why is ionization energy always positive (endothermic)?

[bored]

Ionization = removing an electron. Why does this process "always require energy" according to Kaplan?

Wouldnt removing an electron from an atom like Na release energy, since it wants that sexy octet?

Thanks

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[Synapsis]

Ionization = removing an electron. Why does this process "always require energy" according to Kaplan?

Wouldnt removing an electron from an atom like Na release energy, since it wants that sexy octet?

Thanks

The simple fact that the valence electron is held by sodium is due to the coulombic potential (although weak) between its nucleus and the electron. That energy that's holding the electron in must be overcome in order to ionize it.

This reasoning can be confused with electron affinity, which is the energy released when adding in electron. This can be negative because some atoms (halogens) like to have an extra electron added, so energy is released. And it's positive for atoms that don't want that electron added, like sodium. The reasoning is different because in the case of ionization energy, it costs energy (positive) to take an electron, simply because the nucleus is holding it with some force. Think of stealing versus donating money (electrons) from/to people (nuclei). It always takes some effort to steal money from a person, even if they're rich. But sometimes, it takes essentially no effort to donate money.

 

[bored]

The simple fact that the valence electron is held by sodium is due to the coulombic potential (although weak) between its nucleus and the electron. That energy that's holding the electron in must be overcome in order to ionize it.

This reasoning can be confused with electron affinity, which is the energy released when adding in electron. This can be negative because some atoms (halogens) like to have an extra electron added, so energy is released. And it's positive for atoms that don't want that electron added, like sodium. The reasoning is different because in the case of ionization energy, it costs energy (positive) to take an electron, simply because the nucleus is holding it with some force. Think of stealing versus donating money (electrons) from/to people (nuclei). It always takes some effort to steal money from a person, even if they're rich. But sometimes, it takes essentially no effort to donate money.

Ok but could one argue that the sodium ion (with one electron removed) is more stable than the sodium atom and so energy is released after we have enough energy to overcome the coulombs force?

 

[milski]

Ok but could one argue that the sodium ion (with one electron removed) is more stable than the sodium atom and so energy is released after we have enough energy to overcome the coulombs force?

No ions are more stable than their corresponding neutral atom. Carrying around the additional +/- charge is a huge liability.

 

[QrtrLifeCrisis]

No ions are more stable than their corresponding neutral atom. Carrying around the additional +/- charge is a huge liability.

I don't think this is true. Otherwise, halogens would not have negative electron affinities (i.e., releasing energy when it accepts an electron). The halogen ions are more stable than their neutral counterparts. As a general rule, when something releases energy, it becomes more stable.

In fact, aren't neutral halogens super unstable? They undergo radical reactions.

 

[Synapsis]

Ok but could one argue that the sodium ion (with one electron removed) is more stable than the sodium atom and so energy is released after we have enough energy to overcome the coulombs force?

Which would be a fair argument. But the removal of that electron is analogous to a chemical reaction in that there is an "activation energy" of sorts necessary (overcoming the coulombic interaction). If it didn't require any energy to remove an electron then anytime we just had Na, it would just lose an electron to form Na+! It's like an exothermic reaction where, even though the product is more stable than the reactant, you don't just "get there", it first costs a little bit of energy.

Does this help?

 

[QrtrLifeCrisis]

OP, I would think about it in terms of the Coloumbic Force Equation:

F=-kqq/r^2

For Na and the first ionization energy, it takes a net input of energy to pull a valence electron from its orbital and move it to r=infinity (going from a negative value to zero).

Conversely, Na actually has a negative electron affinity so it releases energy when it becomes an anion.

Essentially, electrons are attracted to the nucleus. They release energy when they get closer to the nucleus. Or, they absorb energy when they get further away from the nucleus. However, this attraction is attenuated due to the effects of other electrons surrounding the nucleus, which is why IE and EA differs for all atoms.

 

[ToldYouSo]

Ionization = removing an electron. Why does this process "always require energy" according to Kaplan?

Wouldnt removing an electron from an atom like Na release energy, since it wants that sexy octet?

Thanks

It always requires energy to separate an electron from a positively charged nucleus.

 

[milski]

I don't think this is true. Otherwise, halogens would not have negative electron affinities (i.e., releasing energy when it accepts an electron). The halogen ions are more stable than their neutral counterparts. As a general rule, when something releases energy, it becomes more stable.

In fact, aren't neutral halogens super unstable? They undergo radical reactions.

The opposite would imply that some species would auto-ionize and stay in that state which normally does not happen.

Halogens will happily share electrons and participate in redox reactions or will stay as ions in a lattice when stabilized by positively charged ions but neither of these is what we're talking about here. Redox is not ionization at all and in the lattice case you cannot ignore the stabilization that includes the lattice energy. X₂ on its own, when not interacting with other species, is quite stable in staying neutral and non-ionized.

 

[QrtrLifeCrisis]

The opposite would imply that some species would auto-ionize and stay in that state which normally does not happen.

Halogens will happily share electrons and participate in redox reactions or will stay as ions in a lattice when stabilized by positively charged ions but neither of these is what we're talking about here. Redox is not ionization at all and in the lattice case you cannot ignore the stabilization that includes the lattice energy. X₂ on its own, when not interacting with other species, is quite stable in staying neutral and non-ionized.

X₂ is not a neutral atom. X₂ is a molecule. It exists in that state because X is indeed unstable as a neutral atom. The same can be said of any element that exists in its diatomic state.

The opposite to my statement would not imply auto-ionization. It would imply that these "neutral atoms" are highly reactive as you pointed out. X does not exist in a vacuum, independent of other atoms in the universe.

If you meant to say, "No ions are more stable than their corresponding neutral element", then I would not have had a problem with this statement..

 

[krogers21]

To think of it in the most generic way, a nucleus is positively charged (neutrons = neutral charge, protons = positive charge). As a result, the electrons are attracted to this positive charge. The outer shell electrons are repelled, however, from the inner shell (inner = closer to the nucleus, outer = whatever your reference) electrons. Ionization energy is based on the energy it takes to pull the valence(typically outer shell unless all natural valence electrons have been removed - very unstable) electrons away from the positive nucleus. To think about whether it will want to remove its electrons or not depends on whether it would be more stable with or without electrons. For instance, Na has an electron configuration of [Ne]3s1. It can become more stable by losing it's electrons to essentially have an electron configuration as that of Ne so the 1st ionization energy is relatively small as say compared to an anion. For an anion, say Cl, it has an electron configuration of [Ne]3s2 3p5 (7 valence electrons). It wants to gain an electron, not lose one to make it have that "sexy octet" so it's going to take a lot of energy to make it lose an electron. In general, all ionizations are endothermic because you're taking something away. I like to think of it in terms of robbery. If a robbery takes a purse from a lady, that's "robbery energy". To take the purse from an old lady (a cation), the robber is going to still put forth an effort because a lady isn't going to just hand over her purse (ignore the fact that guns exist! haha). However, if a robber tries to take a purse from a body builder woman, that robber better use a lot of energy! Sorry if this doesn't help - sometimes I only see the analogy haha.