Physics 30 - Atomic Physics

Cathode ray

A beam of electrons emitted from a negative cathode

By electric and magnetic fields

What can cathode rays be deflected by?

Electric force and magnetic force

What forces to consider when dealing with Thomson's cathode ray experiment?

A centripetal force

When a charged particle moves perpendicular to a magnetic field, what can the magnetic force be considered as?

centripetal force = magnetic force

What forces should be considered when finding charge - to - mass ratio

He sprayed mists of oil into a contraption with two charged parallel plates. When the oil drops passed through the top plate the oil becomes charged. He was then able to adjust the electric field between the plates to make the oil drops to be suspended. This experiment determined the charge of a singular electron

Describe Millikan's Oil Drop Experiment

Force of gravity and electric force

What forces should be considered when dealing with Millikan's Oil Drop Experiment?

Rutherford shot a beam of alpha particles into a piece of thin gold foil. Most of them passed through the foil but there were very few that actually ricocheted or scattered backward. This proved that there was a high concentration of positive charge.

What was Rutherford's gold foil experiment and what did it prove?

Rutherford predicted that electrons orbit around the nucleus. However if this was the case, they would be accelerating because of their constant change in direction. According to Maxwell, accelerating charged particles are the source of electromagnetic radiation. So, an orbiting electron would be losing energy and would then collapse into the nucleus.

What was the flaw in Rutherford's Model of the Atom?

A hot, dense material

What causes a continuous spectrum with no bright or dark lines?

A hot gas at low pressure

What causes an emission spectrum with bright lines at distinct characteristic wavelengths?

Shining white light through the gas at low pressure

What causes an absorption spectrum with dark lines that match the bright lines of the gas?

Fraunhofer Lines

The dark lines seen on the absorption spectrum of the Sun

Excited state

When an electron absorbs energy from an incident photon that'll make it transition to a higher energy level

An absorption spectrum

When an electron gets to an excited state, what kind of spectrum is created?

When the electrons jumps from an excited state back down to a ground state

When is a photon emitted from an atom?

An emission spectrum

When an electron jumps from an excited state back down to a ground state, what kind of spectrum is created?

It depends on the differences in energies between orbits

What does the amount of energy an electron can absorb and/or emit depend on?

Energy of photon = Energy final - energy initial

What equation should be considered when finding the differences in energy of energy levels?

Top number: atomic mass number (number of protons + number of neutrons)

Bottom Number: Atomic number (number of protons)

Symbol: Element symbol

In nuclear notation, what does the top number mean? The bottom number? The large symbol?

Isotopes

Atoms of the same element that have different numbers of neutrons (the # of protons and electrons stays the same)

ex. Uranium - 235 (92 protons, 143 neutrons) and Uranium - 238 (91 protons, 146 neutrons)

1 u = 1.66×10⁻²⁷ kg

What is 1 atomic mass unit equivalent to?

The strong nuclear force. It is much stronger than the electrostatic forces of repulsion between the protons and only acts over a few femtometers

What binds the protons and neutrons together in the nucleus?

Binding energy

The energy requires (work done against the strong nuclear force) to separate the nucleons of a nucleus

∆E = ∆mc²

What is the equation for mass-energy?

Less

Is the mass of the sum of the separate masses in a nucleus more or less than the actual mass of the nucleus?

Mass defect

What is the “missing mass” converted into energy called?

Results from the decay (disintegration) of an unstable nucleus

What does radioactivity result from?

When unstable nuclei emits alpha particles

What is alpha decay?

Unstable nuclei will emit alpha particles when the electrostatic repulsive forces between the protons becomes greater than the strong nuclear force

When will nuclei emit alpha particles?

₂⁴He or ₂⁴α

What is the symbol for alpha particles?

Transmutation

When an atom undergoes alpha decay, it loses 2 protons and 2 neutrons. Since it loses protons, it must turn into another element

AZX → A - 4 Z - 2 Y + 4 2 α

Where:

A = at the top and represents the atomic mass number

Z = on the bottom and represents the atomic number

X = Parent Element

Y = Daughter element

What is the nuclear reaction of alpha decay?

No, since it can’t pass through the skin and can be stopped by a sheet of paper

Is alpha radiation very penetrating?

Beta - negative decay

A beta-particle is emitted when a neutron changes into a proton. When that occurs, a negative charge must be produced to balance the positive charge. This is in form of an electron that is thrown from the nucleus

⁰-₁ β or e⁻

What is the symbol of an electron or beta-negative particle?

Yes because they are high speed electrons. Most beta particles can be stopped by a thin piece of metal

Are beta-particles more penetrating than alpha particles?

You transmute the parent atom into the parent the element with one more proton, one more step on the periodic table. The mass number remains unchanged

What is the effect of beta-decay?

AZX → A Z+1 Y + ⁰-₁ β + antineutrino

What is the nuclear equation of beta negative - decay?

It is emitted when a neutron changes into a proton

When is an antineutrino emitted?

Beta-positive decay

a beta particle is emitted when a proton changes into a neutron in the nucleus. The opposite of beta-negative decay. When this occurs, a positive charge must be created to conserve positive charge of the proton that became a neutron. This is the form of a positron that is thrown from the nucleus.

₊₁⁰ β or e⁺

What is the symbol of a positron?

Positron

The antiparticle of the electron

Antimatter

A form of matter that has a key property, such as charge, opposite to that of ordinary matter

AZX → A Z - 1 Y + ₊₁⁰ β + neutrino

What is the nuclear equation of beta-positive decay?

Gamma rays

Very high energy photons emitted by the nucleus. Sometimes the nucleus can be in an “excited” state, a high energy state. This is possibly due to a collision with another particle, or the nucleus is excited after an α or β decay.

γ

What is the symbol of gamma rays?

Gamma decay

When the nucleus jumps down to a lower energy state, it releases energy as a photon, a gamma ray. No transmutation happens

It is the most penetrating radiation. Several centimeters of lead is required to stop it

How penetrating is gamma radiation?

Radioactive decay series

A process of successive decays that continue until the creation of a stable nucleus

Half-life

The time required for one-half of the atoms in any sample of radioisotope to decay

Activity (A)/Decay Rate

the number of nuclei in a given sample that decay within a given time. It is expressed in becquerels (1 Bq = 1 decay/s)

Fission

The splitting of a nucleus into 2 or more smaller nuclei. It’s accompanied by the release of a tremendous amount of energy. The mass of the products is less than the mass of the target nucleus, and the mass defect is converted to energy.

Fusion

Nuclei with small masses combine to form a nucleus with a larger mass. The larger nucleus is more tightly bound, so its mass is less than the sum of the masses of the smaller nuclei. This mass defect is converted to its

Cloud Chambers (aka Wilson Chamber)

Used for detecting particles of ionizing radiation

Broad and straight because it has double the charge and has more mass

What is the track of an alpha particle in a cloud chamber

Thinner and shows more evidence of deflection

What is the track of an electron in a cloud chamber

An apparatus in which a charged particle passes through a superheated liquid and leaves a trail of gas bubbles that can be illuminated and photographed

What is a bubble chamber

The tracks of the particles would be curved. Positive charges would go one way while negative charges curve the other way. A magnetic field causes charged particles to travel in helical whose radius is determined by the charge-to-mass ratio

What happens if a magnetic field is present?

Electron-positron annihilation

Annihilation results in 2 diametrically emitted photons with a typical energy of 511 keV each

The patient is injected with a radioactive tracer, which is a dye that is undergoing beta decay and emitting positrons. The positrons interact with electrons in the surrounding tissue and the resulting map shows the tissues in which the molecular probe has become concentrated

Describe PET scans

Hadron

Any particle that interacts via the strong nuclear force

They are composed of smaller particles called quarks

What did physicists discover about the composition of protons and neutrons

uud (up, up, down)

What is the quark composition of a proton

udd (up, up, down)

What is the quark composition of a neutron