chemical bonding Flashcards
define hybridisation
mixing of atomic orbitals of different energy to obtain homogenous orbitals of the same energy.
Graphite has a higher melting point than diamond. Explain why in terms of structure and bonding.
Both diamond and graphite have a giant covalent structure and melting involves the breaking of the covalent bonds between the carbon atoms. In diamond, the carbon atoms are sp3 hybridized while in graphite, the carbon atoms are sp2 hybridized. Each carbon atom in graphite has one unhybridized p orbital containing one electron each. The p orbital of one carbon atom overlaps collaterally(side-on) with the p orbitals of its immediate neighbor. Hence, there is additional electron density between the carbon atoms holding the nuclei more closely, hence strengthening the C-C bond.
Describe bonding in a layer of BN
Both B and N are sp2 hybridized and each B and N atom thus has one unhybridized p orbital perpendicular to the plane of atoms. B is from group 13 and its unhybridized p orbital is empty while N is from group 15 and its unhybridized p orbital has 2 electrons. The unhybridized p orbitals overlap side-on continuously with its neighbours, resulting in the delocalization of pi electrons over the whole layer.
Explain why electrical conductivity of BN is lower than that of graphite
BN is an electrical conductor due to the delocalization of pi electrons. However, since N is more electronegative than B, the pi electrons tend to stay with N rather than delocalize over the pi electron cloud, resulting in BN being a poorer electrical conductor than graphite.
Why silicon has a lower melting point than diamond despite having the same structure.
Since the valence orbitals of Si are larger and more diffuse than that of C, there is less effective overlap between the orbitals of Si, resulting in the Si-Si bond being weaker than the C-C bond. Less energy is required to overcome the weaker Si-Si bond in silicon than the stronger C-C bonds in diamond, hence silicon has a lower boiling point than diamond.
explain why melting point of diamond is higher than that of silicon
both have giant covalent structures. However, for silicon, due to its larger size, its valence orbital is more diffuse, and thus the orbital overlap between the valence orbitals of the Si atoms are less effective, leading to weaker Si-Si bonds which require less energy to overcome, resulting in a lower melting point of silicon.
