Why are transition metals such as iron and tungsten mechanically strong?

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Diagram showing four types of crystal bonding: ionic bonding with sodium and chlorine ions, covalent bonding with carbon atoms sharing electrons, metallic bonding with iron ions surrounded by free electrons, and molecular bonding with hydrogen molecules held by weak electrical attraction. — chemical bonding Different types of bonding in crystals.

Transition metals, such as iron and tungsten, are mechanically strong primarily due to the nature of their metallic bonds. Unlike simple metals, which rely on weaker bonds formed by valence electrons in sp-shells, transition metals possess partially filled d-shells.1

The electrons in these d-orbitals are more tightly bound to the metal ions and do not wander freely throughout the crystal in the same way as sp-electrons. Instead, these d-orbitals form covalent-like bonds with the d-orbitals of neighboring atoms. Because these d-bonds are significantly stronger than the bonds found in simple metals, they result in much higher cohesive energy, which directly contributes to the exceptional mechanical strength of these elements.1