O2 – a bond is formed by two non-metallic elements (O and O), therefore, the bond is covalent and not polar (since these non-metals are the same);
HF – the bond is formed by two non-metallic elements (H and F), therefore, the bond is covalent polar (since these non-metals have different electronegativity);
NaF – the bond is formed by non-metallic (F) and metallic (Na) elements, therefore, the bond is ionic;
Ca is a metal, therefore the bond is metallic.
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