Oxygen is highly electronegative, which means it has a strong tendency to attract electrons towards itself when forming chemical bonds. Electronegativity is a measure of the ability of an atom in a chemical compound to attract electrons, and on the Pauling scale, oxygen has a value of 3.44, making it one of the most electronegative elements.
When oxygen bonds with hydrogen to form water (H2O), the difference in electronegativity between oxygen and hydrogen leads to a polar covalent bond. In this molecule, the oxygen atom carries a partial negative charge while the hydrogen atoms carry partial positive charges. This polarity allows water molecules to interact with each other through hydrogen bonds, which are relatively weak attractions between the hydrogen atom of one molecule and the electronegative oxygen atom of another. These hydrogen bonds are crucial for many of water's unique properties, such as its high surface tension and boiling point relative to its molecular weight.
In contrast, sodium, calcium, and potassium are metals that have low electronegativities and tend to lose electrons rather than attract them. These elements typically form ionic bonds rather than the polar covalent bonds characteristic of hydrogen bonding. This fundamental difference in bonding behavior is what distinguishes oxygen from the other