Try these: square Ca+square O_(2)arrow square CaO Ca= Ca=1 O=1 0= square N_(2)+square H_(2)arrow square NH_(3) N= N= H= H= square Cu_(2)O+square Carrow square Cu+square CO_(2) Cu= Cu= O= O= C= C= square H_(2)O_(2)arrow square H_(2)O+square O_(2) H= H= O= O=