effective nuc charge question

[KK82]

anyone want to explain this. i hate zeff.

The ionization energy of H is 1312. approximate second IE of He,

5248, 2624, 656,328. anser is 5248

this is the explaination i dont get at al....
they say zeff for H is 1. firstly i thought zeff was atomic number minus core electrons but H's valence are the core. however i do ge that h just has one proton so is tha why it is 1. the they say IE IS PROPORTIONal TO Z^2, the second IE of helium should be four times as great as If of hydrogen.

oh yeah, they say the zeff for He+ is +2....

anyone elaborate?????????

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[docmd2010]

anyone want to explain this. i hate zeff.

The ionization energy of H is 1312. approximate second IE of He,

5248, 2624, 656,328. anser is 5248

this is the explaination i dont get at al....
they say zeff for H is 1. firstly i thought zeff was atomic number minus core electrons but H's valence are the core. however i do ge that h just has one proton so is tha why it is 1. the they say IE IS PROPORTIONal TO Z^2, the second IE of helium should be four times as great as If of hydrogen.

oh yeah, they say the zeff for He+ is +2....

anyone elaborate?????????

To answer this question, you should employ common sense rather than trying to figure out confusing math equations. Look at the answer choices. The latter two are immediately out-Helium as 2 protons in its nucleus in addition to being a noble gas, hence it's nucleus exerts a larger attraction for electrons than Hydrogen. So it makes sense that the first ionization energy of Helium would be greater than Hydrogen. You should know that the energy to remove an electron increases with the ionization energy number. So obviously, the last two choices can't be correct, as they are lower than Hydrogen's first ionization energy of 1312. You know that the second ionization energy of Hydrogen must be much greater than the first, so let's approximate that the second is also 2600 (approximately double the first). Now look at the second answer choice-2624, it doesn't make sense that helium, being a noble gas with a stable electron configuration, would have the same second ionization energy as hydrogen, (it takes more energy to take away an electron from a stable configuration such as helium) therefore the firsts chocie, 5248 is the only plausible and most logical answer choice.

Sometimes you have to look beyond the equations and simply look at the question, each mcat question on average should usually only take you about 1 to 1.5 mintues, so obviously all the questions should be fairly straight forward when it comes to math.

hope this helps.

 

[DrBowtie]

I agree if you just look at it for a sec. you could easily figure it out.
You know it is harder to pull off electrons as you pull them off so you can eliminate the doubled one. Then the other one is way too large. POE works wonders here.

 

[medstu2006]

anyone want to explain this. i hate zeff.

The ionization energy of H is 1312. approximate second IE of He,

5248, 2624, 656,328. anser is 5248

this is the explaination i dont get at al....
they say zeff for H is 1. firstly i thought zeff was atomic number minus core electrons but H's valence are the core. however i do ge that h just has one proton so is tha why it is 1. the they say IE IS PROPORTIONal TO Z^2, the second IE of helium should be four times as great as If of hydrogen.

oh yeah, they say the zeff for He+ is +2....

anyone elaborate?????????

Zeff = # of protons - non-valence electrons
For H, you get 1-0 = 1. The 1 electron that H has is a valence electron.
Same applies for He since it has 2 electrons in its outermost orbit. So u get +2.

As for ionization energy, it increases as you go from left to right. The second ionization energy is always greater than the first. So you can eliminate the last 2 options. Since He is a noble gas, you know that the nucleus holds onto its electron pretty tightly and so it would require high amount of energy to remove the first electron, let alone the second one. Therefore the ans. is 5248. I have not heard about IE being proportional to Z^2. Where did you get this formula?
I hope this helps.

 

[DrBowtie]

Even so being proportional could require a constant.

 

[KK82]

thanks guys, i do get that, but it was just throwing me off bc Hydrogen only has 1 electron so how can it have a second ionization energy?

secondly, He+ has the same config as Hydrogen (both have 1 electron) so i thought that He+ second IE=Hydrogen's first, and only, IE

btw, the formula was from berkeley review, but they did not expalin anything else

 

[QofQuimica]

I have written a post explaining Zeff: http://forums.studentdoctor.net/showpost.php?p=2720242&postcount=2