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broad and easy explanation eh? how about lower members of group 4 are less able to use their s electrons to form 4 bonds than that of say Carbon which promotes an 's' electron to a 'p' orbital = 4 unpaired electrons = 4bonds. So Pb is stuck with 2 paired s electrons (INERT PAIR) and 2 unpaired electrons = 2 bonds hope that helps

Okay for 6B #1 you have a proton signal at >10 which strongly suggests a caboxylic acid, the remaining 3 signals correspond to a 3 carbon fragment where all the protons are in different environments i.e. two triplets and a hextat (n+1 rule) 6B #2 you have a proton in the 9-10 range, strongly suggesting an aldehyde and the two reamining signals and 3 carbons, this means symmetry in the molecule so two signals are equivalent 6A #4 is very similar to 6B #1 except its a bromoalkane instead of the acid

Good question, you may recall that there exists diagonal relationships in early members of different groups in the periodic table leading to unusual behaviour for that group. Li and Mg share a diagonal relationship because in Li->Na there is a decrease in charge density, but from Na->Mg there is an increase. It is known that Li and Mg share similar chemistry due to the similarity in the degree of covalency in their compounds. Lithium has a comparatively low conductivity because of its high ionization energy relative to the others; that is, it "cheats" (on average, delocalizes less than 1 mole of electrons per mole of atoms), so its conductivity is lower than expected. The same rationale can be applied to the Group 2 elements which are divalent, where the conductivity of magnesium (and Be) is low in comparison to the others. This suggests that this higher ionization energies limit delocalization to fewer than 2 moles of electrons per mole of atoms. This could explain the comparatively low melting point of Mg. If fewer than two electrons are delocalized, then the resulting lattice energy must be lowered accordingly.