Answer:
calcium (Ca)
Step-by-step explanation:
The element most likely to react with oxygen explosively is calcium. Calcium and magnesium are in the same group . Both of them are in group 2 . The electron configuration of magnesium and calcium are as follow;
Magnesium has an atomic number of 12 and can be represented as 2 8 2 .
or 1s2 2s2 2p6 3s2
Calcium has an atomic number of 20 and can be represented as 2 8 8 2.
or 1s2 2s2 2p6 3s2 3p6 4s2
Notice both of them have valency electron of 2. Both can easily go into a reaction as they lose the 2 electron to attain the octet rule.
Most group 2 elements react explosively with Oxygen and are usually reactive. Group 2 elements are also known as Alkali earth metals.