Question

5. This problem deals with a dilute gas of neutral lithium (Z=3) and fluorine (Z=9) atoms, and the molecules that might (or might not) form from them. Please explain your answers briefly.

a) Are any molecules likely to arise from combinations of only lithium atoms? Why or why not? If so, what is the most likely molecule and how can you estimate its binding energy?

b) Are any molecules likely to arise from combinations of only fluorine atoms? Why or why not? If so, what is the most likely molecule, and how can you estimate its binding energy?

c) Are any molecules likely to arise from combinations of both lithium and fluorine atoms? Why or why not? If so, what is the most likely molecule and how can you estimate its binding energy?

d) Of the three possible combinations above, is there any reason to expect that one will have the largest dipole moment? If so, which one? Why? Can you estimate it?

Answer 1

given lithium (Z = 3), fluorine (Z = 9) atoms in a gas

a. lithium atoms may combine to form molecules like Li2, Li6 etc

the most abundant specie of the lithium molecule would be though Li2, at about 1 % of the total gass phase, the other molecules will have furhter lower concentrations

the binding energy can be estimated as the eenrgy required to bring the two +vely charged neucleus close together at a distnace of the bondlength of the covelent Li2 molecule, which will be +ve

b. for fluorine atoms, the fluorine atoms exist in f2 (di fluorine ) state, as a diatomic covelent molecule. the binding energy is the energy required per mole to convert f2 gas into charged f- atoms

c. when fluorine atoms and Li atoms are present, they react to form LiF ( lithium floiride). this is easy to form componund whise binding energy is sufficiently large and negative and can be estimated to be calculable by caluclating the potential energy of the Li+ and F- ions cloase together at the bond length seperation

d. out of the three combinations, LiF has the largest dipole moment as LiF has the ,argest electronegativity differnce among the atoms, giveing rise to a larde charge polarisation in the molecule, this large charge polarisation gives rise to largest dipole moment. This dipole moment for LiF will be of the order of charge of electron*bond length of the LiF molecule