Valence electrons are the electrons in the outermost shell. Those are t<span>he electrons on an atom that can be gained or lost in a chemical reaction. </span>Elements that are left on the periodic table <span> have relatively few </span>valence electrons<span>, and can form ions more easily by losing their </span>valence electrons<span> to form positively charged cations.</span> <span>Nonmetals are further to the right on the periodic table, so they gain electrons relatively easily and lose them with difficulty. </span>
Those would be called indicators , simply google them and you should find many. I'll leave some examples "bromophenol blue" "Methyl red" and "phenol red".
