The four elements occurring in the greatest amount in the human body are oxygen, carbon, hydrogen, and nitrogen. Rather than existing by their lonesome self, they typically occur as an element in a compound. For example, one third of the body is composed of water. Water contains two molecules of hydrogen and one of oxygen. One can logically approximate that one ninth of the body is represented by oxygen contained in water, whereas two ninths is hydrogen.
The human body does not exist in a fixed state. Rather, there are infinite chemical reactions constantly occurring at a molecular level that, on average, achieve a state of equilibrium. The elements in our bodies exist as a part of these changing pathways. Nonetheless, there is conservation with regards to the amount of each element typically contained in a human body.
Oxygen makes up 65% of our body and is present in many types of cells, including bone. Carbon occurs in a smaller amount (18%), but is present in all types of cells. Hydrogen (10%) and Nitrogen (3%) also represent smaller amounts, but occur in all of the same types of cells as oxygen.
Therefore, if someone weighed 100 lbs their body would be comprised of 65 lbs of Oxygen, 18 lbs of Carbon, 10 lbs of Hydrogen, and 3 lbs of Nitrogen.