there are a lot of elements on the periodic table but they are not found in equal amounts in the universe or on earth the two most common elements in the universe by far are hydrogen and helium since all the rest are fused inside stars and are scattered around when they die but on the earth there is very little helium while all the hydrogen is not elemental but rather is bound to other elements to form compounds so what elements can we find on earth well as the earth formed the materials distributed themselves by a process called differentiation where the denser materials sunk towards the center and the less dense materials rose to the top kind of like a centrifuge the result was that Earth has different layers the core of the earth has a radius of about 3400 kilometers and is made predominantly of iron and nickel and trace amounts of other transition metals the core itself can be divided into an inner core which is solid and an outer core which is liquid beyond the core is the region called the mantle this contains a lot of metal silicates and it is less dense than the core then there is the outermost layer which is called the crust and that's what we walk on every day and it has three sections the first is the ground all the dirt and rocks or the lithosphere the second is all the oceans or hydrosphere and then there is Earth's atmosphere or all the gas flying around our heads so those are the layers that comprise the earth now let's get a little more specific about what's in each one of them this periodic table shows exactly how the elements got distributed with these transition metals in purple forming the core due to their high densities these in orange make up the sulfide phase which are slightly lower in density and then there is the silicate phase which are more familiar because they can be found in the crust above that in blue are the elements that are largely found in the atmosphere which is mostly nitrogen plus some oxygen a little bit of argon and trace amounts of other things and then in white are elements that are not commonly found anywhere on earth unless we make them in particle accelerators but of course these elements are not found in equal abundance there is not the same amount of strontium as there is magnesium lying around the earth so what is the precise distribution as best we can tell this table which arranges the elements by decreasing abundance in the Earth's crust shows us that iron is the most abundant in the earth overall at 31.9% as it makes up the majority of the core very close after that is oxygen because it is so abundant in the mantle and crust present in many different silicates from there we have lots of silicon also in the silicates and magnesium and after that we drop below 1% for the remaining elements with many of them being exceedingly rare in comparison all the way down to helium which despite being a quarter of all the atoms in the universe is almost none of the ones on earth so this may give us a better picture of why some materials are so valuable it's because they are so hard to find