What is Arsenic?

Arsenic (As) is an element, which means that it is a chemical that can’t be broken down into simpler chemicals (so it is not a compound or molecule that is made up of other elements).

The elements are arranged in the periodic table of elements shown below. Arsenic is the one in the purple box.

The picture shows a model of the DNA molecule. The orange blobs are the phosphorus atoms that link the nucleotides (individual DNA molecules) together in a long chain. In this model, two chains stick together to form a double helix.

The molecular weight of arsenic is 75, so one mole of As atoms has a mass of 75 grams.

Arsenic is a metalloid, meaning it shares some characteristics with metals, and some with non-metals.

We are mainly interested in arsenic because of its toxicity. Toxic chemicals prevent or affect processes that are required for life. They somehow stop or change a function that is required for the animal, person or plant to live. Arsenic is a very potent toxin.

Arsenic is a naturally occurring compound. It is found in the Earth’s crust (the solid outer layer that we live on) at a concentration of around 2-5 parts per million. It is the 20th most abundant element on Earth.

Arsenic is not evenly distributed though, so some places have much higher arsenic concentrations, and other places are lower. Arsenic is often associated with mineral ores that are mined, like copper, gold and zinc. Arsenic concentrations are also high at hotsprings and other geothermal sources.


Even though it is toxic, arsenic has been used for many purposes, including in medicine (believe it or not!), agriculture, glass production, as a wood preservative, and in the electronics industry. This is discussed further in the "Where arsenic comes from" section.

Arsenic can exist in different forms. Organic forms of arsenic are associated with organic carbon.

Arsenic can be incorporated into organic compounds like monomethylarsonic acid (CH3AsO(OH)2), arsenobetaine ((CH3)3As+CH2COOH), arsenocholine ((CH3)3As+CH2CH2OH), and Paris Green (Cu(CH4COO)2.3Cu(AsO2)2).

Inorganic forms of arsenic include many solid minerals, such as orpiment (As2S3) and arsenopyrite (FeAsS). There are also soluble inorganic forms like arsenious acid (H3AsO3), and arsenic acid (H3AsO4), which are the compounds of concern in drinking water. Arsenious acid has a valence state of +3, which may be written as As(III), and arsenic acid is As(V), with a valence state of +5.

Valence state:

Arsenic acid and arsenious acid are the forms that are normally found in water – though they may lose some of their H+ atoms depending on the pH.

If arsenic acid, or some other acid for that matter, loses a proton (H+) the remaining part of the molecule has a negative charge. At near-neutral pH, which is common for natural waters, arsenic acid loses one or two H+ ions, giving the rest of the molecule a charge of –1 or –2 (H2AsO4- or HAsO42-).

Arsenious acid remains mostly uncharged until the pH is raised to about 9. Above that pH, it will start losing H+ ions.

The difference in charge at normal environmental pH means that the two forms behave differently in the environment.

Negatively charged molecules are attracted to positively charged sites on the surface of soil particles or rocks. Many rocks have positively charged iron, aluminum and manganese binding sites on their surfaces. Negatively charged molecules can associate with the positively charged surface sites, because opposite charges attract each other. That means the negatively charged molecules get “stuck” on the surface and don’t move with the water. They will continue to be trapped in the soil as long as there are free binding sites on the soil surface. Once the sites are all filled or “saturated”, As(V) will also be mobile. The uncharged molecules of arsenious acid are free to travel in the water and are more mobile. That means the uncharged form is more likely to end up in your tap water.

Note that there are also plenty of negatively charged binding sites on soil particles as well - but they won't affect the arsenic species we are discussing here. The arsenic also reacts with surface sites in ways that are not related to charge, but this is the simplest way to imagine the reaction

The surface chemistry of soils is really interesting and complicated. It affects water quality and the movement of all sorts of pollutants in the environment, but that discussion is beyond the scope of this web page.

So you can see the importance of understanding the different forms of arsenic. The different forms will travel differently in the environment. But what we’ve seen so far is only part of the story. Another important factor is that some forms of arsenic are more toxic than others. We will discuss toxicity further in the section on Health effects.


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