Simplify the following text for a Grade 10 native English-speaking person.
Original: Since the number of valence electrons which actually participate in chemical reactions is difficult to predict , the concept of valence electrons is less useful for transition metals than for main group elements .
Simplified: In chemistry , valence electrons are the electrons in the outside or valence electron shell of an atom . Valence electrons are important in how a chemical element reacts with other elements .