_H2+_Cl2->_HCl fill in the blanks if necessary.

H₂   +   Cl₂    →    2HCl

Explanation:

The given reaction expression is shown below:

      H₂   +   Cl₂    →    HCl

We are to balance this given reaction expression.

Let us do this mathematically;

       aH₂   +   bCl₂    →    cHCl

Conserving H :  2a  = c

                   Cl:   2b  = c

Now, let a  = 1, c  = 2 , b = 1

So:

            H₂   +   Cl₂    →    2HCl

the density(ratio between mass and volume as m/v) decreases from top to bottom in periodic table group as the size increases and nuclear charge decreases. but in periods density increases from left to right as atom size decreases and nuclear charge increases.