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Why are alkaline-earth metal oxides generally basic?

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A color-coded periodic table of the elements displays each element's symbol and atomic number, with categories such as alkali metals, noble gases, halogens, transition metals, and actinoid elements distinguished by different background colors. — alkaline-earth metal Modern version of the periodic table of the elements (printable).

Alkaline-earth metal oxides are generally basic because they are ionic compounds that contain the oxide anion ( $O^{2 -}$ ).12

When these oxides react with water, they undergo a chemical reaction to form hydroxides, which are compounds containing the $O H^{-}$ ion. This process is represented by the general equation:12

$M O + H_{2} O \to M (O H)_{2}$

(where $M$ represents an alkaline-earth metal). Because these resulting solutions contain hydroxide ions, they exhibit basic properties. Furthermore, in accordance with their basic nature, these oxides react with acids in neutralization reactions to produce salts and water.12

The basicity of these oxides generally increases as you move down the group in the periodic table, following the trend: $B e O < M g O < C a O < S r O < B a O$ .1