Why Does?

Why Does Sodium Form NaCl?

Sodium reacts with chlorine to form Na+ ions and Cl- ions in spite of the fact that the first ionization energy of sodium is larger than the electron affinity of chlorine. To explain this, we need to divide the reaction between sodium and chlorine into a number of hypothetical steps for which we know the amount of energy given off or absorbed.

The starting materials for this reaction are solid sodium metal and chlorine molecules in the gas phase, and the product of the reaction is solid sodium chloride.

2 Na(s) + Cl2(g) ----->2 NaCl(s)

Let's imagine that the reaction takes place by the following sequence of steps.

Na(s) -----> Na(g) deltaHo = 107.3 kJ
Na(g) -----> Na+(g) deltaHo = 495.8 kJ
1/2 Cl2(g) -----> Cl(g) deltaHo = 121.7 kJ
Cl(g) + e- -----> Cl-(g) deltaHo = -348.8 kJ
Na+(g) + Cl-(g) -----> NaCl(s) deltaHo = -787.3 kJ

If we consider just the first four steps in this reaction, Hess's law suggests that it 5.16 takes 376.0 kJ/mol to form Na+ and Cl- ions from sodium metal and chlorine gas.

Na(s) + 1/2 Cl2(g) -----> Na+(g) + Cl-(g) deltaHo = 376.0 kJ/mol

When we include the last step in the calculation, the lattice energy of NaCl is large enough to compensate for all of the steps in this reaction that consume energy, as shown in the figure below.

Na(s) + 1/2 Cl2(g) -----> NaCl(s) deltaHo = -411.3 kJ/mol


The primary driving force behind this reaction is therefore the force of attraction between the Na+ and Cl- ions formed in the reaction, not the affinity of a chlorine atom for electrons.

Why does the reaction stop at NaCl? Why doesn't it keep going to form NaCl2 or NaCl3? The lattice energy would increase as the charge on the sodium atom increased from Na+ to Na2+ or Na3+. But to form an Na2+ ion, we have to remove a second electron from the sodium atom, and the second ionization energy of sodium (4562.4 kJ/mol) is almost 10 times as large as the first ionization energy. The increase in the lattice energy that would result from forming an Na2+ ion can't begin to compensate for the energy needed to break into the filled-shell configuration of the Na+ ion to remove a second electron. The reaction between sodium and chlorine therefore stops at NaCl.

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Why Does Magnesium Form MgCl2?

If the reaction between sodium and chlorine stops at NaCl, why does the reaction between magnesium and chlorine go on to form MgCl2? To answer this question, let's break the reaction into the following steps.

Mg(s) -----> Mg(g) deltaHo = 147.7 kJ
Mg(g) -----> Mg+(g) deltaHo = 737.7 kJ
Mg+(g) -----> Mg2+(g) deltaHo = 1450.6 kJ
Cl2(g) -----> 2 Cl(g) deltaHo = 243.4 kJ
2 Cl(g) + e- -----> 2 Cl-(g) deltaHo = -697.4 kJ
Mg2+(g) + 2 Cl-(g) -----> MgCl2(s) deltaHo = -2526 kJ

The lattice energy for MgCl2 is large enough to compensate for the energy it takes to remove the second electron from a magnesium atom because we don't have to break into a filled-shell configuration to form Mg2+ ions.

Mg(s) + Cl2(g) -----> MgCl2(s) deltaHo = -644 kJ/mol

The reaction stops at MgCl2, however, because it would take an enormous amount of energy to break into the filled-shell configuration of the Mg2+ ion to remove another electron.

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Why Do Semimetals Exist?

Metals have some or all of the following properties.

Nonmetals have the opposite properties.

The differences in the chemical and physical properties of metals and nonmetals can be traced to differences in their electron configurations, their ionization energies, their electron affinities, and the radii of their atoms and ions. As a rule, metals have relatively few electrons in their outermost shell of orbitals, lower ionization energies, smaller electron affinities, and larger atoms than nonmetals.

There are no abrupt changes in the physical properties discussed in this chapter as we go across a row of the periodic table or down a column. As a result, the change from metal to nonmetal must be gradual. Instead of arbitrarily dividing elements into metals and nonmetals, it might be better to describe some elements as being more metallic and other elements as more nonmetallic. Metallic character decreases as we go across a row of the periodic table from left to right, while nonmetallic character increases.

Metallic character decreases

Na Mg Al Si P S Cl Ar

Nonmetallic character increases ----->

For the same reasons, elements toward the top of a column of the periodic table are the most nonmetallic, while elements toward the bottom of a column are the most metallic. This is evident in Group IVA, where a gradual transition is seen from a nonmetallic element (carbon) toward the well-known metals tin and lead.

arrow metallic
C nonmetallic

If elements become less metallic and more nonmetallic as we go across a row of the periodic table from left to right, we should encounter elements along each row that have properties that lie between the extremes of metals and nonmetals. The eight elements in this class (B, Si, Ge, As, Sb, Te, Po, and At) often look metallic, but they are brittle, like nonmetals. They are neither good conductors nor good insulators but serve as the basis for the semiconductor industry. These eight elements are often known as metalloids, or semimetals.

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