For me it is the fact that our blood contains iron. I earlier used to believe the word stood for some ‘organic element’ since I couldn’t accept we had metal flowing through our supposed carbon-based bodies, till I realized that is where the taste and smell of blood comes from.
That stuff about metal is really counterintuitive, because normally when we talk about iron, gold, copper, nickel, zinc, magnesium, aluminium etc it’s usually about the element in its metallic form. However, when you study chemistry a bit more, you’ll come to realize metals can be dissolved in water and they can be a part of a completely different compound too.
Calcium, sodium and potassium are basically the exact opposite in this regard. Normally when people talk about these metals, they are referring to various compounds that obviously aren’t metallic at all. This leads to people thinking of these elements as non-metallic, but it is possible to purify them to such an extent that you are left with nothing but the metal.
In the case of Ca, Na and K, the resulting metal is highly reactive in our aggressive atmosphere, so that’s why we rarely see these elements in a metallic form. Our atmosphere contains water and oxygen, which makes it an incredibly hostile environment for metals like this. Imagine, we’re breathing this stuff that attacks so many elements mercilessly.
Hydrogen and oxygen and very reactive, which is exactly why they are so necessary for survival. Our bodies function off of chemical reactions, it makes sense to power that off of the most reactive elements it can easily find.