Why some materials are ductile and some brittle?

Answer 1

Materials that contain metallic bonding tend to be ductile because after a small distortion each metal ion is still in exactly the same environment, so the shape is retained.

Materials that are brittle tend to be bonded ionically; the structure is held together by the alternating positive-negative charges, if a distortion brings two like-charges together then they repel and the structure comes apart.

I'm assuming you have the basic knowledge of metallic and ionic bonding that will allow you to understand my answer, if not, and this doesn't make sense, feel free to message me and I'll try and help you further :)

Answer 2

Materials that are ductile have strong metallic bonds that allow atoms to slide past each other when subjected to stress, without breaking. In contrast, brittle materials have weaker or more directional bonds that make them prone to fracturing when stressed, as they cannot plastically deform like ductile materials. Factors such as the crystal structure, atomic arrangement, and type of bonding determine whether a material will exhibit ductile or brittle behavior.