# Can someone explain

This forum made possible through the generous support of SDN members, donors, and sponsors. Thank you.

#### tydt

##### Full Member
10+ Year Member

Here's the question:

Arrange the atoms or ions O,F,F^+, and S in order by ionization energy, from smallest to largest.

A. S<O<F^+<F
B. S<O<F<F^+

Ok, I know that ionization energy increases from left to right and bottom up, which leads me to think that choice A would be correct, because wouldn't positive charge on the F atom make it go back to the left by one on the periodic table and decreases its ionization energy? But, choice B is the correct answer. Someone explain this pls.

Also, is ionization energy trend just another way of saying electronegativity trend?

#### diastole

##### Full Member
10+ Year Member
Ionization energy is the energy required to remove an electron. With F^+, you already removed an electron so now you are looking at the energy required to remove a second electron. The second ionization energy is always greater than the first ionization energy so the answer is B. It even makes sense if you think about it. You have the same amount of protons pulling on fewer electrons, so they will pull them even closer and it will be harder to pull off a second electron and even harder to pull off a third or a fourth. Does that make sense?

#### tydt

##### Full Member
10+ Year Member
Ok, so because F^+ is already had one of its electron removed which puts it at a higher state than F. If you look at the electronegativity trends, C is smaller than N so requires less ionization energy. That's why I was thinking because F^+ has fewer electons which means it would also requires lower ionization energy. Thus, I feel like i'm dealing with a twisted concept here. So which would you consider bigger between C^2+ and Ne? I'm still confuse. Sorry to bother but this is killing me. x_x

#### diastole

##### Full Member
10+ Year Member
Ok, so because F^+ is already had one of its electron removed which puts it at a higher state than F. If you look at the electronegativity trends, C is smaller than N so requires less ionization energy. That's why I was thinking because F^+ has fewer electons which means it would also requires lower ionization energy. Thus, I feel like i'm dealing with a twisted concept here. So which would you consider bigger between C^2+ and Ne? I'm still confuse. Sorry to bother but this is killing me. x_x

I don't consider electronegativity at all when I consider these problems. I think of the pull of the protons on the electrons. As you move across a row, you get more protons pulling in the electrons so they have a stronger pull and the radius gets smaller. Because the electrons are more tightly held, it is harder to remove one so the ionization energy increases as you move across the row too. If you try to remove a second electron, it is even more tightly held so it requires more energy than taking off the first.

When you go down a column, it works differently. There are more protons so the pull is stronger but the electrons in the valence shell are farther away and the other shells shield them from the charge. Thus it is easier to remove an electron from a element farther down the row.

For the problem, you asked I would think that C^2+ is going to have a higher ionization energy than Ne. With the carbon cation, six protons are pulling on just four electrons so they are going to hold them very tightly. It will require a lot of energy to remove the electron. With Ne, you have ten protons pulling on ten electrons so they aren't held as tightly.

I know that there is more going on than just the pull of the protons on the electrons. The atoms are also have higher ionization energy if you are removing an electron with a full octet. For example, Ne has a higher ionization energy than Mg^+ even though for Ne, it is the first ionization energy and for Mg^+, it is the second ionization energy because in the first case you are breaking the full octet and in the second case you are creating one. However, I don't think the PCAT goes into that level of detail. I think you will be okay if you picture how much the protons are pulling on the electrons. But maybe a chem major can come in with a better explanation.

#### tydt

##### Full Member
10+ Year Member
Things are finally coming together for me now, I don't know why I would consider electronegativity, which is clearly another concept. The PCAT is starting to take its toll on me and I'm over thinking way too much. The worst that can happen, is me missing one question. lol which I doubt now. Thnx your help!