mixed bonding
The four bond types — ionic, covalent, metallic, and secondary — are useful boxes, but nature rarely commits to just one. Most real bonds are blends, sitting somewhere between the extremes. Mixed bonding is the honest acknowledgement that a bond can be, say, 60 percent ionic and 40 percent covalent at the same time. Think of it as a colour wheel rather than three separate paint pots: pure red, pure blue, and everything in between.
Electronegativity is the dial that sets the mixture. When two atoms differ a lot in electronegativity, electrons transfer nearly completely and the bond is mostly ionic; when they are equal, electrons are shared evenly and the bond is purely covalent; in between, the bond is partly ionic and partly covalent. Pauling gave an approximate formula for the percent ionic character based on the electronegativity difference: for sodium chloride the difference of about 2.1 gives roughly 70 percent ionic character (so even table salt keeps some covalent sharing), while silicon carbide, with a small difference, is largely covalent with just a little ionic flavour.
A helpful way to organise this is the bonding tetrahedron: place ionic, covalent, metallic, and van der Waals bonds at four corners, and every real material falls somewhere inside. Semiconductors like gallium arsenide are mixed covalent-ionic; intermetallic compounds are mixed metallic-ionic or metallic-covalent; many ceramics are mixed ionic-covalent. Recognising the blend is what lets you predict properties that a single-box label would get wrong.
Gallium arsenide, a workhorse semiconductor, has bonds that are mostly covalent (directional sharing, four bonds per atom like silicon) but slightly ionic because gallium and arsenic differ a little in electronegativity. That small ionic tilt subtly changes its band gap and is part of why it outperforms pure silicon in some devices.
Most bonds are blends; the electronegativity gap sets the recipe.
Percent ionic character is an estimate, not a measurement — the formula is a useful guide, and different methods give somewhat different numbers. The real point is directional: bigger electronegativity difference means more ionic, and that shift genuinely changes hardness, brittleness, and conductivity.