Why are elements coming after Iron in the periodic table less abundant?

Q: Why are elements coming after Iron in the periodic table less abundant?

Krishna: In general, elements up to iron are made in large stars in the process of becoming supernovae. Iron-56 is particularly common, since it is the most stable nuclide (in that it has the highest nuclear binding energy per nucleon) and can easily be made from alpha particles (being a product of decay of radioactive nickel-56, ultimately made from 14 helium nuclei). Elements heavier than iron are made in energy-absorbing processes in large stars, and their abundance in the universe (and on Earth) generally decreases with increasing atomic number.

                                                           Periodic table

When you bring two hydrogen nuclei together, they fuse and give off energy: the energy of two protons bound together is lower than that of two protons on their own. If you add another proton, you get lithium, and some more energy. 

This goes on, giving you progressively less energy when you add protons, until you reach iron. From there on, you need to add energy when you add more protons. Or if you look at it from the other direction: if you have a cobalt atom and peel off a proton, you get iron and some energy.

From hydrogen to iron, you get energy out by fusion—putting atoms together. From iron and up to uranium,  you get energy out by fission—splitting atoms apart.

As to why this is so, that’s a complicated combination of the strong nuclear force and the electromagnetic force. The strong nuclear force pulls protons together, the electric charge of protons push them apart, neutrons isolate electric charge and adds to the strong nuclear force. When you put it all together, it works out as described above.

Iron's abundance in rocky planets  like Earth is due to its abundant production during the runaway fusion and explosion of type Ia supernovae, which scatters the iron into space.

Iron  is a chemical element with symbol Fe (from Latin: ferrum) and atomic number 26. It is a metal that belongs to the first transition series and group 8 of the periodic table. It is, by mass, the most common element on Earth, right in front of oxygen (32.1% and 30.1%, respectively), forming much of Earth's outer and inner core. It is the fourth most common element in the Earth's crust.

Iron atoms are the most stable atoms: when you have iron, you need to add energy to turn it into anything else; if you have anything else, turning it into iron will release energy.