What is the most abundant element on Earth?
Oxygen is the most abundant. It comprises 49.5% of the total mass of the Earth’s crust, water, and atmosphere. The second most abundant is Silicon (Si) at 27.7% of Earth’s mass.
The other elements in decreasing order of approximate abundance: Aluminum (8.1%), iron (5.0%), calcium (3.6%), sodium (2.8%), and potassium at 2.6%.
Source: about.com’s, howthingswork.
Be aware that these abundances are only applicable for the Earth’s crust. Iron is much more abundant if one is considering the whole Earth, which is what I would interpret the question to be asking.
Point well taken. If the question asked “What is the most abundant in the Earth’s crust?” the first answer would have been a fair estimate. As written, Professor Schott’s answer is a better one. Thanks for the clarification!