moment-g.comical Bonding

Why do moment-g.comical bonds form? In large part, it is to lower the potential energy (PE) ofthe system. Potential energy arises fromthe interaction of positive and negative charges. At an atomic level, positive charges arecarried by protons and negative charges are carried by electrons.The PE can be calculated using Coulomb"s Law, which is theproduct of two charges, Q1 and Q2 dividedby the distance between the charges, d. If the two charges have the same sign (+ class=GramE>,+or -,-) the PE will be a positive number. Like charges repel each other, so positivePE is a destabilizing factor. If the two charges have different signs, the PE will be negative. This indicates an attractive forcebetween the charges and is a stabilizing factor. moment-g.comical bonding leads to a loweringof the PE and formation of more stable moment-g.comical species.

Ionic bonding

Ionic bonds form between metals and non-metals. Metals are the elements on the leftside of the Periodic Table. The mostmetallic elements are Cesium and Francium. Metals tend to lose electrons to attain Noble Gas electron configuration. Groups 1 and 2 (the activemetals) lose 1 and 2 valence electrons, respectively, because of their low Ionizationenergies. Non-metals are limited to the elements in the upperright hand corner of the Periodic Table. The most non-metallic element is fluorine. Non-metals tend to gain electrons toattain Noble Gas configurations. Thehave relatively high Electron affinities and high Ionization energies. Metals tend to lose electrons and non-metals tend to gainelectrons, so in reactions involving these two groups, there is electrontransfer from the metal to the non-metal. The metal is oxidized and the non-metal is reduced. An example of this is the reaction betweenthe metal, sodium, and the non-metal, chlorine. The sodium atom gives up an electron to form the Na+ ion andthe chorine molecule gains electrons to form 2 Cl- ions. The chargeson these anions and cations are stabilized by forming a crystal lattice,in which each of the ions is surrounded by counter ions.
The sodium ions, Na+, are represented by the redspheres, and the chloride ions, Cl-, by the yellow spheres. The formula for the product, NaCl,indicates the ratio of sodium ions to chloride ions. There are no individual molecules ofNaCl.

Covalent Bonding

Covalent bonding takes place between non-metals. There is no transfer of electrons, but a sharingof valence electrons. The non-metals allhave fairly high ionization energies, meaning that it is relatively difficultto remove their valence electrons. Thenon-metals also have relatively high electron affinities, so they tend toattract electrons to themselves. So,they share valence electrons with other non-metals. The shared electrons are held betweenthe two nuclei. The formula of covalentcompounds represents actual numbers of atoms that are bonded to form molecules,like C6H12O6 for glucose. Covalent species exist as individualmolecules.

Metallic Bonding

Metallic bonding exists between metal atoms. Metals have relatively low ionizationenergies (easily removed electrons) but also low electron affinities (verylittle tendency to gain electrons). So,metals will share electrons. However, itis a different sort of bonding than covalent bonding. Metals share valence electrons, but these arenot localized between individual atoms. Instead, they are distributed throughout the metal and are completely delocalized. They are often described as being a"sea" of electrons which flow freely between the atoms. The graphic, below, attempts to showthis. The darker gray spheres are themetal nuclei and core electrons. Thelighter gray areas are the loosely held valence electrons, which areeffectively shared by all of the metal atoms.

Ionic bonding - Lattice Energy

Metals and non-metals interact to form ionic compounds. An example of this is the reaction between Naand Cl2. 2 Na(s) + Cl2(g) → 2 NaCl (s)

The link, below (which sometimes works and sometimes doesn"t) shows this reaction taking place. 2 Na (s) + Cl2 (g) → 2 NaCl (s) It is an extremely exothermic reaction. A great deal of heat is given off, indicating a large decrease in the PE of the system.  The product, NaCl, is much more stable than the reactants, Na and Cl2. This reaction can be broken down into a few steps, to determine the source of this energy. We expect a large negative number as the final answer.

First, the sodium is ionized: Na (g) → Na+ + e-I1 = 494 kJ/molEnergy needs to be added in order to remove the electron. Chlorine is ionized: Cl(g) + e- → Cl-sup> (g)Electron affinity = -349 kJ/mol Energy is given off when chlorine gains an electron. The sum of these two is positive. There must be another step involved. That step involves assembling the ions into a crystal lattice, so it is called the Lattice Energy. For NaCl, this equals class=GramE>-787 kJ/mol.

This represents the strong attraction between the anions (Cl-)and anions (Na+) held in close proximity. The interaction is coulombic, proportional to the size and sign of the charges, and inversely proportional to the distance between them.

Lewis electron-dot symbols

The moment-g.comist, G.N. Lewis, devised a simple way to account for the valence electrons when atoms form bonds. Lewis electron-dot symbols represent the valence electrons on each atom. The element symbol itself, represents the nucleus and core electrons and each "dot" represents a valence electron.These are shown below:
With the metals, (to the left of the red line) the totalnumber of dots represent electrons that the element can lose in order toform a cation. In the non-metals (to theright of the red line) the number of unpaired dot
represents the numberof electrons that can become paired, through the gain or sharing ofelectrons. So, the number of unpaireddots equals either the negative charge on the anion that forms,from electron transfer with a metal, or the number of covalent bondsthat the element can form by sharing electrons with other non-metals. Mg, with two dots, tends to form the Mg2+ion. Carbon, with 4 unpaired dots, canform the carbide ion, C4-, when reacting with metals, or can formfour bonds when reacting with non-metals. The reaction between Na and Cl2 can be written interms of their Lewis electron dot structures.2 Na (s) + Cl2(g) → 2 NaCl (s)
Chlorine gains one valence electron to form Cl-and sodium loses one electron to form Na+. Both now have Noble gas electronconfigurations.

Ionic radii

When atoms lose electrons
to form cations, theionic radius is always smaller than the atomic radius. There are fewer electrons, with an unchangednuclear charge, Z. This means that theremaining electrons will be held more strongly and more closely to thenucleus. When atoms gain electronsto forms anions, the ionic radius is always larger than theatomic radius. With more electrons, the electron/electron repulsion term is larger, destabilizing the atom and leaving the electrons farther from the nucleus. Shown below is a chart ofionic radii.
Elemental sodium is larger than elemental chlorine. However, when they are ionized, theirrelative sizes reverse. It is verydifficult to predict absolute sizes. Relative sizes can be predicted for isoelectronicseries, species which have the same number of electrons. For example O2- and F-both have 10 electrons. The nuclearcharge on oxygen is +8 and the nuclear charge on fluorine is +9. The positive charges increase, but thenegative charges stay the same (-10). So, F- will be smaller due to the increased attraction(+9/-10 versus +8/-10). The series of In3+, Sn4+and Sb5+ show the same trend. They all have 46e-, but have nuclear charges of +49, +50 and+51, respectively.

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Sb5+ is thesmallest of the three.