Chemistry 102 – Chapter 8 Summary and Review Answers 1. What is ionization energy? Show the first and second ionization energy for a sodium atom. Answer: Ionization energy is the energy required to remove an electron from a gaseous atom in the ground state. Na(g) → Na+(g) + 1e– IE2 Na+(g) → Na2+(g) + 1e– IE1 2. What is the periodic trend for metallic character? Using this, what has a greater metallic character, sodium or potassium? Answer: The periodic trend for metallic character is it decreases across a period from left to right and increases down a group from top to bottom. Therefore because sodium and potassium are in the same group, with potassium further down in the group it will have the greatest metallic character. 3. Using an argon atom, give three ions which are isoelectronic to argon. Answer: Argon has 18 electrons with a configuration 1s2 2s2 2p6 3s2 3p6. Cations that are isoelectronic to argon are in period 4 and examples include K+ and Ca2+. Anions that are isoelectronic to argon are in period 3 and examples include S2– and Cl–. 4. What is the periodic trend for electronegativity? Use electronegativities for chlorine, hydrogen and sodium to approximate the type of bond (ionic bonds, polar covalent bonds or nonpolar covalent bonds) in the element chlorine, and compounds hydrogen chloride and sodium chloride. Answer: The trend for electronegativity is increasing across a period from left to right and increasing up a group from bottom to top. As the difference in electronegativity increases, the ionic character of the bond increases. Therefore, chlorine (Cl2) has a difference of zero and contains a nonpolar covalent bond. Hydrogen chloride has a difference of 0.9 and contains a polar covalent bond (or ~20% ionic character per figure 9.7 in your text). Sodium chloride has a difference of 2.1 and contains an ionic bond (or ~75% ionic character – see same figure). 5. Explain ionic and atomic size for an isoelectronic series (you can use the series you gave for the medium exercise above). Answer: For an isoelectronic series, as the atomic number increases, the size decreases. The atomic numbers of the atom (Ar) or ions (K+, Ca2+, S2–, Cl–) are: S2– Cl– Ar K+ Ca2+ Z Size 16 17 18 19 20 2– –
+
S > Cl > Ar > K > Ca2+ 6. For the first ionization plot shown to the right, how many noble gases are shown? How many alkali metals? Explain the two first ionization energies in the circled region. Answer: There are 3 noble gases shown (the highest values) and similarly there are three alkali metals shown (the lowest values). The circled ionization energies correspond to the removal of the electron in Be (which has the configuration of 1s2 2s2) and B (which has the configuration of 1s2 2s2 2p1). Because of the pairing of the two electrons in the 2s sublevel, the first ionization energy of beryllium is higher than that of boron even though boron is further to the right.