The most common elements on Earth are oxygen, silicon, aluminum and iron. Oxygen makes up about 46 percent of Earth by weight, while silicon makes up about 28 percent, and aluminum and iron make up approximately 8 and 5.5 percent, respectively.
After the most abundant elements, most elements become much harder to find. Other than the top four elements, plus calcium, sodium, magnesium and potassium, no other element makes up more than 1 percent of Earth. The ninth most common element is titanium, at roughly 0.5 percent, followed by hydrogen, at 0.14 percent. This is in stark contrast to the elements in the Milky Way Galaxy, where hydrogen is by far the most abundant at approximately 74 percent.