basic oxide

...Therefore, these are called acidic oxides. On the other hand, oxides of metal elements with very low electronegativity and high electropositivity, i.e., oxides of metal elements in the lower left of the periodic table, tend to form ionic crystals containing O ^2-. For example, sodium oxide Na ₂ O and calcium oxide CaO are called basic oxides because they are strongly alkaline when dissolved in water. In contrast, elements that are intermediate between nonmetallic elements and metal elements, such as those mentioned above, have low electronegativity even for nonmetallic elements, and as their metallic nature increases, their oxides tend to form macromolecules rather than single molecules (becoming less soluble in water), and their acidity weakens (Sb ₂ O ₅ , TeO ₃ , etc.). Even for metal elements, as their electronegativity increases, the bond with oxygen becomes less ionic and more covalent, decreasing their basicity, making them less soluble in water and more acidic. ...

*Some of the terminology that mentions "basic oxide" is listed below.

Source | Heibonsha World Encyclopedia 2nd Edition | Information

…したがって，これらは酸性酸化物acidic oxideといっている。また逆に電気陰性度がきわめて低く，電気陽性の高い，すなわち周期表左下方の金属元素の酸化物ではO^2－を含むイオン結晶をつくりやすく，たとえば酸化ナトリウムNa₂Oや酸化カルシウムCaOでは，水に溶かすと強い塩基性を示すので塩基性酸化物basic oxideといっている。これらに対し上記のような非金属元素と金属元素との中間にある元素では，非金属元素でも電気陰性度が低くなり，金属性が増すと，その酸化物は単分子からなるよりも巨大分子をつくりやすくなり(水に難溶性となる)，酸性は弱まってくるし(Sb₂O₅,TeO₃など)，金属元素でも電気陰性度が増してくると酸素との結合はイオン性を減じ，共有性が増してきて塩基性が減り，水に溶けにくくなり，酸性が増す。…

※「basic oxide」について言及している用語解説の一部を掲載しています。

出典｜株式会社平凡社世界大百科事典 第２版について | 情報