Rare gases belong to group 18 of the periodic table and are chemical elements with similar properties. All noble gases have a complete outer envelope. None of them have any color, odor and all have very low chemical reactivity. There are six: helium, neon, argon, krypton, xenon and radon. HeliumHelium has an atomic number of 2. It has no color, odor or taste. It is also an inert monatomic gas. It is the first of the noble gases in the periodic table. Its melting and boiling points are the lowest among all elements and exist only as a gas except in extreme conditions. Helium is the second most abundant element in the universe, although it is not normally found naturally on earth, but is created by radioactive decay. Helium is one of the only elements whose escape velocity is such that when released into the atmosphere, it flies into space. Indeed, helium is the only element present in space before being found on Earth. In 1868, an astronomer named Pierre Janssen was in India to observe a solar eclipse. He was using a spectroscope to help him with his observations, and while using it, he saw a yellow line in the spectrum, which no one had ever heard of or seen. This indicated the existence of an unknown element. Janssen named it helium after the Greek god Helios, also known as Apollo, associated with the Sun. Janseen met with a world-renowned English astronomer, Sir Joseph Lockyer, best known for his work analyzing light waves. Lockyer, like Janssen, also believed that Janseen had discovered a new element. A few months later, he also observed the same spectrum as that discovered by Janseen. The spectroscope was still a new invention that most of the world's scientists made... middle of article... this is not valid. Photo of Xenon Radon Radon is the last element of the rare gases. Its symbol is 86 and its atomic number is 86. It is colorless, tasteless, odorless and gaseous. It is one of the densest substances that remains as a gas under normal conditions. Radon is part of the radioactive decay chains of thorium and uranium as they decay into lead. As radon decays, it produces decay products. These products adhere to elements present in the air, such as dust particles. If these dust particles are inhaled, they can stick to the lungs and cause lung cancer. Several studies have proven that those who breathe radon are more likely to get lung cancer than those who do not breathe it. Radon therefore poses a considerable danger in workplaces such as mechanical workshops and shipyards. Radon poisoning ranks second in deaths caused by lung cancer, behind smoking.