Elements of the same mass number but of different atomic numbers are known as ____.

Complete and balance the following equations:

Cl₂ + OH^- $$\to$$ Cl^- + ClO^-

Complete and balance the following equation:

HNO₃ + HCl $$\to$$ NO + Cl₂

The unit vector perpendicular to the plane determined by $$P\left( {1, - 1,2} \right),\,Q\left( {2,0, - 1} \right)$$ and $$R\left( {0,2,1} \right)$$ is ...........