Hold your own hand up in front of your face. Squeeze a finger, feel the skin slide over knuckle and tendon, and consider what you are touching: about seven octillion atoms — a seven with twenty-seven zeros after it — folded into skin, blood and bone. That is more atoms than there are stars in the observable universe, and every last one belongs to some element of the same periodic table you are studying for your exam. The astonishing part is how few of them matter. Almost all of you is built from one short shopping list.
The big four
By weight you are roughly 65% oxygen, 18% carbon, 10% hydrogen and 3% nitrogen — four elements, 96% of a human being. The oxygen and hydrogen are mostly a polite way of saying you are a bag of water: makes up about 60% of your body weight, which is why a body sinks when the lungs empty and floats when they fill. Count atoms instead of kilograms, and hydrogen becomes your most numerous element — it simply weighs almost nothing. Carbon is the scaffold of everything you would call living: carbohydrate, fat, protein, the DNA coiled in every cell. Nitrogen, barely 3% of you, is stitched into every protein and every letter of your genetic code — the reason farmers spend so much on nitrogen fertiliser.
The builders
Next come calcium, about 1.5% of you, and phosphorus, about 1.2%. Nearly all your calcium is banked in bones and teeth, a living vault the body deposits into and borrows from for a lifetime — yet the small remainder floating in blood is so vital that without it your muscles would not contract and your cuts would not clot. Phosphorus sits in the backbone of DNA, in every cell membrane, and in ATP, the molecule your body spends for every heartbeat, every blink, every thought. When you sprint for the bus, you are burning phosphorus coin.
Three grams of iron, a whisper of iodine
Then the traces — microscopic by weight, catastrophic to lack. Your entire store of iron comes to about three grams, the weight of a small coin, yet it equips the roughly two million red blood cells your marrow builds every second, each of them packed with haemoglobin molecules coiled around an iron atom. That iron is the seat where oxygen boards for its ride from lungs to toes. Iodine you need in specks — around 150 micrograms a day — but your thyroid turns it into the hormones that set how fast you burn food into energy. Where iodine runs short the thyroid swells into a goitre, which is why the salt in your kitchen is iodised.
Older than the Sun
And where did the shopping list come from? Hydrogen is the eldest: its protons condensed in the first moments after the Big Bang, 13.8 billion years ago. Your carbon and oxygen were cooked inside stars, and so were your calcium and phosphorus. Iron has the strangest biography of all. Fusion up to iron releases energy; past iron it consumes energy — so a growing iron core is a star's death sentence. Every iron atom in your blood was forged in the heart of a massive dying star and flung across the galaxy by the supernova that killed it, ages before the Sun was born.
Look at your hand once more. You have been carrying a piece of ancient sky all your life, every atom of it on loan from the universe. To say you are made of stardust is not poetry. It is simply the chemistry.