hondritic meteorites (chondrites), the oldest rocks in our solar system, provide a significant record of the processes that transformed a disk of gas and dust into a collection of planets, moons, asteroids, and comets.
They are considered to be the building blocks of the inner planets, Mercury, Venus, Earth, and Mars.
The igneous activity that produced such intrusions...
...calculation was based on the assumption that the substance of the Earth is inert and thus incapable of producing new heat.
Heavier elements can have several isotopic numbers, meaning different numbers of neutrons, but the same number of protons.