Why are postassium and calcium usually combined with other substances and not found as pure elements?
1. The thing to remember here is that elements like to have a full valence shell of electrons (the octet rule). Looking at the periodic table, you can see that potassium is in group 1 with 1 electron in it’s outer shell, and calcium is in group 2 with 2 electrons in it’s outer shell. Because of this, Calcium and Potassium want to donate their electrons and form ionic compounds in order to have a full octet. They are extremely good reducing agents, and really want to lose electrons and become oxidized. You don’t find them in nature because of their reacting with other elements (usually from group 6 and7) to form a stable octet. 2. Both calcium and potassium have equal shielding (equal number of electron shells in between other valence shell and nucleus). However, the calcium atom has one more proton than the potassium atom, this means it has a greater nuclear charge, and thus, the more positively charged nucleus pulls on the outer shell more, leading to it’s smaller radius.
