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=

$\square Ca+\square O_{2}\rightarrow \square CaO$<br />$Ca=1$<br />$O=1$<br />$\square N_{2}+\square H_{2}\rightarrow \square NH_{3}$<br />$N=1$<br />$H=3$<br />$\square Cu_{2}O+\square C\rightarrow \square Cu+\square CO_{2}$<br />$Cu=2$<br />$O=1$<br />$C=1$<br />$\square H_{2}O_{2}\rightarrow \square H_{2}O+\square O_{2}$<br />$H=2$<br />$O=2$