Magnesium has 2 valence electrons, and oxygen has 6 valence electrons, which type of bonding is likely to occur between a magnesium atom and an oxygen atom?

The reactivity of atoms is governed by their "desire" to obtain a stable electron configuration for their outermost energy shell.

As you know, this happens when an atom has #8# electrons, also called valence electrons, in its outermost energy shell, i.e. when the atom has a complete octet.

Now, magnesium, #"Mg"#, has #2# valence electrons. In order to complete its octet, it must get to #8# valence electrons. The only way for magnesium to accomplish this is to give up its two valence electrons and have one of its filled energy levels become the valence shell, i.e. the outermost energy level.

On the other hand, oxygen, #"O"#, has #6# valence electrons, so it's much easier for oxygen to add two electrons to its outermost shell in order to complete its octet.

When a magnesium atom reacts with an oxygen atom, magnesium gives up its two valence electrons and becomes a positively charged ion, i.e. a cation, #"Mg"^(2+)#. Oxygen accepts these two electrons and becomes a negatively charged ion, i.e. an anion, #"O"^(2-)#.

The cation and the anion will then be attracted to each other by way of the electrostatic force of attraction #-># an ionic bond is formed.