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    The United Nations proclaimed 2019 to be the International Year of the Periodic Table of Chemical Elements. This is to commemorate the 150th anniversary of Dmitri Mendeleev’s efforts to create a periodic table of elements.

    The Periodic Table of Chemical Elements is one of the most significant achievements in science, capturing the essence not only of chemistry, but also of physics and biology.

    United Nations Educational, Scientific and Cultural Organization

    Humans have known about and used pure forms of elements like sulfur for thousands of years, but the science of isolating and identifying elements really took off in the 1700s. As scientists gathered more information, they recognised patterns in the properties of the elements. Being scientists, their next steps were to categorise the elements and create some order – as detailed in the article Development of the periodic table.

    An element is an element is an element

    An element is a substance that consists of atoms with the same atomic number. The element iron is made only of iron atoms, and iron atoms are the same everywhere – iron atoms on Earth are the same as iron atoms on Mars.

    This information – and the orderliness of the period table – may make the elements appear to be somewhat predictable and dull, but many of the elements have some unusual aspects about them. Following are just a few weird and wonderful elemental facts.

    Solid elements

    Most of the elements are solid at standard conditions (room temperature and pressure):

    • Iodine (I) is a solid, but when it is heated, it easily sublimes – it goes straight to a gaseous state – and gives off a purple vapour.
    • Gallium (Ga) can turn from a solid to a liquid by merely sitting in your hand. It has a melting point of 29.8℃. The average temperature of the human body is 37℃. However, its boiling point is quite high – 2,229°C – making gallium useful in high-temperature thermometers.
    • Almost all of the elemental metals are silver in colour. Gold (Au) and copper (Cu) are the only two exceptions.
    • The rarest naturally occurring element is astatine (At). There are about 28 g of it in the Earth’s crust. Astatine got its name from the Greek word astatos, meaning unstable.
    • Carbon (C) reacts with other elements to produce 10 million different compounds. Carbon makes up 20% of the mass of living things.
    • The elements fermium (Fm) and einsteinium (Es) were discovered as a result of the first hydrogen bomb test in 1952. Drones carried filter paper to collect samples from the atmosphere, and the two elements were found in debris on the paper. Both elements are synthetic and radioactive and are only produced in very, very tiny amounts. Both elements are also named after famous scientists.
    • Potassium (K) is so reactive that it is not naturally found on its own. Pure potassium has to be stored in oil or kerosene to keep it from reacting in air.
    • Caesium (Cs) is another reactive element. It ignites spontaneously in air and reacts explosively with water. It is usually stored and shipped in mineral oil.
    • Magnesium (Mg) is needed for more than 300 biochemical reactions in our bodies. Magnesium wheels – better known as mag wheels – were used in the 1960s. They were lightweight, making them ideal for racing. The downside was they were prone to corrosion and (incorrectly) thought to be flammable. Competitive racing rims are now made of a magnesium alloy.

    Liquid elements

    There are only two elements that exist in liquid form at standard conditions – mercury and bromine:

    • Mercury (Hg) dissolves and/or corrodes many metals, and as a result, it is not usually allowed aboard aircraft.
    • Bromine (Br) is harmful to the atmosphere. It is responsible for up to half of the loss of ozone above the Antarctic. Humans are responsible for about 30% of bromine in the atmosphere.

    Gaseous elements

    There are 11 gaseous elements at standard conditions:

    • Helium (He) is the second most abundant element in the universe but is actually quite rare on Earth. The speed of sound in helium is around three times the speed of sound in the air. This causes people's voices to get high pitched and squeaky when they breathe helium.
    • Hydrogen (H) is the lightest element and the most abundant chemical substance in the universe. It provided lift for the first reliable form of air travel. It is also highly flammable so most hydrogen-enabled flight services ceased in the late 1930s.
    • We associate neon (Ne) with bright signs. Neon is responsible for reddish-orange light, but another noble gas called krypton (Kr) may actually be responsible for multicoloured ‘neon’ signs. The tubes are painted with the desired colour, and krypton’s white discharge creates the glowing effect.

    Elements with an unknown state

    The elements in the 7th period (row) from rutherfordium (Rf) to tennessine (Ts) are so short-lived that scientists are not able to reliably classify them as solids, liquid or gases:

    • Tennessine (Ts) was officially named in 2016. It is artificially produced, and its most stable isotope has a half-life of about 80 milliseconds.

    Activity ideas

    Element rap – in this activity, students become familiar with the names and symbols of the chemical elements by creating a rap or poem.

    Symbol find – in this activity, students become familiar with symbols of the chemical elements by creating them using letters from a phrase or sentence.

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      Published 1 May 2019 Referencing Hub articles