The element commonly used in thermometers is mercury, which has unique properties that make it effective for measuring temperature. Mercury remains liquid at a wide range of temperatures and expands uniformly when heated, allowing for accurate and precise temperature readings. However, due to concerns about mercury's toxicity and environmental impact, many modern thermometers have shifted towards using alternatives such as alcohol or digital sensors. Despite this, mercury thermometers were historically popular for their reliability and were widely used in both medical and laboratory settings until regulations began limiting their use.