Why is sodium more reactive than magnesium

That is because Sodium it is more up in the scale unlike Magnesium. Sodium's radioactive because it has a lesser number then Magnesium, Sodium has 1 and Magnesium is 2.

Mg has two valence electrons

Explanation:

Na has only one valence electron, so it readily donates that electron to other atoms to achieve an octet.

Mg must lose two valence electrons to get an octet. It takes extra energy to remove the second electron. In addition, the two electrons are in a filled s subshell, which has added stability.