There are 92 elements listed as naturally occurring on the periodic table.
In real terms, that number is probably closer to 88 or 89.
Radioactivity converts heavy elements into daughter species so it is difficult to say they are "naturally occurring."
Technetium, for example, has never been observed in any minerals on Earth but occupies a spot on the table.
For the most part, 92 elements are all that is required to make up everything we see. (Another 26 elements have been synthesized over the past 60 years.)
However, not all elements are equally created. The relative abundance of the elements depends on where you look and what you are looking for.
The universe, as a whole, is primarily composed of hydrogen. On a mass basis, it makes up roughly 74 per cent of everything.
Helium is No. 2 at 24 per cent.
Everything else - all of the other elements in the periodic table - make up less than two per cent.
On an atomic basis, this gets even more lopsided. Hydrogen has an atomic mass of roughly one as it has only a single proton. Helium has two protons and two neutrons in its nucleus and is therefore four times heavier than hydrogen.
Counting atoms, hydrogen is closer to 90.8 per cent of the universe, helium about 8.9 per cent and all of the other elements make up slightly less than one per cent of all the atoms that exist.
Yes, from a Douglas Adams perspective, everyone and everything around you is merely the leftovers in the universe.
All of the elements beyond hydrogen really are debris.
Helium is formed by fusing hydrogen atoms in the core of a star.
Lithium is formed by fusing helium and hydrogen. But forming beryllium is tricky as you simply can't take two helium nuclei and smash them together.
Beryllium is a choke point in nucleosynthesis.
Heavier elements are synthesized through a triple play in which three helium nuclei fuse to give a carbon atom. Adding helium to carbon gives oxygen.
Adding another helium nucleus yields neon. And so on.
Of course, these elements are formed within the core of a star.
They are not released until the star explodes either in its death throes as it forms a smaller, hot body or in the form of a super nova. It is only at the end of a star's life that it spreads this debris into the surrounding space.
The planets of the solar system, and everything living on them, are made from stardust.
This is why the abundance of elements on Earth is very different from the abundance of elements in the rest of space.
Earth formed from the same matter cloud as the sun, but is formed predominantly from the heavier constituents of the original solar cloud.
Earth is heavy. It weighs 5.98 x 10^24 kilograms (roughly a six followed by 24 zeros) and it is predominantly composed of iron.
Indeed, the iron content of the planet is calculated to be 32.1 per cent. I say calculated because we don't have any real way to measure the metal content of the core.
Still, all of the measurements we have made about the Earth are consistent with iron being about one third of its mass. Further, the process of nucleosynthesis occurring in stars hits an end point at iron so having a lot of iron makes sense. Iron will eventually be the only element in the universe. (The heavier elements are formed only during the explosion of a super nova, due to the binding energy of the nucleons.)
The second major component of the Earth is oxygen at 30.1 per cent. Most rocks are some form of oxide or generated from oxygen bearing anions.
Silicon (15.1 per cent, magnesium(13.9 per cent), sulphur (2.9 per cent), nickel (1.8 per cent), calcium (1.5 per cent) and aluminum (1.4 per cent) round out the top eight elements.
Everything else makes up the remaining 1.2 per cent.
But these are the ingredients for the planet as a whole. The crust upon which we live is composed predominantly of oxygen (46.0 per cent), silicon (27 per cent), and aluminum (8.2 per cent).
Iron is fourth on the list at 6.3 per cent. Calcium, sodium, potassium, magnesium, titration and hydrogen round out the top ten.
Notice carbon has not made an appearance. It is simply not as abundant as some of the other elements until we get to the biosphere. It is here carbon dominates, almost.
The most abundant element in living organisms is still oxygen as most creatures are composed primarily of water. Human cells are between 65 per cent and 90 per cent water, by mass, depending on where they are in the body. Brain cells are the high end while bone is very low.
Carbon is the second most abundant element in the human body accounting for 18 per cent of our mass. Arguably it is also the most important as it allows for the complex self-replicating molecular structures we define as living organisms.
Of the 92 natural elements, carbon is the most important for life as we know it.
