Gen Chem destroyer Q

[MedHopeful234]

Clarification Question:

Organize in decreasing order with respect to atomic radii:

a. Br->I->F-
b. K+>Ca2+>Al3+
c. Al3+>Na+>O2-
d. K+>Ca2+>Br-
e. O2->O+>O2-

It was between B and D for me... but the answer was B... I read this same question on another thread and someone explained that you cannot compare cations with anions (as in the case of D)... Is that ALWAYS the case? As in the only time you can compare ions is for them to be in isoelectric series?

Also, can someone explain why atomic radius decreases across the periodic table? I think it has something to do with effective nuclear charge, but I never quiet understood what that meant. I get confused only because we are adding both protons and electrons going across and the decrease in radius doesn't make as much sense to me.

Thanks!

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[StumpMT]

Yes, the only time you can compare ions is if they're in isoelectric series. Actually you could but that's beyond the scope of what the DAT will cover, according to Chad on his "Periodic Trends" video. If you have his videos this one will answer your question perfectly, but I'll try to explain it for you to the best of my ability.

Atomic radius decreases going up and to the right on the periodic table. This is because the effective nuclear charge (overall attraction to nucleus) increases. There is a simple equation for this, Zeff = Z - S. Zeff is the effective nuclear charge (overall attraction to the nucleus), Z = # protons and S = # core electrons. A core electron is any electron that is not a valence electron. For example Sodium, Na, has 11 protons, 10 core electrons and 1 valence electron. The effective nuclear charge felt would be 11-10=+1. Mg has 12 protons, 10 core electrons and 2 valence electrons so its effective nuclear charge would be 12-10=+2. So Mg has a greater effective nuclear charge than Na so that is why Mg is smaller than Na.

I hope this helps

 

[MedHopeful234]

Thanks a lot! His vide this help explain but so did your explanation. Appreciate it