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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?

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    $\begingroup$ It also has 18 core electrons. $\endgroup$ – orthocresol Dec 27 '15 at 7:06
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    $\begingroup$ It's not a full level. It's but an s-sublevel. $\endgroup$ – Ivan Neretin Dec 27 '15 at 7:08
  • $\begingroup$ 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. $\endgroup$ – Sikander Dec 27 '15 at 8:24
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The electronic configuration of Ca is $\ce{1s^2 2s^2 2p^6 3s^2 3p^6 4s^2}$

It has a total 4 Energy levels. The outermost energy level has 2 electrons called valance electrons. As, Calcium is metal and it can easy donate two electrons and becomes cation (Ca+2). Now, this cation is stable.

Calcium is reactive because it can easy donate electrons by providing energy to it called ionisation energy.

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    $\begingroup$ Calcium does not have four energy levels. It has 10 occupied electronic states at 6 different energetic levels. (the 2p and 3p states are triple degenerate). Furthermore, the reactivity of Ca is not explained because Ca can provide an energy termed the ionisation energy. Ionisation costs energy, it does not provide energy. The fact that Ca is reactive is because those two valence electrons are relatively unstable and can go to a lower energetic state by reacting. If under this reaction the electrons effectively migrate to another atom, Ca becomes ionised. $\endgroup$ – Ivo Filot Dec 30 '15 at 13:21

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