The elemental composition of an adult human body by mass is
- A.C > O > H > N
- B.O > C > H > N
- C.N > C > H > O
- D.N > C > O > H
Correct answer
Explanation
The correct answer is B, O > C > H > N. By mass, an adult human body has the most oxygen, then carbon, hydrogen and nitrogen.
Oxygen makes up about 65 percent of body mass, carbon about 18 percent, hydrogen about 10 percent and nitrogen about 3 percent. Oxygen leads because roughly 60 percent of the body is water, and oxygen supplies most of the weight of each water molecule. Carbon is the backbone of proteins, fats, carbohydrates and nucleic acids. Calcium and phosphorus, mostly in bones and teeth, come next. These six elements together account for about 99 percent of body mass.
A is wrong because carbon, though vital, comes second to oxygen by mass. C is wrong because nitrogen, at about 3 percent, is the smallest of the four, and oxygen is the largest. D is wrong because it also puts nitrogen first, when it is last among these four.
Exam tip: by mass the order is O > C > H > N > Ca > P, but by number of atoms hydrogen is the most common element in the body.
Practice Questions
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