system
Given the following post, respond as if you're commenting on Reddit:
human
What's the difference between hard and brittle?

I mean from the molecular (or bond?) standpoint. Why is it that something can be very hard and yet brittle?  It seems like both properties are essentially a strong bond between molecules.
gpt
So lets get some technical definitions in there first:

Hardness is the ability to resist deformation. Brittleness is the tendency to undergo sudden catastrophic structural failure instead of plastic deformation.

While they *seem* to be related, they are in fact not. 

Lets talk in terms of metals for discussing hardness. Hardness essentially measures the force required to deform the molecules. From a molecular standpoint, it is how much free space there is in the crystal lattice that can be compressed for a relatively small energetic cost. This can take the form of shortening of every bond in the cubic cell, or the movement of an atom from a lattice point into a hole that is not on the lattice proper (there are other mechanisms in coarse grain materials too). If your metal is made of small hard spheres (like titanium or aluminium) it will be very hard, since these things are very difficult to compress. If your metal is something big and soft like gold, then it can easily be squished and it is soft.

Lets talk in terms of polymers for discussing brittleness. Brittleness tells you how something behaves beyond the elastic limit, when the tensile strength of your material has been exceeded. A plastic deformation is a rapid rearrangement of your material that results in a permanent deformation, a long unbranched polymer will readily undergo plastic deformations, since the chains can rearrange to slip over each other. If your material if brittle then it doesn't undergo rearrangements, it just fractures into smaller pieces to relieve the stress (force per unit area) being applied. Glass is a massively cross-linked amorphous polymer, its chains can't slide over one another, since they are all cross-linked, so it can't deform, it simply shatters instead.

Hope that helps a bit.