introduction during the formation of planet earth, there were elements that did not adapt to the crystalline structures of the minerals present in the mantle, preferring instead to be in the liquid phase which would then form the crust. these elements were said to be incompatible in the mantle compared to the crust, hence the name incompatible elements. these elements include the alkali metals: K, Rb, Cs, the transition elements: Y, Zr, Nb, Hf, Ta, the actinides: Th, U, and all the lanthanides. According to Goldschmidt, all these elements combined are called lithophilic elementsCHEMISTRYAlkali metals- K, Rb, CsThese are the elements found in the first row of the periodic table. They have a single electron in their outer shell that they will easily lose in an ionic bond with other elements to form a positively charged ion or cation. The removal of this single electron gives these alkali metals an inert gas configuration which is consequently stable. They also have ionization potentials that are very low in their respective periods. All these metals are highly reactive, therefore they rarely occur in elemental form in nature. It is also due to this highly reactive nature that, when some of these elements are exposed to water, they explode. Alkali metals have a low melting point and low density. they are also soft and ductile and also good conductors of electricity. Rubidium Rb is silvery white and appears as a solid at room temperature. Its melting point is approximately 39°C. its atomic number, with a density of 1533 kg.m. PotassiumK with an atomic number of 19, also present as a solid at room temperature, is so reactive that it will even react and burn a hole in the iceCeicio...... half of paper......easily combines with other ligands to form stable complexes. These elements are all unstable and very reactive due to atomic numbers above 83. Thorium and uranium are the only actinides that occur naturally in the Earth's crust. Thorium With atomic number 90, Th has a melting temperature of 1750˚C and a density of 11725 kg/m ᵌ.UraniumU is considered the heaviest element present in nature, with a nucleus that attracts its electron shells closer to itself because it is so full of protons and neutrons. This causes the relativistic effect to occur. This effect influences the orbital energies of the electrons so the internal electrons will quickly rotate towards the nucleus to better protect it. As a result, the outer valence orbitals will be shielded and expanded, forming hybrid molecular orbitals. Works Cited www.rsc.org/periodic-table/element/39/yttrium