Wednesday, July 29, 2009

A Few Words About Neutrons And Isotopes*

I learned this today from Wiki:
The term isotope was coined in 1913 by Margaret Todd, a Scottish doctor, during a conversation with Frederick Soddy. Soddy, a chemist at Glasgow University, explained that it appeared from his investigations as if several elements occupied each position in the periodic table. Todd suggested the Greek term meaning "at the same place" as a suitable name. Soddy adopted the term and went on to win the Nobel Prize for Chemistry in 1921 for his work on radioactive substances.

The concept of isotopes confounded the builders of the Periodic Table in Soddy's time. Things got even worse after J. J. Thompson showed that he could resolve purified neon into neon of two different masses, Ne-20 and Ne-22. It took the birth of quantum mechanics and Chadwick's neutron to put things back together again.

Today we know with confidence that different isotopes of the same element differ in number of neutrons within their atomic nuclei. Neutrons add heft and stability (or instability) to atomic nuclei, without changing the "place" of the element at the table; in other words, what fixes an element's place is the number of protons in its nucleus, not the sum of its protons and neutrons. Thus the concept "at the same place" makes perfect sense for different atomic mass versions of the same element. All naturally occurring elements have isotopes, for example, hydrogen, which has three isotopes so important that they're given quasi-chemical symbols of their own: H, D, and T, corresponding to protium, deuterium, and tritium, having 0, 1, and 2 neutrons respectively.

Our government (and others) have long been in the business of separating isotopes: uranium-235 was the fission fuel for the first atomic bomb, and plutonium-239 was the fission fuel for the second one. The first hydrogen bomb (code-named Ivy Mike) used liquefied deuterium-tritium gas as fusion fuel, i.e., hydrogen molecules consisting of the two heavier isotopes of hydrogen. Ivy Mike weighed around 62 tons, the bulk of which was dedicated to cooling the liquefied fusion fuel. Practical weaponization of the H-bomb was not achieved until lithium deuteride (which doesn't require cryogenics) became the fusion fuel of choice.

Iran is actively pursuing uranium isotope enrichment, ostensibly to collect enough U-235 for either peaceful electrical power generation or for a fission weapon. Less talked about is the concomitant accumulation of so-called depleted uranium (DU) which is the non-radioactive U-238 “waste” obtained during enrichment. DU is both an effective tank armor and a lethal component of bullets or rounds. While travelling at high velocity, DU or DU-coated shells burn into uranium oxide, literally forming a burning projectile. DU weapons and armor were fielded with spectacular results by the US in the First Gulf War: Iraqi tank shells literally bounced off the Abrams tanks equipped with DU armor. You can bet the Iranians were watching that with keen interest.

Isotopes also have many, many peaceful uses: think of radiochemical uses in medicine and biology and their use in determining the geologic age of materials (radiocarbon dating). Stable isotopes like deuterium and carbon-13 also find broad use as detectable labels which can also be introduced into controlled experiments and followed where they go and don't go. Moreover, subtle effects on the rates (speed) of chemical reactions gives insight into how the reactions proceed.

I once worked around neutrons as part of a scientific collaboration. Our endeavors were peaceful, despite occurring in part at Los Alamos National Laboratory. While determining the molecular structure of a certain substance, we needed the help of neutrons to locate hydrogen atoms using a technique called neutron diffraction which uses beams of free neutrons. To make a long story short, we solved the structure, but I went on to show how one could get the same essential information using more conventional instruments, but that’s another story. And that's the closest I ever want to get to loose neutrons.

*My creds include working with neutrons and co-writing a book chapter on isotopes in chemistry.

Saturday, July 18, 2009

It was all just Rocket Science!

This weekend commemorates the historic trip to the moon by the heroic Apollo 11 astronauts 40 years ago. Let's also pause to remember the passion and drive of the men who designed and built the vehicles that put them there, in particular Dr. Wernher von Braun, designated rocket visionary.

Let's pause and also give thought to the victims of the German V-2 rocket program and to the slaves who died making those rockets under appalling conditions (The V in V-2 stands for Vergeltungswaffe = vengeance weapon). And spare a thought for cranky old Robert Goddard, our own homegrown rocket hero, who at least appeared on a stamp:

I am unconvinced by allegations that the Germans stole secrets from Goddard, having read the account of the V-2 program in Michael Neufeld's excellent The Rocket and the Reich. Neufeld, no fawning acolyte of von Braun, correctly points out that the Germans merely used Goddard's published ideas. In science and technology, success builds upon free and open communication.

The Smithsonian in DC has (or used to have) a collection of scale model rockets lined up side by side, showing the historical progression of rocket design. The models may have even been owned by von Braun himself (first photo above). I don't recall exactly where the collection begins and ends, however, a V-2 rocket stands in the lineup. What struck me then was that there were two V-2's next to each other in the collection: a German one and a V-2 that had been rebranded with American insignia.

Soviet advances in the spring of 1945 halted the V-2 program at Peenemünde. Von Braun and his team relocated to a safer location in the Bavarian Alps while the Third Reich collapsed. On May 2, 1945, with Hitler already dead and Berlin under Soviet control, von Braun surrendered to the Americans. He said later:
We knew that we had created a new means of warfare, and the question as to what nation, to what victorious nation we were willing to entrust this brainchild of ours was a moral decision more than anything else. We wanted to see the world spared another conflict such as Germany had just been through, and we felt that only by surrendering such a weapon to people who are guided by the Bible could such an assurance to the world be best secured.
Von Braun and his team, criminally liable in some eyes for the V-2 rocket attacks on European capitals, were given a second chance. Goddard had died in August of 1945 and America needed rocket science. And did we ever get some. Von Braun first headed a secret team located outside of El Paso, TX, where under a sort of house arrest, he and his team reassembed captured V-2 rockets. In 1950, von Braun led the Army's rocket development program team that resulted in the Redstone, the rocket used for the first nuclear ballistic missile. Von Braun and his German wife became naturalized American citizens in 1955.
Von Braun's career really took off after the Soviets launched Sputnik. He was appointed director of the newly created George Marshall Space Flight Center in Huntsville, Alabama. The ballistic missile team, still including many of the old school Peenemünder, all now worked for NASA. And they succeeded splendidly.

My own recollections of the Apollo heydays are still pretty clear. I recall as a boy visiting the Kennedy Space Center in the summer of 1968 on a family vacation to Florida. The giant Saturn V rocket used to launch Apollo 7 was then under construction inside the massive Vehicle Assembly Building . My dad took super 8 mm film of this which I have to just dropped off to convert to digital format. I recall that hot and sweaty Wisconsin day a year later when the moon-landing happened. Relatives were visiting and we cousins had been playing tackle football in the backyard all day. The grown-ups called us inside to watch the historic landing on TV in the cool of the basement.

I also recall seeing von Braun on TV with Walter Cronkite. My memory is fuzzy exactly when that was, but surely it must have been between Apollo missions or perhaps during the long flight time of one of the historic moon missions; von Braun would have been too preoccupied during the take-off and landing phases of each mission to be chatting it up with the avuncular Walter. I do wish I could find that clip on Youtube. Maybe it will turn up as part of a Walter Cronkite retrospective.
Added: Hector at Kiarian Lunch wonders if we will ever go back.
Added much later: Lou Minati linked some really cool old Apollo 11 footage Link

Wednesday, July 15, 2009

He Watches Over Us

First a few words about spectroscopy. That link has a cool animation of what looks like Pink Floyd’s Dark Side Of The Moon album cover where a prism splits a beam of white light into the colors of the rainbow--the spectrum (pl = spectra) of white light.
Early chemists used colored lines in spectra for the identification of materials when placed in flames.* The flame spectra of individual elements lack the full spectrum of the sun, and more or less resemble a bar code, that is, the spectra have one or several discrete lines separated by spaces. The discrete lines are a “fingerprint” or spectroscopic signature, unique for each element. With that brief introduction, we go back in time to 1868:
On 18 August 1868, a total eclipse of the sun was visible in India, and a number of scientists went there to make observations of the solar prominences. One who examined photographs of the spectra was Joseph Norman Lockyer (1836-1920) who although a civil servant at the War Office had already in his spare time done valuable work in astronomical spectroscopy.
Lockyer was particularly interested in a so called D3 line in the yellow region of solar spectra that had been obtained during the eclipse in India. It was known that the well-known sodium D line was in fact two lines close together, called the D1 and D2 lines. The D3 line could not be obtained from any substance in the laboratory, and Lockyer boldly suggested that it was caused by a new element, found in the sun but apparently not on earth. He gave this new element the name helium, from the Greek helios, the sun.
~The World Of Physical Chemistry, Keith J. Laidler
Lockyer’s hypothesis illustrates one of two ways to advance a theory in science: The first is to amass so much data that the subsequent explanation almost sounds obvious; the second is to boldly assert something with little or no support, and await experimental confirmation.

Helium is the second most abundant element in the universe after hydrogen. Where it does occur naturally on earth, it originates from the radioactive decay of heavier elements. The bulk of our domestic helium supply comes from deposits underground found with gas and oil. Helium is a non-renewable resource: even if made synthetically using radio-decay processes, the supply could not meet demand: link. We recognize helium's use in filling balloons but it is also used in welding and to replace nitrogen in synthetic breathing gas for deep-water diving because its lower solubility in blood minimizes occurrence of the often fatal "bends." However, its greatest use is in liquefied form to cool instruments and for cryogenic research. I used to use lots of it to cool the NMR supercon magnets found associated with nearly every modern chemistry lab.

Helium sits atop the northeast corner of the Periodic Table. From that vantage point, it is possible to look downwards through the eastern border of the chart all the way to the bottom. The elements directly beneath He are the so-called noble or inert elements on account of their general failure to interact chemically with other elements. The other related elements were all given Greek names: Neon (new), Argon (inert) Krypton (hidden), Xenon (strange), and Radon (named after radium but with its suffix changed to conform to the others). The discovery of the noble elements at first confounded the construction of the table--was there another family further to the right? But it was eventually recognized that the noble gas family perfected an understanding of the physical nature of the elements (more on that when we get to Lithium next).

So how many helium balloons would it take to lift a man? Mythbusters apparently did this experiment (I didn’t see it) with helium weather balloons and used about 45 of them, and their balloons were 2.5 meters diameter. I once tried to fill one of those inflatable love dolls (a gag gift) with helium to get it to float for a Halloween party- it didn't work. :(
*Robert Bunsen was a 19th century German chemist interested in the flame emission spectra of the elements--guess what he invented?

Sunday, July 12, 2009

H Is For Humble Hydrogen

The sun consumes about a half billion tons of hydrogen every second, fusing mass into helium and radiating the excess as energy. We just sit back on sunny days and bask in the afterglow of the nuclear holocaust at a very safe distance, thinking nothing of it. Our nonchalance towards any solar dimming is justified by considering that the sun should last another 5 billion years or so.

Hydrogen fuel cells (chemical, not nuclear) are already used in spacecraft, and modern rocket engines burn liquid hydrogen and liquid oxygen. But back on earth, there is talk of using hydrogen as an energy source to replace hydrocarbon fuels. Hydrogen gas burns cleanly, as the very name reminds us: hydrogen = water generating; the catch is that hydrogen gas has to be made because little is found naturally on earth.

By far the cheapest way to make hydrogen gas is from natural gas, CH4, using a process that co-produces CO2 (the carbon atom has to go somewhere). But another little appreciated fact is that a big consumer of hydrogen gas is the fertilizer industry—hydrogen is used to make ammonia from nitrogen—and another big user is the food industry—it is used it to hydrogenate vegetable oils. Any large-scale diversion of existing hydrogen to transportation fuels will ultimately raise the price of food via the costs of ammonia fertilizer and food processing costs. Sound familiar?

What’s really needed is a new and different way to cheaply make hydrogen gas—something like the efficient photolysis of water or the electrolysis of water using electricity from nuclear power plants. Both technologies exist, but they are economic nonstarters. For my money, I’d rather see cars run on methane, rather than going through the additional process hoops of converting the methane to hydrogen gas. A similar argument holds for bio-fuels, which I will discuss when I get to carbon and oxygen.

Hydrogen is the most promiscuous chemical element, pair bonding with nearly every element and even forming special bonding threesomes called hydrogen bonds. Hydrogen bonds are the principle force binding the two strands of DNA together. Arguably, hydrogen bonds are present at the conception of human life: when the two single strands of DNA, one from the mother, one from the father, join for the first time, those strands are united by about 3 billion hydrogen bonds. Each one is worth a small amount, but together, summed over the entire double helix, amounts to a formidable binding glue.

The themes of family and weak and strong chemical forces reminds me of some lines from the David Lynch movie “The Straight Story." Richard Farnsworth says (while demonstrating with sticks):
When my kids were young I played a game with them. I'd give each of them a stick. One for each of 'em, and I'd tell them to break it. They'd do that easy. Then I'd tell them to make one bundle of all the sticks and try to break that. And course they couldn't. I used to say that was family, that bundle.

Tuesday, July 7, 2009

Elemental Musings

The other day, my son asked me what makes fireworks different colors. "It's the elements" I explained: "different chemical elements in fireworks give different colors when they burn." Fortunately, I remembered* a couple of examples: red (strontium), blue (copper), and green (barium). Because he has a periodic table on his wall, those names were at least familiar to him.

I googled up a cool spiral version of the Periodic Table of the Elements (the original is here. I like this chart because the spiral line tracks the series of the known elements, ranging from 1 to about 107 (there are actually now 118 elements). Also, the radial arrangement indicates some of the “rhyming” between related elements (so-called periodicity).

Mentally uncoil the spiral and imagine a number line beginning with 1 (hydrogen), continuing with 2 (helium), then 3 (lithium), etc., and ending at 118. This is the alphabet of matter--just as 26 characters are the alphabet of our language. But just as the alphabet alone cannot capture the compositional richness of language--words, sentences, paragraphs, and books--the elements alone cannot capture the richness of the physical world. And yet the table of elements is still a marvel to contemplate.

*Added: a website link that tells you how to color flames with common household materials: Link.