Sodium forms a +1 ion while fluorine forms a -1 ion. Please explain why they do not form +2 and -2 ions, respectively. Use electron configurations and periodic trends to support your answer.

It is because sodium and fluorine are closer to the electronic configuration of neon(1s2 2s2 2p6 ).
Sodium---1s2 2s2 2p6 3s1
Fluorine---1s2 2s2 2p5
Thus it is easy to achieve the electronic configuration of neon rather than anything else so they loose and gain one electron easily to become stable

When the magnesium was added to water and phenolphtalein, what was observed?