Questions

When atoms react they do so in order to reach the lowest energy, which is the most stable. The electrons orbitals will overlap in the way that reduces the energy the most. Orbitals of similar energy will produce the greatest stabilisation.The answer you gave isn't the whole story, it depends what species is reacting with what. The effective nuclear charge on the atom will affect the energy of the valance electrons.

Hence, the reactivity of an atom arises from the existence of unpaired electrons in the valence shell.

In the given puzzle, the logic behind is

9 = 72 => 9 x 8 = 72

8 = 56 => 8 x 7 = 56

7 = 42 => 7 x 6 = 42

6 = 30 => 6 x 5 = 30

5 = 20 => 5 x 4 = 20

Similarly,

4 = 12 => 4 x 3 = 12

3 = 6 => 3 x 2 = 6

Sum of numbers in the outer circle in before Arc should come in inner circle patterns in the clock wise direction next Arc.

i.e, 9+6 = 15 comes in next arc of inner circle

7+3 = 10 comes in next arc of inner circle

In the same way, 4+2 = 6 comes in place of '?'.