The electronic configuration of chromium is [Ar] 3d5 4s1 and that of manganese is [Ar] 3d5 4s2 i.e., in manganese-all the ‘d’ orbitals are singly filled and the 4s orbital is completely filled. As the ‘d’ electrons are more tightly held by the nucleus, the electrons are not available for bonding resulting in weaker metallic bonding in manganese compared to chromium.
