Element number 4 on the periodic table happens to be the lightest structural metal and it would be an absolutely fantastic option if it wasn't so outrageously rare and expensive. A few years ago a hi-tech firm took the trouble to make a bike frame out of beryllium which cost an astounding $25,000 each. In solid form the metal is lighter than aluminum but much harder and stiffer. It would be possible to make a skyscraper ten times taller than the Empire State if it was made of Be. The metal however is toxic when shaved down to dust as it lodges in the lungs causing a disease that thankfully very few people need to worry about since, again, the metal is too rare and valuable to be used in anything outside of very specialized applications.