Question

a) why can noble gases bond with other atoms under certain conditions?

b) why does MgO fail in a brittle manner even though the crystal structure of MgO is FCC?

Answer 1

a) At normal conditions, the noble gases are not reactive because their outer orbitals are completely filled with electrons.

For example Xenon: 1s²2s²2p⁶3s²3p⁶3d¹⁰4s²4p⁶4d¹⁰5s²5p⁶

But at certain conditions they can react to form compounds. Here are the reasons.

The atoms is large. Hence the electrons in the outer orbitals are not held as strongly as the inner ones. Now when these electrons are targeted by highly electronegative elements like Fluorine, they can lose the electron and form compounds. In other words, the loosely held electrons in the outer shells, can be taken by the high electronegative Fluorine and thus form a bond with the noble gas Xenon.

b) MgO is a ceramic material with an FCC (rocksalt) structure. The structure is due to ionic bonding, and resistant to slip. Due to the inherent presence of dislocations in the structures, when such a dislocation moves in the structure under applied stress, it causes the charge balance between the anions and the cations to disrupt and this leads to the breakage of bonds in the oppositely charged ions. When slip occurs, the movement of the slip plane may lead to the anion and the cation to move past each other causing repulsion. This leads to the Burger's vector more than that in the metals and other alloys, causing brittle fracture.

The brittle fracture thus is because of the flaws in the crystal structure and the movement of these dislocations.