In chemistry, a correlation can be found between elements and their position on the periodic table. Phosphorous can be found in the 3rd period and arsenic in the 4th period. However, arsenic and phosphorous are both found in group 15 of the periodic table. The primary determinant of an element’s chemical behavior is its valence electron shell, or its electron configuration. This primarily determines reactivity. Elements are grouped in the periodic table according to their valence shell electrons. Those elements within a group demonstrate similar chemical properties. This is one basis for the theory that arsenic can substitute for phosphorous within biological systems, a theory that is consistent with a recent study. A highly controversial publication by Wolfe Simon et.al. claims that arsenic can replace phosphorous in DNA and RNA and that this particular strain of bacteria used, the Halomonadaceae, can survive solely on arsenic.
• Found in most all biological systems, a key element of cellular function
• Smaller atomic radii
• Electron Configuration [Ne] 3s2 3p3
• Found in the 3rd Period of the Periodic Table
• Direct contact can cause local irritation and dermatitis.
• Overexposure has been associate with an increased risk of skin, liver, bladder, kidney, and lung cancer (Merck Index)
• Larger atomic radii
• Electron Configuration [Ar] 4s2 3d10 4p3
• Found in the 4th period of the Periodic Table
When looking at the ability of an organism to replace its dependency on phosphorous with a dependency on arsenic, the stability of arsenic-containing biomolecules must be considered. The potential of an organism to incorporate arsenic as a replacement for phosphorous would be quite useful, both in an explanation of evolution and in the advancement of science. Arsenic and phosphorous, in fact, do share chemical properties, primarily when considering their oxidation states. Both phosphorous and arsenate share a most common oxide, 5+. In this oxide, Arsenate (HAsO4-2) exhibits very similar pKa values to those of phosphate (HPO4-2) and forms analogous esters.  Although they exhibit the same oxidation states and form analogous esters, arsenic and phosphorous differ in stability. Phosphorous has a stronger chemical stability than Arsenic. For example, Arsenic is much more easily reduced from As (V) to As (III) than phosphorous, which permits phosphorous the greater stability in its oxidation state.  Arsenate esters tend to be more reactive as well, especially in water environments. They tend to rapidly hydrolyze and break the high energy bonds. This would waste the energy stored in the bonds that could be more efficiently utilized by the organism.
To what extent does this concept of arsenic-dependent life apply to our world? When considering the strength of such a claim, we must also consider the application of this claim. It has been found that arsenic rich locations were around when life is believed to have begun. The question becomes why did evolution choose phosphorous when arsenic was so prevalent in these areas? . The answer from most scientists today is that it simply cannot work; arsenic cannot replace phosphorous. This recent paper by Wolfe Simon et. Al makes a strong claim that it can. The truth of the matter is that nobody knows for sure whether arsenic is stable enough to serve as a permanent replacement for phosphorous in life, but if it can, this could serve to explain how life on other planets could have evolved.
 Arsenate Replacing Phosphate: Alternative Life Chemistries and Ion Promiscuity
 Wolfe-Simon, F., Davies, P. C., and Anbar, A. D. (2009) Did nature also use arsenic? Int. J. Astrobiol. 8, 69–74.
 Dixon, H. B. F. (1997) The biochemical action of arsonic acids especially as phosphate analogues. Adv. Inorg. Chem. 44, 191–227.
 Science. ISSN 0036-8075 (print), 1095-9203 (online)