The chemical formula of a person depends on a number of factors, most specifically whether we’re talking about a male or a female.
Male bodies contain more water than female bodies, which have extra lipids.
By weight, oxygen amounts to about two-thirds of the body, followed by carbon at 20%t, hydrogen at 10% and nitrogen at 3%.
Elements originating from pollutants would only be present in trace amounts.
If a human body were broken into single atoms, we would arrive at an empirical formula H15750 N310 O6500 C2250 Ca63 P48 K15 S15 Na10 Cl6 Mg3 Fe1.
The relative numbers of atoms in this differ from the composition by weight because atoms have different masses.
The composition of an alien life form would depend on two key factors.
First, the element that forms the ‘skeleton’ of its macromolecules. All life discovered so far is based on carbon, which can form long chains to which other elements bind. The most likely alternative building blocks for macromolecules would be silicon, phosphorus or nitrogen.
Second, the solvent for the biochemical reactions that drive the body. The most likely alternative to water is probably ammonia (NH3) because it can dissolve most organic molecules. It is also liquid well below water’s freezing point and is prevalent in space. So an alien life form might be silica and ammonia based.
The chemical elements in an adult human are distributed in various molecular and atomic species.
An accurate formula could be expressed in the standard form: 71025H2O+91024C6H12O6+21024CH3(CH2)14+ and so on.
However, such a series would fill a book and we cannot possibly identify all species. Metabolism, defined as the chemical and energy exchanges in a living body, means that any such chemical formula is continually changing.
Having a chemical formula for a process can be useful.
If we find all the elements and determine all the mathematical expressions applying to them, the whole process can be determined. But this is not the whole story.
Life is characterised by extensive, adaptive self-regulation of its own structural order, and utilises feedback control.
An organism uses its resources in its own emergent way. The chemical reactions work, but how they are brought together is a matter of emergent control systems. This means that not only is it impossible to write an accurate formula for a human being, it is unnecessary and can be misleading to try.
Life is what it does with chemical species, not just which ones it is made from.
The same would go for any alien life form we might encounter. We spend considerable time searching the electromagnetic spectrum to detect their signals, and we receive a lot of signals. But how will we know if any of them are life?
Only, if they show the characteristic of life: I’m in control, and I’m not solely a bottom-up deterministic chemical process.