KEY Standard 8 Assessment 2011
1. Predict which element Mg or Cl has a larger atomic radius and justify your reasoning.
Mg has a larger atomic radius than Cl. Both elements have valence e- in the same shell, so the reason for the difference is
due to the difference in effective nuclear charge (p+ in the nucleus). Mg has an effective nuclear charge of 2 and Cl has an
effective nuclear charge of 7 accounting for the difference in atomic radii. The greater the effective nuclear charge the
stronger the attraction for its electrons and thus the smaller the atomic radius.
2. Place the following isoelectronic series in order of increasing atomic/ionic radius and justify your reasoning:
Ca2+, S2-, Ar, K+
Ca2+< K+<Ar<S2- ; all of these have the same # of electrons and thus the same e- config. The reason for the trend of
increasing radii is due to the differences in protons in the nucleus. The greater the number of protons, the stronger the
attraction for its electrons, and smaller the atomic radius. Calcium ion has 20 protons, Potassium ion has 19 protons, Ar has
18, and Sulfide has 16 which accounts for the small to large radii trend listed above.
3. Justify that the first ionization energy of Be is larger than Ba.
Beryllium’s valence e- are in the 2nd shell, where Barium’s valence e- are in the 6th shell. The difference in ionization
energies are due to the differences in the sizes of the valence shells, and resulting attraction between the e- and the nucleus.
The attraction of the e- to the nucleus increases with smaller distances, requiring more energy to remove an e-. Since Be has
its valence e- closer to the nucleus more energy is required to remove an e- accounting for the larger ionization energy than
that of Ba.
Questions 4-8 refer to the valence electron configurations of elements; choose the letter (A-E) that best represents the
relationship.
(A) 2s2 2p5
(B) 5s2 4d10 5p5
(C) 4s1 3d10
(D)3s2
(E) 3s2 3p6
1. Has the lowest electronegativity briefly justify your reasoning
(E) since this is the valence e- config of Ar. Noble gases have an electronegativity of 0 due having a complete shell of
electrons and no attraction for additional electrons. Plus in general they do not form chemical bonds.
2. Has the largest exergonic electron affinity briefly justify your reasoning
(A) since this is the valence e- config of F. Flourine has the smallest atomic radius and largest effective nuclear charge of all
the elements listed and thus the greatest attraction for adding an additional e-. This results in the large exergonic change
when an e- is added, resulting in a stable species with a filled shell of e-.
3. Has the largest third ionization energy briefly justify your reasoning
(D)since this is the valence e- config. of Mg. When Mg2+ Mg3+ + e- the result is an e- is taken from a complete shell of e, requiring a very large amount of energy. This would be larger than that of (E) since has a greater effective nuclear charge.
4. Would behave chemically most like Silver and forms blue and green compounds. briefly justify your reasoning
(C) since this is the valence e- config of Cu. Copper forms blue compounds (ex. CuSO4, Cu(NO3)2) and copper oxide is green
in color (ex. Statue of liberty, or oxidized penny). Copper is also in the same vertical column as Ag accounting for the similar
low reactive metal properties as silver.
5. Which two of the elements listed above are from the same group? Identify the group and list three unique properties of
that group.
(A)&(B) since both exist in the same vertical column on the periodic table (F and I). They are halogens and have unique
properties of being diatomic as elements, having variable states g, l, s, form -1 oxidation states, and high exergonic e- affinity.