There is such a "group" of metal elements, they are often accompanied by two in nature, can be described as "inseparable", and the physical and chemical characteristics of each pair is so surprisingly similar, that when they were found, people mistook the two symbiotic elements for one. And these a few "twin brothers" : zirconium and hafnium, niobium and tantalum, molybdenum and tungsten, and a can pair with them any symbiosis rhenium, in addition to each pair of small commonality, their whole circle, there is a big commonality: high hardness, high melting point, high acid resistance, so commonly known as "three high" elements.
As for the melting point, the figures are clear: none of the seven elements has a melting point below 2200 ° c, and tungsten has the highest melting point at 3410 ° c. Not to say that the ordinary fire cannot burn them, not even the billowing flames of a steel furnace can. As a result, they are more than enough material for rocket shells to cope with the heat generated by friction with the atmosphere during lift-off.
Most metal enters acid to melt namely, even if do not melt, also be corroded get mottled pothole, only these 7 "tough guy", let strong acid such as strong hydrochloric acid, strong sulfuric acid take them helpless. Tantalum and tungsten, in particular, cannot be harmed even by aqua vita, the king of acids. Therefore, they are the most reliable engineering materials in the field of advanced science and technology such as atomic energy and aerospace.
