AP Exam Questions

Explain each of the following observations using principles of atomic structure and/or bonding

a) Potassium has a lower first-ionization energy than lithium

b) The ionic radius of N^3– is larger than that of O^2–.

c) A calcium atom is larger than a zinc atom.

d) Boron has a lower first-ionization energy than beryllium.

Explain each of the following observations using principles of atomic structure and/or bonding

a) Potassium has a lower first-ionization energy than lithium

K (1s²)(2s²,2p⁶)(3s²,3p⁶) (3d⁰) (4s¹) Z^* = 19 – (10 · 1.00 + 8 · 0.85) = 2.2

Li (1s²)(2s¹) Z^* = 3 - (2 · 0.35) = 2.3

Bassed on these calculations of effective nuclear charge experienced by the 4s electron in potassium and the 2s electron in lithium there is not a significant difference in their effective nuclear charges. So we should not base an explanation on the difference in effective nuclear charge. The primary reason is related to the energy of the s electron in the n = 4 level in potassium and the electron in the n = 2 level in lithium. The electron in the n = 4 level is higher in energy compared to an electron in n = 2 level and therefore less energy is required to remove the 4s electron in poatassium compared to the 2s electron in lithium.

b) The ionic radius of N^3– is larger than that of O^2–.

N^3– (1s²)(2s²,2p⁶) Z^* = 7 – (2 · 0.85 + 7 · 0.35) = 2.85

O^2– (1s²)(2s²,2p⁶) Z^* = 8 – (2 · 0.85 + 7 · 0.35) = 3.85

The electrons in N^3– experience a smaller effective nuclear charge and are less bound (greater electron-electron repulsions) to the nucleus compared to the electrons in oxygen. The result is the electrons in nitrogen will take up a occupy volume.

c) A calcium atom is larger than a zinc atom.

Ca (1s²)(2s²,2p⁶)(3s²,3p⁶) (3d⁰) (4s²) Z^* = 20 – (10 · 1.00 + 8 · 0.85 + 1 · 0.35) = 2.85

Zn (1s²)(2s²,2p⁶)(3s²,3p⁶) (3d¹⁰) (4s²) Z^* = 30 – (10 · 1.00 + 18 · 0.85 + 1 · 0.35) = 4.35

The electrons in the 4s level experience a greater effective nuclear charge in Zn than in Ca. The electrons in Zn are therefore attracted to the nucleus to a larger extent than the electrons in calcium. So the atomic radius is smaller. In general the Z^* increases more slowly across a d block compared to a p block. The reason is the electrons being added in the d block are inside the ns electrons, while for the p block the electrons are not inside the electrons. This would not be asked on the AP Chemistry exam.

d) Boron has a lower first-ionization energy than beryllium.

Be (1s²)(2s²) Z^* = 4 – (2 · 0.85 + 1 · 0.35) = 1.95

B (1s²)(2s²,2p¹) Z^* = 8 – (2 · 0.85 + 2 · 0.35) = 2.6

We would expect from Z^* arguments that it should require more energy to remove the first electron in boron compared to beryllium. However, the electron that is removed from boron is in the 2p level which is higher in energy compared to the 2s level, therefore it requires less energy to remove a 2p electron in boron compared to the 2s electron in beryllium.

Question #2

The diagram shows the first ionization energies for the elements Li to Ne. Briefly, explain each of the following in terms of atomic structure.

a) In general, there is an increase in the first ionization energy from Li to Ne.

b) The first ionization energy of B is lower than that of Be.

c) The first ionization energy of O is lower than that of N.

d) Predict how the first ionization energy of Na compares to those of Li and of Ne. Explain.

Question #3

Account for each of the following in terms of principles of atomic structure.

a) The second ionization energy of sodium is about three times greater than the second ionization energy of magnesium.

 

16. Which of the following are reasonable values for the first four ionization energies for Al?

9. Which of the following are reasonable values for the first four ionization energies for Mg?

	1st	2nd	3rd	4th
A)	496 kJ	4562 kJ	6912 kJ	9543 kJ
B)	578 kJ	1817 kJ	2744 kJ	11,577 kJ
C)	738 kJ	1451 kJ	7733 kJ	10,540 kJ
D)	657 kJ	1269 kJ	2136 kJ	2752 kJ