7.6 Metals, Nonmetals, and Metalloids — Key Concepts for Quick Review

Electron Affinity Trends

• Noble gases have EA > 0; electron affinity is positive for Ar(g) + e^- → Ar^-(g) indicating no stable anion formation for noble gases.
  $\mathrm{Ar(g) + e^- \rightarrow Ar^-(g)}$
• Halogens have the most negative EA; gaining one electron yields a noble-gas configuration.
  $\mathrm{X(g) + e^- \rightarrow X^-(g)}$
• Be and Mg have positive EA because added electrons would enter a previously empty p subshell (higher in energy).
• Group 5A elements have larger electron-electron repulsions due to half-filled p subshells, leading to EA that is either positive or less negative than the neighbors to the left.
• Electron affinities do not change greatly down a group. As the added electron goes into successively higher-energy p subshells (2p, 3p, 4p, …), the decreasing electron-nucleus attraction is offset by reduced electron-electron repulsions, yielding only small overall changes.

Metals, Metalloids, and Nonmetals

• Elements can be broadly categorized as metals, metalloids, and nonmetals (Fig. 7.13).
• Metallic character increases down a group and decreases across a period (red stepped line separates metals from nonmetals).
• Key properties:
  • Metals: shiny luster, malleable, ductile, good conductors; oxides are basic; tend to form cations; low first ionization energy.
  • Nonmetals: lack luster, brittle or soft solids; poor conductors; oxides are molecular and form acidic solutions; tend to form anions or oxyanions.
  • Metalloids: intermediate properties.
• First ionization energy is the best single indicator of metallic vs nonmetal behavior.

Oxidation States and Bonding

• Alkali metals (group 1) form M^+ ions; alkaline earth metals (group 2) form M^{2+}.
• Groups 3A–7A: cations form by losing outer s electrons or outer s and p electrons (e.g., Sn^{2+}, Sn^{4+}).
• Transition metals: can form multiple cations; Fe^{2+} and Fe^{3+} are common examples.
• Compounds of metals with nonmetals are typically ionic solids (e.g., NiO).

Metal Oxides and Reactions

• Metal oxides + water generally form metal hydroxides; oxides are basic.
  • Example:
  • Sodium oxide hydrolyzes: $\mathrm{Na<em>2O(s) + H</em>2O(l) \rightarrow 2\,NaOH(aq)}$
  • Calcium oxide hydrolyzes: $\mathrm{CaO(s) + H<em>2O(l) \rightarrow Ca(OH)</em>2(aq)}$
• Oxide anions react with water to form hydroxide:
  $\mathrm{O^{2-}(aq) + H_2O(l) \rightarrow 2\,OH^-(aq)}$
• NiO is insoluble in water but reacts with acids (e.g., HNO3) to form Ni^{2+} salts: $\mathrm{NiO(s) + 2\,HNO</em>3(aq) \rightarrow Ni(NO<em>3)</em>2(aq) + H_2O(l)}$

Nonmetals

• Nonmetals exist as diatomic molecules (excluding noble gases): H2, N2, O2, F2, Cl2 (gases); Br2 (l); I_2 (s).
• Nonmetal oxides are typically acidic and tend to form molecular substances.
• Nonmetals tend to gain electrons to reach a noble-gas configuration when reacting with metals, forming salts or covalent compounds.
  • Example: Al with Br2 forms ionic AlBr3:
    $\mathrm{2\,Al(s) + 3\,Br<em>2(l) \rightarrow 2\,AlBr</em>3(s)}$
  • Example: Br^- gains an electron to fill its p subshell:
    $\mathrm{Br([Ar]\,3d^{10}4s^23p^5) + e^- \rightarrow Br^-([Ar]\,3d^{10}4s^23p^6)}$
• Most nonmetal oxides dissolve in water to form acids; many dissolve in bases to form salts and water.
  • CO2 + H2O → H2CO3
    $\mathrm{CO<em>2(g) + H</em>2O(l) \rightarrow H<em>2CO</em>3(aq)}$
  • CO2 + 2 NaOH → Na2CO3 + H2O
    $\mathrm{CO<em>2(g) + 2\,NaOH(aq) \rightarrow Na</em>2CO<em>3(aq) + H</em>2O(l)}$

Sample Exercise 7.8 (Properties of Metal Oxides)

• (a) Sc2O3 is an ionic solid at room temperature with a very high melting point: mp = 2485 °C.
• (b) Reaction with nitric acid:
  $\mathrm{Sc<em>2O</em>3(s) + 6\,HNO<em>3(aq) \rightarrow 2\,Sc(NO</em>3)<em>3(aq) + 3\,H</em>2O(l)}$

Quick Takeaways

• Electron affinity trends are less pronounced than ionization energy trends; noble gases resist electron attachment, halogens strongly attract electrons.
• Metallic character rises down a group and falls across a period; first ionization energy is a strong metal/nonmetal indicator.
• Metals form cations and basic oxides; nonmetals form anions or covalent compounds and acidic oxides.
• Metal oxides react with water or acids to form hydroxides or salts; nonmetal oxides react with water to form acids or with bases to form salts.