Meitnerium is a synthetic element with the atomic number 109 and the symbol Mt. It is named after the Austrian physicist Lise Meitner. the atomic mass of Meitnerium has not been experimentally determined due to its short half-life and difficulty in producing enough of it for precise measurements. However, its atomic mass is expected to be around 278 atomic mass units (amu). Please note that scientific information can change over time, so it's always good to consult up-to-date sources for the latest data on Meitnerium or any other element.