The human body is composed of about 60 distinct chemical elements, though the vast majority of body mass is accounted for by just six: oxygen (O), carbon (C), hydrogen (H), nitrogen (N), calcium (Ca), and phosphorus (P). The four most abundant by mass — O, C, H, N — collectively represent approximately 96% of total body weight, reflecting the fundamental chemistry of biological systems built primarily from water and organic carbon-based molecules. Oxygen leads at roughly 65% because water (H₂O, approximately 60-70% of body weight) has a molecular weight dominated by oxygen (16 of the 18 molecular weight units of water), and because organic biomolecules invariably contain multiple oxygen atoms in their functional groups (hydroxyl, carboxyl, carbonyl, ester, peptide bonds, etc.). Carbon at ~18% reflects its central role as the structural backbone of every organic molecule in the body — proteins, carbohydrates, lipids, and nucleic acids all have carbon skeletons. Hydrogen at ~10% is present in water and in the C-H bonds forming the majority of bonds in organic molecules. Nitrogen at ~3% is specifically incorporated into amino groups of amino acids, the purine and pyrimidine ring systems of nucleotide bases in DNA and RNA, and various other nitrogenous biological compounds.

The preponderance of oxygen (65%) over all other elements in the human body reflects two reinforcing factors: the sheer abundance of water in biological systems, and the oxygen-rich nature of organic biochemistry. Water constitutes approximately 60% of body weight in adult humans (higher in infants, lower in elderly and obese individuals), and oxygen contributes approximately 89% of water's mass (atomic mass of O = 16, of H = 1, so in H₂O the ratio is 16:2 = 8:1 by mass). Beyond water, organic biomolecules are extensively oxygenated: carbohydrates have a general formula approximating (CH₂O)n where one-third of atoms are oxygen; proteins contain oxygen in every peptide bond (CO-NH) and in side chain hydroxyl, carboxyl, and amide groups; lipids contain oxygen in ester bonds (triglycerides) and phosphate groups (phospholipids); nucleic acids contain oxygen in the sugar-phosphate backbone and in carbonyl groups of pyrimidine and purine bases. This pervasive oxygen chemistry of life, combined with water's dominance as the biological solvent, explains why oxygen leads all other elements in body mass contribution despite being less prominent in terms of number of atoms (where hydrogen, being so light, actually outnumbers oxygen atoms substantially).

Carbon holds a unique position in chemistry that makes it the ideal element for constructing the molecules of life: it forms four covalent bonds (tetravalent), allowing it to build extraordinarily complex three-dimensional structures; it can bond with itself to form chains, rings, and branched structures of essentially unlimited length and complexity; carbon-carbon bonds are strong enough to be stable under biological conditions but can be selectively broken by enzymes; and carbon can bond to hydrogen, oxygen, nitrogen, sulfur, phosphorus, and halogens with similar energy levels, allowing diverse chemistry. The approximately 18% of body mass represented by carbon encompasses essentially all organic biomolecules — the carbon skeleton of every amino acid, every nucleotide base, every lipid fatty acid chain, and every glucose and other sugar molecule. Silicon, just below carbon in the periodic table and similarly tetravalent, theoretically could form the backbone of alternative biology, but silicon-silicon bonds are weaker, silicon-oxygen bonds are very strong (tending to form insoluble silicates rather than soluble compounds), and silicon cannot effectively form double bonds — these properties make carbon far superior for the complex, flexible, soluble, and enzymatically manipulable molecules that life requires.

Hydrogen, despite being third by mass at ~10%, is actually the most numerous element by atom count in the human body, reflecting its very low atomic mass of 1 (versus 16 for oxygen, 12 for carbon, 14 for nitrogen). Hydrogen is present in virtually every organic bond (C-H bonds are the most common bond type in organic chemistry) and in every water molecule. Nitrogen (3%) is essential for the biochemistry of genetic information and protein function: the nitrogen-containing aromatic rings of adenine, guanine, cytosine, thymine, and uracil carry the genetic information in DNA and RNA through specific hydrogen bonding patterns (A-T/A-U and G-C base pairs), while the amino groups (-NH₂, -NH₃⁺) of all 20 amino acids give proteins their characteristic chemical versatility, buffering capacity, and capacity for hydrogen bond formation in protein secondary structure. Calcium (~1.5%), the most abundant mineral in the body, is concentrated in bones and teeth (as hydroxyapatite, Ca₁₀(PO₄)₆(OH)₂) and also serves as a critical intracellular second messenger (Ca²⁺ influx triggers muscle contraction, exocytosis, and numerous cellular signalling cascades). Phosphorus (~1%) in the phosphate form (PO₄³⁻) is essential for DNA and RNA backbone formation, ATP energy currency, phospholipid membrane structure, and bone mineralisation.