What do parentheses around a group of atoms in a chemical formula indicate?

Five students had to answer the question how are elements arranged in a periodic tabledamon: i think the elements are arranged by increasing massflo: i think the elements are arranged according to their properties sienna: i think the elements are arranged by when their discovers kyle: i think the elements are arranged according to how common they areglenda: i don't agree with any of themwho is right

What is the empirical formula of c6h18o3? ch3o c2h5o c2h6o c2h5o5

What is the effect of returning nuclear reactor cooling water back into bodies of water?