Why does magnesium have a higher third ionisation energy than aluminium?

Answer 1

Because aluminum has 3 electrons in its valence shell and when you get to the 3rd ionization energy it has an empty valence shell with a configuration as the noble gas Neon. Aluminum wants to stay at this state and will take a lot of energy to loose another electron from next energy level.

Answer 2

This explanation covers a lot of mid-level chemistry, for those who don't know a lot about it and may need some background.

• The electrons in an atom are arranged into orbitals, then further divided into sublevels. The order of sublevels is 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d... The cause of the discrepancy in 4s/3d being reversed is due to electrons in 4s having less energy than those in 3d; it is close, though, so they can easily flip back and forth when excited (the science word for 'given lots more energy').
• Each type of sublevel holds a fixed maximum amount of electrons: s-levels hold 2, p-levels hold 6, d-levels hold 10 (and f-levels hold fourteen, but that's another unnecessary, unrelated story). The electrons in each sublevel are then "organised" into pairs; one "up" spin, one "down" spin - their spins then cancel (another related, but unnecessary story to that is how iron is magnetic).
• Now, silicon has 14 electrons while aluminum has 13. Their arrangements are:(for silicon) 1s2, 2s2, 2p6, 3s2, 3p2 and (for aluminum) 1s2, 2s2, 2p6, 3s2, 3p1. Note that aluminum has only one electron in its 3p sublevel and that silicon has two. Because 3p has a maximum size of six, neither is full.
• When talking about ionisation energies (IEs), we refer to the energy needed to remove an electron only. So for the first IE, we remove one electron from each.
• The arrangements are now 1s2, 2s2, 2p6, 3s2, 3p1 for silicon and 1s2, 2s2, 2p6, 3s2 for aluminum. Again, note the number in the 3p sublevel: silicon has only one, but aluminum now has zero. Since that sublevel is empty, we go to the next one down, aluminum's 3s.
• When we do the second IE, we remove a second electron. This is quite easy to do to silicon, because it has only one electron in its valence (outermost) sublevel and it is unpaired. But because aluminum's valence sublevel is now 3s, which is full and has both its electrons paired, we must break up the pair and break up a full sublevel. This takes more energy than just one electron from 3p; so aluminum's second IE is higher than silicon's.

Answer 3

Removing the third electron from magnesium, or the fourth electron from aluminium, means removing an electron from a filled shell. This is much more difficult than removing an electron from an unfilled shell.

Answer 4

The second ionization energy of calcium is higher than its first ionization energy. You are, after all, removing an electron from a positively charged ion; however, the Ca+ ion does not have a noble gas configuration, so it easily loses a second electron to achieve one. If you were to look up calcium's 3rd ionization energy, you would find it to be tremendously higher than the second, just as potassium's second ionization energy is much higher than its first.

Answer 5

As silicons electrons pair up in the 3s orbital hence there is mutual repulsion between electrons and the 2nd ionisation energy is lowered.

Answer 6

Magnesium has higher 3rd ionisation energy. It is difficult to remove 3rd electron from completely filled orbit.

Answer 7

Just for the lol's.