I don't understand why arsenic is toxic and why it does damage to humans. Could it be that it is so similar to phosphorous? If so, would that make antimony toxic as well?

Arsenic is toxic to humans due to the fact that it imitates phosphorous almost identically in its bonding patterns, as both elements are in the same group.

However, why would this have any affect? The most likely reason for this is due to the fact that when the phosphorous is substituted by arsenic in many of a cells vital molecules such as DNA and other phosphorous containing groups, the properties of the new substance is slightly different in terms of its polarity, reactivity, etc. Because the proteins and enzymes within a cell interacts with these phosphate containing groups in such a specific manner, these slight irregularity in chemical nature can have detrimental effects (you can think of the processes of these proteins and enzymes between the phosphate as clockwork, i.e. if the parts are slightly different, it will not work).

I am not as sure about the chemistry of antimony, as I have not researched that topic greatly, but I would guess it being in the same group adds to its toxicity.

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    Phosphorous in the human body is primarily in the +5 oxidation state. A heavier element in Group 15 would have to match that for maximumcsimilarity, and when we get as heavy as antimony in that group relativistic effects start to interfere with that. – Oscar Lanzi Nov 19 at 2:00

Wikipedia has a fairly good article outlining arsenic poisoning. To give a bsic overveiw arsenic imitates phosphorus in numerous enzyme formations, namely acetyl-COA which participates in numerous biochemical reactions including the Kreb's cycle. The Kreb's Cylce is the process used to produce adenosene triphosphate (ATP) which is used for cellular energy. ATP contains 3 linked phosphate groups (triphosphate) and removing a phosphate to form adenosene diphosphate (ADP) is where cell derive energy from. when one of these phosphates is an arsenate instead the cell, cannot use it for productive energy and starts to die.

Antimony is toxic for similar reasons, but antimonate ions are much larger and antimony tends to form in the 3+ state so antimony's oxides do not imitate phosphorus as well as arsenic does. This is reflected in antimony's oral LD50 of $\pu{7000 mg/kg}$ which is 200 times that of arsenic's oral LD50 of $\pu{14.6 mg/kg}$. It should be noted that this decreased toxicity is due to poor absorbtion of antimony and the LD50 for inhalation exposure is substantially lower, though not comparable to arsenic. Also antimony salts tend to even more toxic reflecting that antimony's biggest obstacle to killing you is entering the bloodstream.

Bismuth is even less toxic (non-toxic really) for the same reasons as the properties for bismuth compounds are even more distant from phosphorus than antimony.

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