What is the atom that represents the greatest weight in the human body?

Short Answer:

By weight, oxygen makes up approximately 65% of the human body.

Numerically, hydrogen approaches 65% of the total number of atoms in the human body, but it is probably closer to 63% for most people.

The compound that contributes most to the weight of a human is water which averages around 60% of the weight of an adult.

Accounting by Mass:

The mass of the human body is provided mostly by oxygen, carbon, hydrogen, nitrogen. Together these elements account for about 96% of the mass of a human.

Add in about a kilogram of calcium in the bones and body fluids and you are up to 97.4%. All of this depends on the particular person since water, our major component, can differ by 10% from one individual to another.

That said, typically the breakdown goes like this.

Oxygen: 65% of the mass

Carbon: 18% of the mass

Hydrogen: 10% of the mass

Nitrogen: 3% of the mass

Calcium: 1.4 % of the mass

Phosphorus 1.1% of the mass

With phosphorus, one has accounted for 98.5% of the mass of the typical human and that will change by a couple of tenths from person to person.

Accounting by Numbers:

Numerically we have nearly twice as many hydrogen atoms in our bodies as compared to all other elements combined. Specifically, hydrogen counts as 63% of our atoms by number, give or take a bit, but being so light it barely gets to 10% of our mass. Oxygen atoms count as 24% and carbon atoms are a mere 12% of the populations. Together, our three basic organic building blocks account for 99% of all the atoms in the body.

Caveat:

As was said above, the water content of the human body weight varieties between 55 to 65 % from one person to the next and being the major component changes the numbers above a little for each individual.

Summary:

Almost 99% of the mass of the human body is made up of the six elements oxygen, carbon, hydrogen, nitrogen, calcium, and phosphorus.

Most of the last 1% is made up of five elements: potassium, sulfur, sodium, chlorine, and magnesium, but a tenth of a percent or so is made up of another dozen or so elements which include those recognized as essential to health such as iron, iodine and fluorine as well as beneficial trace elements like manganese, copper and zinc.