<span>Find a periodic table of elements. ... <span>Label each column on the periodic table of elements from 1 to 18. ... </span><span>Find your element on the table. ... </span>Use the group numbers to determine the number of valence electrons. </span><span>imgres</span>
Velocity is speed and Direction. It does not include direction
There is a shortcut trick while doing such fill in the blanks of nuclear reactions of hydrogen and helium
Let a,b,care elements of set N

Now
for our question
Hence b=4-3+1=1+1=2
So
The missing place should b e deuterium of heavy water
In nuclear reactions energy is released so it's mentioned on product side not reactant side