Element 1:Hydrogen

The first element of the Mendeleev's Periodic Table is Hydrogen. It is gas at room temperature and has a melting and boiling point of -259.16°C (13.99 K) and -252.869°C (20.271 K) respectively, making it difficult to keep in liquid form. It is the least dense and lightest of all elements on the periodic table with a density of 0.0000899 and an atomic weight of 1.00794, and one of the smallest with a diameter of 106pm (one picometer = 1 trillionth of a meter)

Hydrogen's usage in pure form is minimal, but the most prominent usage is its usage in stars for fuel, with our sun alone consuming six hundred million tons of hydrogen and converting all but four million tons into helium, the second element on the Periodic table (the rest is converted into energy according to E=mc^2). It was also used in zeppelins, starting in WWI, and its high flammability was a huge problem as they could burst into flames, as very famously happened in 1937 in what is now known as the Hindenburg Disaster.
Mainly only Hydrogen-1 exists naturally in the world, with trace amounts of Hydrogen-2 and even less of Hydrogen-3. Hydrogen-3, also known as tritium, is a radioactive isotope of hydrogen and is used as a long-lasting power source, and is often used in glow-in-the-dark watches, as well as (together with hydrogen-2, AKA deuterium) in boosting the explosive power of nuclear fission weapons and hydrogen bombs.