Calcium reactivity vs electronic configuration

Calcium is an alkaline earth metal, so it is reactive. But, it has two valence electrons. Don't those two electrons fill up the first energy level? Isn't an atom with a full energy level considered stable? Why is it reactive then?

• It also has 18 core electrons. – orthocresol Dec 27 '15 at 7:06
• It's not a full level. It's but an s-sublevel. – Ivan Neretin Dec 27 '15 at 7:08
• The concept of stability is significant where exchange energy is involved which also forms the basis of Hund's rule,but for $s$-orbital exchange energy is not considered since there are no different spacial distribution unlike $p,d,f$-orbitals.Moreover Calcium is more electro positive atom due to less ionization energy so it would be more stable in compound form when it reacts with most of the electronegative elements. – Sikander Dec 27 '15 at 8:24

The electronic configuration of Ca is $\ce{1s^2 2s^2 2p^6 3s^2 3p^6 4s^2}$