AP Exam Help?

Question

Use principles of atomic structure and/or chemical bonding to answer each of the following.

(a) The radius of the Ca atom is 0.197 nanometer; the radius of the Ca2+ ion is 0.099 nanometer. Account for this difference.

(b) The lattice energy of CaO(s) is -3,460 kilojoules per mole; the lattice energy for K2O(s) is -2,240 kilojoules per mole. Account for this difference.


Ionization Energy (kJ/mol)
First Second
K 419 3,050
Ca 590 1,140



(c) Explain the difference between Ca and K in regard to

a. their first ionization energies,

b. their second ionization energies.

(d) The first ionization energy of Mg is 738 kilojoules per mole and that of Al is 578 kilojoules per mole. Account for this difference.

Answer 1

This is a good question to learn up on core charge effects and periodic trends of materials. Make sure you revise that part if you don't have it down pat because it sure will come up.

(a) Ca atom has electronic configuration 1s2 1p6 2s2 2p6 3s2 3p6 4s2.

Ca2+ atom has all of before except 4s2. As the 4s shell is the furthest away, losing those 2 lonely electrons reduce the atomic size, as the furthest shell now is only the 3p shell.

(b) Ca = O has double bonds, K2O only single bonds. Double bonds are stronger as distance between elements are smaller. There might be another explaination I've missed.

(c) K has higher 1st ionisation energy than Ca because it only has 1 outermost electron, so it's more easily lost. However, 2nd ionisation energy is much higher because it means one of the 3p electrons has to go. As there are 8 electrons in the outermost shell of the K+ ion, it is extremely reluctant to give up one electron. In contrast, it's much easier for Ca+ because it's only got 1 left. It wants to avoid that lone electron like the plague.

(d) This one's interesting. Usually we expect Al to have higher 1st ioniz. energy because of the same reasons as above. Perhaps having 3 electrons and being on the diagonal staircase means that Al already has some metalloid (semiconducting) type characteristics and does not form cations as readily.

Answer 2

a) The radius of the neutral atom is always smaller than that of its cation. This is because in the cation the number of protons is higher than that of electrons. In case of Ca+2, 20p vs. 18 e-s. These extra protons exert additional attraction on the electron cloud causing a smaller radius.
b)The lattice energy is a function of several factors. In this case, the cationic charge and the geometry of the crystal. The cationic charge of Ca is +2 while that of K is +1. The CaO crsytallizes in a different lattice geometry leading to a different madelung constant.
c)a.The first ionization potential(energy) of K is smaller than that of Ca2+, because of the electronic configuration of K: 1s2 2s2 2p6 3s2 3p6 4s1. Lossing the electron in the last sheild would lead to a stable noble gas configuration. Losing the second electron would be very difficult for two reasons; the noble gas configuration and the difficulty of removing an electron from a positive ion.
For Ca, losing the first electron is easier, and losing the second electron would leave an ion with a noble gas configuration; i.e. expected to be easier than in K.
d)Same electronic configuartion criteria is applied here. The valance electron in Al is in a 3p orbital, and losing this electron would leave an ion with a filled 2s orbital. Removing the electron from the filled orbital is harder than removing it from a semi-filled orbital. Hence, first ionization energy of Mg is higher than that of Al, as the values indicated.

Answer 3

i'm no longer taking any AP checks this year yet i have taken all those in previous years. I took AP Euro my sophomore year and it grow to be the first AP exam I took. i grow to be soo anxious! the subsequent year I took the different 2 and that i grow to be only dreading being in that room for 4 hours! AP Euro compared to APUSH is a lot a lot less complicated. AP Euro is more suitable huge and APUSH is like countless little element. AP Lang&Comp.. ugh. I did o.k. on the MC yet poor on 2 of the three essays. good success!