Name and state the fundamental law that every equation must fulfill.

Write the valency of fluorine in ${\mathrm{CaF}}_2$.

Write the valency of sulphur in ${\mathrm{SF}}_6$.

Write the valency of phosphorus in ${\mathrm{PH}}_3$.

Write the valency of carbon in ${\mathrm{CH}}_4$.

Write the valency of nitrogen in ${\mathrm{N}}_2{\mathrm{O}}_3$.