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IYPT Elements

IYPT 2019 Elements 053: Iodine: Seaweed and LCD screens

Element 53 in our International Year of the Periodic Table series is iodine. This element is found in (and can be extracted from) seaweed, and is also used in LCD screens in phones and televisions.

Iodine is one of the more unusual elements in the periodic table in terms of its appearance. At room temperature, it’s a blue-ish black solid. However, if this solid is heated, a vibrant purple iodine vapour forms. Allowed to cool, it will redeposit tiny iodine crystals on any surface it contacts.

The shift straight from a solid to a gas is known as sublimation. There’s actually some debate as to whether iodine can truly be considered to sublime at room temperature; there’s a detailed discussion of this here (albeit slightly complex for non-chemists). In short, iodine does go through the liquid state at atmospheric pressure, though this can sometimes be difficult to spot, so it can’t really be held up as an example of sublimation.

Iodine is found in seaweed, particularly kelp, and in the past, this was the main source for iodine extraction. Today, it’s less economical to extract iodine from kelp, and it’s extracted from brine or caliche (a type of sedimentary rock) instead.

Iodine finds use in the polarising films in LCD screens. These screens are widely used in phones and televisions, meaning you’re likely carrying this element around with you most of the time.

The human thyroid takes up iodine, and potassium iodide tablets are sometimes taken after nuclear incidents to prevent the uptake of radioactive iodine isotopes. If you watched the recent Chernobyl series, you’ll have heard them referring to these tablets at various points. Taking them can guard against thyroid cancer which can be caused by exposure to radiation.

Remember, you can keep track of all of the previous entries in this series on the site here, or on the Royal Society of Chemistry‚Äôs dedicated page.