Elements can be identified from the color of light they give off when they’re ionized: their emission spectra. Ms. Wilson’s chemistry class today set fire to some metal salts to watch them burn.
A hydrogen atom's electron is bumped up an energy level/shell by ultraviolet light, but releases that light when the electron drops back down to its original shell.
She placed the salt crystals into petri dishes, submerged them in a shallow layer of alcohol, and ignited the alcohol. As traces of the salts were incorporated into the flames, the metal atoms became “excited” as they absorbed some of the energy from the flame by bumping up their electrons into higher electron shells. Since atoms don’t “like” to be excited, their excited electrons quickly dropped back to their stable, ground state, but, in doing so, released the excess energy as light of the characteristic wavelength.
PhD Comics does a wonderful job of explaining of sub-atomic particles: what we know, what we don’t know. What’s particularly great about this video is that it goes into how physicists are using the Large Hadron Collider to try to discover new particles: by making graphs of millions of collisions of particles and looking for the tiniest of differences between different predictions of what might be there.
I also like how clear they make the fact that science is a processes of discovery, and what fascinates scientists is the unknown. Students do experiments all the time and if they don’t find what they expect — if it “doesn’t work” — they’re usually very disappointed. I try my best to let them know that this is really what science is about. When your experiment does not do what you want, and you’re confident you designed it right, then the real excitement, the new discoveries, begin.
Emission and absorption spectrum of Hydrogen. Image adapted from the one by Wikimedia Commons User:Adrignola.
When a photon of light hits an atom three things can happen: it can bounce off; it can pass through as if nothing had happened; or it be absorbed. Which one happens depends on the energy of the light, and which atom it is hitting. Hydrogen will absorb different energies from helium.
The interesting thing is that each atom will only absorb photons with exactly the right energy. You see, when the light hits the atom, the atom will only absorb it if it can use it to bump an electron up an electron shell.
An oxygen atom with two electrons in the innermost shell and six in the second shell.
An oxygen atom, for example, has eight electrons, but these fit into two different electron shells. The innermost shell can only hold two electrons, so the other six go into the second shell (which can take a maximum of 8). This is the “ground state” of the atom, because it takes the least amount of energy to keep the electrons in place.
However, if the atom gets hit by just enough energy, one of the electrons can be bumped up into a higher shell. The atom will be “excited” and “want” the electron to drop back down to the lower shell.
And when the electron drops back, it will release the exact same amount of energy that it took to move it up a shell in the first place.
A hydrogen atom's electron is bumped up an energy level/shell by ultraviolet light, but releases that light when the electron drops back down to its original shell.
For hydrogen, the energy to bump up an electron from its first to its second electron shell comes only from light with a wavelength of 1216 x 10-10m (see here for how to calculate). This is in the ultraviolet range, which we can’t see with the naked eye.
However, the energy absorbed and released when the electron moves between the second and fourth shells is 6564 x 10-10m, which is in the visible range. In fact, it’s the red line on the right side of the emission spectrum shown at the top of the page.
The visible part of the emission spectrum of hydrogen and its corresponding electron shell jumps. Note that different colors of light have different energies, and that the blue light is more energetic (and has a shorter wavelength) than red light.
Since this emission signature is unique for each element, looking at the colors of other stars, or the tops of the atmospheres of other planets, is a good way of identifying the elements in them. You can also do flame tests to identify different elements.
Copper.Lithium.Sodium.
The University of Oregon has an excellent little Flash application that shows the absorption spectrum of most of the elements in the periodic table (NOTE that the wavelengths they give are in Angstroms (Å) which are 1×10-10m; or tenths of a nanometer (nm)).
Interactive app that shows the absorption spectrum of the elements in the periodic table. From the University of Oregon. Wavelengths are in Angstroms (Å), and 1 Å = 0.1 nm.
Equations
The wavelength of light emitted for the movement of an electron between the electron shells of a hydrogen atom is given by the Rydberg formula:
where: = wavelength of the light in a vacuum = Rydberg constant (1.1×107 m-1) and are the electron shell numbers (the innermost shell is 1, the next shell is 2 etc.)
Brian Cox explains (on the BBC) explains how electron shells are like standing waves, and how that relates to the sizes of atoms, and explains why atoms are mostly space.
I have a neat little tea strainer that sits inside my almost perfect teacup, yet I’m usually at a loss about what to do with it when I take it out of the cup. When the lid is upside down, the strainer can sit nicely into a circular inset that seem perfectly designed for it; however, if I want to use the lid to keep my tea warm — as I am wont to do — I have to move the strainer somewhere else.
One option is to just put the strainer in another cup, but then air can’t circulate around it, and instead of drying, the used tea leaves stay wet and, eventually, turn moldy. A flat saucer would be better, but not perfect.
Of course, I could just empty out the strainer, wash and dry it as soon as I’m done steeping the leaves, but there are a few ancillary considerations with respect to time that make this a sub-optimal solution.
So, since we have a kiln on campus that sees regular use, I thought I’d sit in on the Middle School art class and make my own ceramic tea strainer holder. Since I’ve also been thinking about Philip Stewart’s spiral, and de Chancourtois‘ helictical periodic tables, and been inspired by Bert Geyer’s attempts at making sonnets tangible, it eventually occurred to me that an open helictical form would work fairly well for my purposes.
I’ve cobbled together a design using Inkscape, and layered it onto a cylinder in Sketchup to see what it would look like.
Draft model of a tea strainer holder.
So far the reactions from students has been quite diverse. I have one volunteer who’s wants to help, and I’ve sparked some discussion as to if what I’m doing actually qualifies as art. There is a lot of curiosity though. The middle-schoolers will probably be doing some type of physical representation of the periodic table, so I’m hoping this project gets them to think more broadly about what they might be able to do.
Physicists at CERN believe they’ve measured neutrinos moving faster than the speed of light. Since most of modern physics is based on the speed of light being the upper speed limit for practically everything, (remember, in E=mc2, c is the speed of light) this is somewhat of a big deal. NPR has an article:
Notes
1. Neutrinos themselves are quite fascinating and elusive particles. Sciencemadefun has a nice video explaining what is a neutrino.
2. Victor Stenger provides an interesting perspective on these results. He points out that the theoretical particles, tachyons, move faster than light, but they can’t move slower than light, so, seen from the point of view of a tachyon, time would move backward. Only photons move at the speed of light.
If you pull apart an atom, the individual parts will weigh more than the atom you started with. The extra mass is the binding energy, which is released when the nucleus of atoms break apart (nuclear fission).
If you pull apart the nucleus of an atom, you’ll find that the mass of its parts is greater than the mass of the original nucleus. That extra mass is where nuclear energy comes from; it’s called the binding energy.
How so?
An alpha particle is the nucleus of a helium atom.
Take a helium atom for example. Its nucleus typically has two protons and two neutrons*, which in nuclear physics is usually called an alpha particle (α).
While we usually say that the mass of a proton is 1 atomic mass unit (u), its actually a little heavier. The mass of a proton is 1.00728 atomic mass units (u), while neutrons weigh 1.00866 u.
The alpha particle (helium nucleus) has less mass than sum of the masses of the individual particles that make it up.
The combined mass of the two protons and two neutrons in the helium nucleus is 0.03035 atomic mass units more than the mass of a helium nucleus made up of the very same particles.
Why?
The one equation that everyone remembers from Einstein (perhaps from all the t-shirts) is:
Energy (E) is equal to mass (m) times some constant (c is the speed of light) squared. What it means is that mass is energy, and vice-versa.
When the four nucleons combine, the extra mass is transformed into the energy that holds them together in the nucleus of the atom. The mass can be directly converted to energy, the binding energy of the atom.
How much energy is released?
Somewhere around 10,000 times more energy is released from a single nuclear reaction compared to a single chemical reaction (like the combustion of TNT).
Binding energy per neuleon for the naturally occuring elements. (image from Science in School).
Footnotes
* Helium with two neutrons would be written , where the bottom number is the number of protons and the upper number is the atomic mass, which is the sum of the number of protons and the number of neutrons.
The Auroras are a great natural phenomena that relates to elements, the structure of atoms, and ionization. They also tie into the physics of charged particles in magnetic fields. The video below provides and excellent overview and also brings up nuclear fusion and convection.
This video explains how particles originating from deep inside the core of the sun creates northern lights, also called aurora borealis, on our planet.
See an extended multimedia version of this video at forskning.no (only in Norwegian):
http://www.forskning.no/artikler/2011/april/285324
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This video is produced by forskning.no in collaboration with the Department of Physics at the University of Oslo.
Production, animation and music: Per Byhring
Script: Arnfinn Christensen
Scientific advisors: Jøran Moen, Hanne Sigrun Byhring and Pål Brekke
Video of the northern lights: arcticlightphoto.no
Video of coronal mass ejection: NASA
= wavelength of the light in a vacuum
= Rydberg constant (1.1×107 m-1)
and 
are the electron shell numbers (the innermost shell is 1, the next shell is 2 etc.)
= Energy (J),
= 6.6×10-34 Js,
= speed of light in a vacuum (3×108 m/s).
, where the bottom number is the number of protons and the upper number is the atomic mass, which is the sum of the number of protons and the number of neutrons.