from hyper dictionary.
Webster's 1913 Dictionary
Definition:
\Mal"le*a*ble\, a. [F. mall['e]able, fr. LL. malleare
to hammer. See {Malleate}.]
Capable of being extended or shaped by beating with a hammer,
or by the pressure of rollers; -- applied to metals.
The property you are looking at is
Brittle \Brit"tle\, a. [OE. britel, brutel, AS. bryttian to
dispense, fr. bre['o]tan to break; akin to Icel. brytja, Sw.
bryta, Dan. bryde. Cf. Brickle.]
Easily broken; apt to break; fragile; not tough or tenacious
Malleabe and brittle are often but not always antonyms
The electrons are the reason for conduction but it's not that simple for malleability. Metals are malleable because there is no natural affinity for the atoms to remain in place with respect to each other (referred to as plastic flow). Brittle substance are usually so because there is an ionic structure that prevents the individual atoms from being rearranged without causing a failure.