Steven Dutch, Professor Emeritus, Natural and Applied Sciences, University of Wisconsin - Green Bay
Although almost any chemical reference will have data on ionic radii and electron configuration, and tabulations of energy levels are not hard to find, there is little readily available data on the relative sizes of orbitals. Ionic radii are useful in crystal modeling, electron configuration and energy levels for all kinds of stuff, but sizes of orbitals don't seem to have much practical use.
It's impossible to measure the sizes of orbitals and even of atoms directly. You measure macroscopic objects basically by bouncing light off them (say off the object and a ruler) but wavelengths tiny enough to show atomic structure in detail (nanometers and less) are energetic enough to destroy what you want to see. It's like measuring the size of a figurine by swinging a hammer. Ionic radii are calculated from atomic spacings in materials. Orbital radii principally come out of theories whose main purpose is to account successfully for the other properties of atoms. These drawings are based on data from J. T. Waber and D. T. Cromer, Orbital Radii of Atoms and Ions, Journal of Chemical Physics, 15 June, 1965, vol. 42, no. 12, p. 4116-4123. The sizes shown are the radii of the maximum electron density, so the actual extent of significant electron density is maybe 50% larger.
All the drawings are to common scale. For orbitals with lobes, the principal lobes are shown but there is no attempt to portray the complex inner structure. It gets plenty busy in toward the center of the atom anyway.
|At francium we begin adding a 7s orbital. Francium has one 7s electron, radium has two.
Radium is a direct alpha descendant on the U and Th decay chains, and is naturally abundant enough to be a problem in drinking water in places. Fr forms mostly by alpha decay in the U-235 decay chain, and has only very unstable isotopes, so at any given time there are only a few grams of it in the crust.
|Like with the lanthindes, we start adding f orbitals, in this case 5f, but since the 5f and 6d orbitals are so similar in energy, the addition of orbitals is even less orderly than the lanthanides.
Contrary to widespread misconception, uranium is not the last naturally occurring element. Tiny amounts of primordial plutonium 244 (half-life 80 million years) have been discovered, and beta decays can probably create natural elements as high as americium or curium, in extremely tiny amounts.
Americium is the isotope used to ionize air in smoke detectors. Einsteinium has been made in milligram amounts and is the heaviest element made in macroscopic amounts.
At some point, we have to ask when the elements cease to exist in any meaningful sense. Plutonium-244 has a most respectable half life of 82.6 million years, and in fact a little primordial plutonium still exists on earth. Americium-243 has a half-life of 7380 years, and the isotope used in smoke detectors, Americium-241, has a half-life of 432.2 years. Einsteinium 254 has a half life of 276 days and Fermium 257 has a half life of 80 days. Lawrencium-262 has a half life of 4 hours, Dubnium-268 16 hours, Hassium-277 12 minutes and even Element 112 has an isotope of mass 285 with a half life of 10 minutes. Those could all potentially be created in amounts big enough to do actual chemistry with. The few atoms ever made beyond that have half-lives of seconds or less. In fairness to the guys at Berkeley, Dubna, and Darmstadt, the problem may be that we can't supply enough neutrons to make stable nuclei for very high atomic numbers.
An interesting feature is that the atoms don't get extremely large, due to the increasing attraction of the larger number of protons. But the mass increases, so ultra-heavy elements can be ultra-dense as well. Bohrium (107), Meitnerium (108) and Hassium (109) are the densest known elements with predicted densities of 37.1 g/cm3, 37.4g/cm3 and a whopping 41g/cm3 respectively. None of these elements have been produced in macroscopic amounts, so the densities are predicted theoretically. Hassium would be more than twice as dense as gold or uranium, and nearly twice as dense as osmium and iridium, the densest stable elements, which weigh in at a featherweight 22 g/cm3.
Somebody recently asked on a discussion site about making things out of bohrium. Apparently that was mentioned on some video game. The obvious answer is that it hasn't been made in macroscopic amounts. I also suspected the critical mass would be absurdly tiny. That turns out not to be the case. Critical mass depends on neutron capture cross section and even einsteinium (99), the heaviest element for which there's data, has a [predicted] critical mass of kilograms. Einsteinium is also the heaviest element made in macroscopic amounts. Since it has a critical mass of kilograms and it's only been made in milligram amounts, and then only sporadically, the likelihood of an einsteinium nuke is pretty slim.
What Do Atoms Really "Look Like?"
What Atoms of Hydrogen Through Xenon Really "Look Like"
What Atoms of the Heavy Elements Really "Look Like"
Scale Drawings of Atoms and Orbitals: Hydrogen Through Krypton
Scale Drawings of Atoms and Orbitals: Rubidium Through Xenon
Scale Drawings of Atoms and Orbitals: Cesium Through Radon
Scale Drawings of Atoms and Orbitals: Francium Through Lawrencium
What the Atomic Structures of Some Simple Materials Really "Look Like"
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Created 26 April 2006, Last Update