Questions and Answers on Atoms

My responses have not been proof read.

Q
The most important theory
Scientists have always debated over the question: which is more important, the initial theorem that creates the field of science or the later theorems that allow the field to develop. In terms of the atom, which theory do you believe has been the most influential on the progress of atomic theory? (Dalton's theory of the atom, the discover of the electron, the theory behind the current wave mechanical model of the atom, etc_) Why is that theory more influential than the other ones?

When electrons are confined in a potential well (e.g. electrons in an atom), why do we consider them to have negative energy? Why does the energy become positive when energy levels are no longer quantized?
A
A theory is the most important under certain set of circumstances. In our pursuit of science, we usually do not believe (base on faith), but we look at each for its own merit. All these theories have their importance depending on the point of view.

Q
Why is there a negative sign in the equation En=-R n-2. What does this negative energy mean?
A
Since we take the energy of a free electron (not in an atom) as zero, and the energy of an electron in an atom is less than that of a free electron, we designate it with a negative sign. See the energy diagram to be shown later.

Q
Spctroscopy
It mentioned that spectroscopy help to confirm a substance as a chemical element because spectra serve as fingerprints of elements and they have been used to confirm the presence or absence of a certain element in a sample. But can you explain in details that how they are related? Would a lab for this course use spectroscopy to identify elements? Is there a modern day spectroscope and what does it consist of?
A
As we shall see that the sepctra of an element depends on the atomic number in another module. Moseley's law regarding X-rays will give you more insight.

Yes, there are many modern spectrometers, IR, UV-visible, X-ray, and Gamma- ray spectrometers. The radiation is analyzed to measure intensity versus frequencies.

Q
X-rays
What else can X-rays penetrate (excluding paper, wood, aluminum and flesh)? Why only aluminum but not other metals?

When high-energy electrons (cathode-rays) striking a metallic or fluorescence plate, X-rays are generated". What is a fluorescence plate?

Why are X-rays and gamma rays able to penetrate materials made up of light elements more easily than heavier elements? Is it related to the size of the atomic radii? In general, do thicker materials consisting of light elements prevent penetration as well as thinner materials composed of heavier elements?
A
Actually, all electromagnetic waves penetrate some thing, just they penetrate with different degree. The classification or description at some point is oversimplified.

X-rays are generated whenever high-energy electrons strike any target, not necessary fluorescence material.

A fluorescence material absorbs high-energy radiation, and emit low energy photons. Your TV and cathode-ray-tube computer screens have fluorescence material in it. So do the walls of fluorescence light tubes.

When a e-m wave is absorbed, it does not penetrate. When it is not, it penetrate. In general, heavy elements have higher absorption.

Q
Charactersitic X-rays
Why are characteristic x-rays produced? Also what, if any, applications are there for characteristic x-rays?
A
When electrons from a higher energy state go to a lower energy state, the excess amount of energy got off as characteristic X-rays.

Characteristic X-rays have know wavelength, and they are most useful in diffraction studies. They are useful as X-rays of course.

Q
X-ray Diffraction
How did Crick and Watson use X-rays to determine the structure of DNA?
A
Well, there is a long answer to this question - almost as long a a course. However, in short, it's a combination of observation and thinking power that led to their conclusion.

Q
Absorption of X-rays
We know that heavy metals, such as lead, are good at absorbing (or deflecting?) X-Rays. Is this ability related to the size of the nuclide? If so, what is the relationship between x-ray wave length and nuclide diameter, if any?
A
X-rays are generated due to electrons of atoms shifting from higher energy level to lower energy level. Their absorption is due to moving electrons to higher energy levels. Keep this in mind when we talk about interaction of ionizing radiation with matter. The size of atomic nuclei has little to do with the absorption.

Q
Mass of photons
If E = mc2 and E = h*freq then mc2 = h*freq or mc = h / wavelength. This would suggest all particles have wave characteristics and all waves have particle characteristics. f this is the case, then photons should have a measurable mass. Radiation is emitted when electrons drop from one energy level to another in the form of a photon. Would it not then follow that electrons are made up of photons?
A
In deed, your suggestion is excellent. However, we usually consider rest mass of photon zero. This is one of the assumption of Einstein's. The other assumption is that light travels with a universal constant speed.
Your suggestion shows that every photon has a different mass associate with its energy.

Q
Standing light waves
Is resonance possible with things like light waves? Can light exist in a standing wave?
A
I asked this very question when I was young. Later, I learned that light in a LASER tube goes back and forth and the phonomena is very much a resonance. That is why, LASER not only have a unique frequency, the waves travel in a coherent fashion - see page 15 of the lecture notes.

Q
Mollecules
It was said in the lectures that atoms are usually studied by the spectrum of its hot gas. Is it possible to study the atomic makeup of a compound in this fashion?
A
The study of atomic make up of a compound is called elemental analysis. There are several methods for this. When a compound is heated up, the molecules dissociate and you will have a mixture of atoms. However, molecules absorb certain lights, and the study of absorption of light has some use in determining certain things about compounds.

Q
Superstring
In the lectures, it is said that protons and neutrons are composed of the smaller particals, quarks. Yet, due to the conflict between Quantum Mechanics and General Relativity, scientists have proposed that all matter and transmitters of force are further composed strings. Do you believe that string/superstring theory will unite the rift between Quantum Mechanics and General Relativity and eventually lead to the Theory of Everything?
A
I don't see real conflict between quantum theory and general relativity, and I have not understood string theory to predict the future direction of its development.

Q
Quarks
Do quarks and leptons physically exist, or they are nothing more than particles that only exist in theories to help in explaining the material world? In the plot of X-Ray spectrum, why does the peak shift to the right when higher voltages are applied, is this related to the existence of quarks and leptons in the physical world?

Do quarks have masses? Do quarks have any wave-like properties? How accurate is the standard model of particles using quarks and leptons? Is the model proven by convincing experimental evidences?

The top and bottom quarks are the high-energy collisions of particles. How were these particles accelerated, and at what comparative energy values/ 'speeds' were they finally able to be observed?

Are quarks currently the smallest natural unit of matter? If so, are there attempts under way to further subdivide quarks into smaller particles? How would one go about proving that quarks are divisible through experiment?
A
Leptons are observed but no single isolated quark has ever been observed.

We shall discuss quarks in another module.

Particles are produced in high-energy particle collision experiments. Protons and antiprotons are accelerated by electro-magnetic field, and when they collide, particles are produced. From these particles, presence of quarks are inferred. The sentense The top and bottom quarks are the high-energy collisions of particles is improper.

Particles do exhibit wave properties. Since no isolated quarks have been observed, their wave properties have not be observed.

Will smaller particles be proposed in the future as fundamental particles? Well, the future is not ours to see and the answer is a MAY BE.

Q
Isotope
Is there a model for hypothesizing stability of any given isotope?
A
Stability of isotopes will be discussed in the module "Nuclides".

Q
Particle Accelerator
What is the significance of particle accelerator? What is the fastest speed up to date?
A
Particle accelerators are tools for the study of particles and atoms. They are essential for many recent discoveries and theories. We usually speek of the energy of accelerated particles, not their speed. The speed of light is the limiting speed, and high energy particles travel at speed close to but not equal to the speed of light.

Q
Electron spin
What do the electron states +1/2 and -1/2 mean? Quantum mechanics says that electrons aren't "solid" particles, but rather complex wavefunctions. How can a wavefunction have a "spin"?
A
The effect of a magnetic field on an electron indicate 1/2 as a proper indicator. Since there are two states, we use + and - to designate them.

The second part of the question indicates a misunderstanding of the wavefunction. In quantum, the wavefunction represent the state of an electron, it is not the electron. Certainly, the electron is treated as a wave. In fact, spin is a wavefunction itself, and it has two forms.

Where do the values of +1/2 and -1/2 come from in the spin of the electron? Do these come from "advanced" quantum mechanics calculations, or are they arbitrary?

A

These values do come from physical experiments designed to measure its magnetic moments etc.

Q
Harmful particles
Which particle emissions are more dangerous to humans and why: Alpha, Beta or Gamma particles?
A
We shall discuss the relative harm of various high-energy particles in the module Radiation Safety.

Q
Periodic Table of Elements
The modern periodic table of elements as we know it today consist of elements which contain a significant number electrons and specific energy levels. It was stated in the text that Mendeleyev and Meyor might not recoginze the modern periodic table. Can we expect that in our life time will there be other major changes to the periodic table that might affect it's organization? For example, could the discovery of subatomic particles which are smaller than electrons and protons, affect the organization of the modern periodic table?
A
The soiety is constantly changing. At the moment, the Periodic Table of Elements serves us well and I do not see any dramatic change. However, I really cannot predict what you will see in your life time.
Physicists do have a table regarding particles. This one is given in a later module, and physicists consider it as their "periodic table of particles".