Question 3

[sleepyzboy2302]

im a little confused, how do we know what IE is? Thanks

[mcat_premed3832]

IE is Ionization Energy which is described with equation in the last paragraph of the passage.

Also, if you have the GS book, check PHY 12.4 since it is an equation/concept that comes up rather frequently on real MCAT tests.

[jsfkt78927]

its pretty obvious how to figure out the answer, i guess thats why so many people get it right.

but the table is odd, i wasnt sure why there were so many ionization energies for some of the elements in the table....

any thoughts anyone?

[mohsinalam5969]

Ionization energy is the energy needed to remove an electron from an atom.
The first four ionisation energies of aluminium, for example, are given by

1st I.E. = 577 kJ mol-1
2nd I.E. = 1820 kJ mol-1
3rd I.E. = 2740 kJ mol-1
4th I.E. = 11600 kJ mol-1
In order to form an Al3+(g) ion from Al(g) you would have to supply
577 + 1820 + 2740 = 5137 kJ mol-1
That's a lot of energy. Why, then, does aluminium form Al3+ ions?

It can only form them if it can get that energy back from somewhere, and whether that's feasible depends on what it is reacting with.

For example, if aluminium reacts with fluorine or oxygen, it can recover that energy in various changes involving the fluorine or oxygen - and so aluminium fluoride or aluminium oxide contain Al3+ ions.
and similarly
If it reacts with chlorine, it can't recover sufficient energy, and so solid anhydrous aluminium chloride isn't actually ionic - instead, it forms covalent bonds.

Why doesn't aluminium form an Al4+ ion? The fourth ionisation energy is huge compared with the first three, and there is nothing that aluminium can react with which would enable it to recover that amount of extra energy.
the reason for this is that The electronic structure of aluminium is 1s22s22p63s23px1. The first three electrons to be removed are the three electrons in the 3p and 3s orbitals. Once they've gone, the fourth electron is removed from the 2p level - much closer to the nucleus, and only screened by the 1s2 (and to some extent the 2s2) electrons.
that is why diffrent ionization energies are given for same elements, it only depends upon which electron is going to b removed and from which orbital it is going to b removed