which element has the larget ionization energy?

[Smooth Operater]

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a. K
b. Cs
c. Ti
d. As
e. Se

I thought it is Se since it has the most proton and most electrong in outer shell. But the correct answer is As. Why?

 

[Streetwolf]

'As' has a p3 configuration while Se has a p4 configuration. Since p orbitals hold 6 electrons max between 3 of them (x, y, z), each p orbital holds 2. The most stable way to fill up p orbitals is to do px, py, pz, px, py, pz or in other words, fill up all 3 orbitals with 1 electron each and then go back and fill them up with the 2nd electron in each. Therefore, the p3 configuration of 1 electron in each of px, py, and pz is relatively stable.

Since Se has p4, it has 2 in px, and 1 in py and pz. Meanwhile, As has p3 so 1 in px, py, and pz. Ionization energy is basically telling you how hard it is to remove one electron from the ground state. It is EASIER to remove the electron in the px orbital of Se because you would be bringing it to a more stable state (ie 1 electron in each of px, py, and pz). It is HARDER to remove the electron in any orbital of As because you are already in a stable state.

Hence the ionization energy of As is higher than Se.

 

[RHONDAROBINSON]

It is Se because ionization energy increases from left to right and also increase as it goes up the periodic table just like the electronegativity.

 

[celeron_266]

arsenic is higher

cuz if you ionize Se to Se+, it has a half filled p subshell, which makes it stable, meaning that its ionization energy should be pretty low....conversely, As already have a half filled p subshell, which is stable...you pull one electron away from it, you are basically destabilizing it, meaning that the ionization energy will be quite high

 

[Smooth Operater]

so RHONDAROBINSON's explanation doesn't work in this situation? This is like a special case.

 

[smuwillobrien]

Ionization energy increases as you move to the right and up along the periodic table. If you look at the halogens, F would have the highest energy of ionization. As you move down the column, the ionization energy decreases. Although the proton number increases, so does the shielding from core and valence electrons.

 

[smuwillobrien]

arsenic is higher

cuz if you ionize Se to Se+, it has a half filled p subshell, which makes it stable, meaning that its ionization energy should be pretty low....conversely, As already have a half filled p subshell, which is stable...you pull one electron away from it, you are basically destabilizing it, meaning that the ionization energy will be quite high

celeron_266 has a good point here about considering subatomic orbitals - sometimes an ion can distribute them in a favourable unpairing configuration (i.e. five electrons in a d shell). You should try and take that into account because it might suggest a non - obvious answer.