has come along way through history…

Name Points PictureDemocritus

• Different for different elements.

• Smallest possible “object.”• Indivisible.

Dalton • Spherical.• Combine in set ratios.

Thomson

• Positive “pudding.”• Negative “plums.”

Rutherford

• Small, dense, positive nucleus.

• Electron cloud.• Divisible.

Bohr • Electrons orbit nucleus.• Electrons have set energy

levels.

Review

Up & Atom

Models of Atoms Through the Years.

This model of the atom may look familiar to you.

This is the Bohr model. In this model, the nucleus is orbited by electrons, which are in different energy levels. http://www.colorado.edu/physics/2000/quantumzone/bohr.html

What a BohR

Electrons Have Specific Energy Levels

• Bohr’s model aimed to explain spectral lines.

• When electrons lose energy they emit particular frequencies of light.

• Bohr showed these particular energies as “orbitals,” similar-looking to the solar system.

An electron orbital is a region around an atomic nucleus (not seen) in which one or a pair of electrons is most likely to exist. For each orbital, The red area is where an electron has a positive wavefunction, and the blue area is where the wavefunction is negative. The number and distribution of electrons in an atom's orbitals plays a major role in determining the reactivity and chemical properties of the atom.

Let’s look at a few elements…

Hydrogen 1= Proton 1= electron

Let’s look at a few elements…Helium 2=protons 2=neutrons

2=electrons

Let’s look at a few elements…Lithium 3=protons 4=neutrons

3=electrons

Let’s look at a few elements…

Fluorine9=protons 10=neutrons

9=electrons

Let’s look at a few elements…

Argon18=protons 22=neutrons

18=electrons

Decay types

Decay

What Happens? How’s the Nucleus?

Alpha

An alpha particle emitted from nucleus

2 less protons2 less neutrons

Beta (minus)

A nucleus emits an electron 1 neutron changes to a proton1 less electron

Beta (plus)

A nucleus emits an positron 1 proton changes to a neutron1 less electron

Gamma

Excited nucleus releases a high-energy photon (gamma ray)

Parts remain the same, but nucleus is less excited

Neutron

A neutron ejected from nucleus

1 less neutron

𝑬=∆𝒎𝒄𝟐

Energy cannot be created nor destroyed, and energy, in all of its forms, has mass. Mass also cannot be created nor destroyed, and in all of its forms, has energy.

Energy and mass are exchangable

For example, a water molecule weighs a little less than two free hydrogen atoms and an oxygen atom; the minuscule mass difference is the energy that is needed to split the molecule into three individual atoms (divided by c²), and which was given off as heat when the molecule formed (this heat had mass).

In this case the mass difference is the energy/heat that is released when the dynamite explodes, and when this heat escapes, the mass associated with it escapes, only to be deposited in the surroundings which absorb the heat (so that total mass is conserved).

𝑬=∆𝒎𝒄𝟐 Energy and mass are exchangable

but this is true only so long as the fragments are cooled and the heat removed….

Likewise, a stick of dynamite in theory weighs a little bit more than the fragments after the explosion.

𝑬=∆𝒎𝒄𝟐

• A spring's mass increases whenever it is stretched or compressed. Its added mass is the added potential energy stored within it, which is bound in the stretched electron bonds linking the atoms within the spring.

Whenever energy is added to a system, the system gains mass.

𝑬=∆𝒎𝒄𝟐

• Raising the temperature of an object (increasing its heat energy) increases its mass.

For example, consider the world's primary mass standard for the kilogram, made of platinum/iridium. If its temperature is allowed to change by 1°C, its mass will change by 1.5 pg (1 pg = 1 × 10−12 g).

Whenever energy is added to a system, the system gains mass.

𝑬=∆𝒎𝒄𝟐• A spinning ball will weigh more than a ball that is not

spinning. Its increase of mass is exactly the equivalent of the mass of energy of rotation,

• Which is itself the sum of the kinetic energies of all the moving parts of the ball).

For example, the Earth itself is more massive due to its daily rotation, than it would be with no rotation. This rotational energy (2.14 x 1029 J) represents 2.38 billion tonnes of added mass.

Whenever energy is added to a system, the system gains mass.

Note that no net mass or energy is really created or lost in any of these examples and scenarios. Mass/energy simply moves from one place to another.

Fission and Fusion

Atoms are the building blocks from which matter is formed. Everything around us is made up of atoms. Nuclear energy is contained within the centre of the atom in a place known as the nucleus. Particles within the nucleus are held together by a strong force. If a large nucleus is split apart (fission), generous amounts of energy can be liberated. Small nuclei can also be combined (fusion) with an accompanying release of energy. Using this strong force that holds the nucleus together to produce energy is essentially what the field of nuclear power generation is about.

The Power of the sun

Fission In The Sun

You tube video: http://www.youtube.com/watch?v=TOErr4xntHE&feature=related Reactions:http://science.howstuffworks.com/sun2.htm Reactions in Detail: http://zebu.uoregon.edu/~soper/Sun/fusionsteps.html Fate of the Sun: http://science.howstuffworks.com/sun6.htm

Nuclear Power

Power Derived from Nuclei

http://www.howstuffworks.com/nuclear-power.htm

Control rods

They absorb the neutrons

Boron is another common neutron absorber. Mechanical properties of boron in its elementary form are unfavourable, therefore alloys or compounds have to be used instead.

Cadmium alloys, generally 80% Ag, 15% In, and 5% Cd, are a common control rod material for pressurized water reactors. It has good mechanical strength and can be easily fabricated. It has to be encased in stainless steel to prevent corrosion in hot water.

They absorb the neutrons

In The Reactor

The graphite core slows the neutrons down which increases the likelihood of a collision.

Critical massA critical mass is the smallest amount of fissile material needed for a sustained nuclear chain reaction. The critical mass of a fissionable material depends upon its nuclear properties: its density, its shape, its purity, its temperature and its surroundings.

Hydrogen bombEnergy produced by fusion of lighter elements.

Binding Energy

Energy produced by fusion of lighter elements and fission for heavier elements

Black BodyAll matter emits electromagnetic radiation when it has a temperature above absolute zero. The radiation represents a conversion of a body's thermal energy into electromagnetic energy, and is therefore called thermal radiation. It is a spontaneous process of radiative distribution of entropy.Conversely all matter absorbs electromagnetic radiation to some degree. An object that absorbs all radiation falling on it, at all wavelengths, is called a black body.A black body is absorbs all wavelengths of light, and is also the perfect emitter of light.

At Earth-ambient temperatures this emission is in the infrared region of the electromagnetic spectrum and is not visible. The object appears black, since it does not reflect or emit any visible light.

As the temperature increases past a few hundred degrees Celsius, black bodies start to emit visible wavelengths, appearing red, orange, yellow, white, and blue with increasing temperature. When an object is visually white, it is emitting a substantial fraction as ultraviolet radiation.

Black Body Radiation

Max Planck

Plancking to the max since 1858 (before it was mainstream)

Planck was convinced that matter was quantised.Planck turned his attention to the problem of black-body radiation. He had been commissioned by electric companies to create maximum light from lightbulbs with minimum energy. The question was this: How does the intensity of the radiation emitted by a black body depend on the frequency and temperature? The question had been explored experimentally, but no theoretical idea agreed with experimental values.Planck proposed that electromagnetic energy could be emitted only in quantized form, in other words, the energy could only be a multiple of an elementary unit , where is Planck's constant.

The Planck constant, was first described as the proportionality constant between and .

Louis de Broglie generalized this relation by postulating that the Planck constant represents the proportionality between the momentum and the quantum wavelength of not just the photon, but any particle. This was confirmed by experiments soon afterwards.

𝑝∝h 𝜆

Electron voltIs approximately 1.602×10−19joule (symbol J). By definition, it is equal to the amount of kinetic energy gained by a single unbound electron when it accelerates through an electric potential difference of one volt.

– Thus it is 1 volt (1 joule per coulomb) multiplied by the electron charge (1.602176565×10−19 C). Therefore, one electron volt is equal to 1.602176565×10−19 J

By mass-energy equivalence, the electron volt is also a unit of massWe use this

as a much more convenient unit instead of dealing with tiny numbers.

Escapist Electrons

How do electrons remove themselves

from the strong hold of the nucleus?

As you know: Electrons are “bound” to the nucleus by the weak nuclear force, this is similar to the way the earth is bound in orbit by the sun, although the weak nuclear force is actually much stronger than the gravitational force!• Too small an amount and the rocket would fall back to Earth,

the electron would fall back into it’s orbit.• Just the right amount and the electron will escape the nucleus

but with no extra kinetic energy, i.e. . • Any amount of energy the electron has extra to the particular

energy needed to break free of that metal will be transformed into kinetic energy.

Electrons must do work to escape the nucleus, just as a rocket must do work to escape the gravity of the Earth.

The Work Function Work function is the energy (or work) required to withdraw an electron completely from a metal surface. This is a measure of how tightly a particular metal holds its electrons.The more energy needed to remove an electron, the higher the work function.𝒉𝒇 =𝝓+𝑬𝑲

Functions of different elements

Compare Silver and Gold on the periodic table to

Calcium and Sodium

emission spectraThese are the specific frequencies of light that different elements emit.

Scientists were puzzled for many years, they decided to focus on trying to explain the “simplest” atom: Hydrogen.

Fun Fact:Sodium is used in many street lamps, you can see the emission spectra shows yellows, hence the tell-tale yellow of the street lamp.

Hydrogen spectrumAbsorption spectra show all the frequencies the element absorbs.

Emission spectra show all the frequencies the element emits.

Hydrogen spectrum

Because these lines show not only different frequencies, but different energies.http://www.ucolick.org/~

bolte/AY4_00/week2/atomic_spectra.html

Hydrogen spectrum

http://www.ucolick.org/~bolte/AY4_00/week2/atomic_spectra.html

Hydrogen spectrum

Schrödinger's Cat The Cat that Defies Logic

http://www.tcd.ie/Physics/Schools/what/atoms/quantum/cat.html

Bohr

He suggested that the interpretation to use depends on what apparatus are used to view the object. • Electrons look like particles if probed with

photons, • but like waves if diffracted through a

crystal lattice. Bohr dragged the ideas of matrix mechanics, the Heisenberg uncertainty principle.

Towards the end of his career Bohr took a more interpretative role and struggled more and more with the philosophical issues of quantum mechanics First, he came up with the idea of complementarity.• He noted that the wave and particle views of an

object exclude each other totally but conceded that both are needed in order to fully understand the properties of the object.

Experimental resultsFor a given metal, with a particular work function () and incident radiation, with frequency ():• The rate at which photoelectrons are ejected is directly

proportional to the intensity of the incident light.• There exists a certain minimum frequency of incident

radiation below which no photoelectrons can be emitted. This frequency is called the threshold frequency.

• Increase in intensity of incident beam increases the the photoelectric current, though stopping voltage remains the same. It does not change the kinetic energy of the photoelectrons.

• Increase in frequency of incident beam increases the maximum kinetic energy with which the photoelectrons are emitted. Thus the stopping voltage increases.

(In practice the number of electrons does change because the probability that each photon results in an emitted electron is a function of photon energy).• The time lag between the incidence of radiation and the

emission of a photoelectron is very small, less than 10−9 second.

ExplanationThe photons of a light beam have a characteristic energy determined by the frequency of the light. • In the photoemission process, if an electron within some material

absorbs the energy of one photon and thus has more energy than the work function, it is ejected.

• If the photon energy is too low, the electron is unable to escape the material.

Increasing the intensity of the light beam increases the number of photons in the light beam, and thus increases the number of electrons excited, but does not increase the energy that each electron possesses. The energy of the emitted electrons does not depend on the intensity of the incoming light, but only on the energy or frequency of the individual photons. It is an interaction between the incident photon and the outermost electron.• Electrons can absorb energy from photons when irradiated, but they

usually follow an "all or nothing" principle. All of the energy from one photon must be absorbed and used to liberate one electron from atomic binding, or else the energy is re-emitted. If the photon energy is absorbed, some of the energy liberates the electron from the atom, and the rest contributes to the electron's kinetic energy as a free particle.

The threshold frequency is typically visible light for alkali metals, near ultraviolet for other metals, and extreme ultraviolet for non-metals.

Photoelectric effecthttp://www.nobelprize.org/educational/physics/quantised_world/ The intensity of the light

had no effect on the energy of the ejected electrons. Moreover, experiments showed that there was a threshold frequency below, which not a single photoelectron was ejected. Below this frequency, the brightness of the incident light made no difference at all! Classical physics had failed again – it could not explain either of these observations

PhotoVoltaic effect In the photoelectric effect, electrons are ejected from a material's surface upon exposure to radiation of sufficient energy.

The photovoltaic effect is the creation of a voltage (or a corresponding electric current) in a material upon exposure to light.

Though the photovoltaic effect is directly related to the photoelectric effect, the two processes are different.

Hydrogen spectrumIn fact, these precise spectral lines, have precise frequencies and therefore precise wavelengths.A man by the name of Balmer presented a formula, but could not explain it. Twenty years later, Einstein and Planck explained it for him using quantum mechanics.

This was later generalised to the Reidberg formula:

for UV, for visible, for Infrared

is the orbital (Starting value for is)

is the Reidberg constant.

Different Series

Ultraviolet

Visible Light

Infrared

Bohr’s ModelBohr came to the conclusion that a circular orbit would be unstable: The electron would simply spiral into the nucleus (like water down a drain).

He proposed orbitals as standing waves. Each orbital having wavelengths.

FailingsWhile the Bohr model was a major step toward understanding the quantum theory of the atom, it is not in fact a correct description of the nature of electron orbits. Some of the shortcomings of the model are:1. It fails to provide any understanding

of why certain spectral lines are brighter than others. There is no mechanism for the calculation of transition probabilities.

2. The Bohr model treats the electron as if it were a miniature planet, with definite radius and momentum. This is in direct violation of the uncertainty principle which dictates that position and momentum cannot be simultaneously determined.

The Bohr model gives us a basic conceptual model of electrons orbits and energies. The precise details of spectra and charge distribution must be left to quantum mechanical calculations, as with the Schrodinger equation.