CHEMICAL BONDING AND COMPOUND FORMATION One of the most important requirements in chemistry is to become familiar with the ways in which elements react to form compounds.  We will investigate the types of chemical bonds, ways to predict compositions and the conventions of naming the compounds.

The octet rule It is well known that the elements in group 8, the noble gases, are extremely unreactive. Examination of the electronic structures of these elements shows that the outer shells are full of electrons; they do not lack for electrons to fill the shells.  None of the other elements has a filled outer shell.

Chemical bonds make atoms more stable than they are if non-bonded. Bond formation involves changes in  the electrons on two atoms This is achieved by one of two methods Electron transfer Electron sharing

Electron transfer involves creation of ions, which bond via ionic bonds to form ionic compounds.  A Familiar compound like table salt, sodium chloride, is a classic example of an ionic compound. Electron sharing involves the sharing of electrons between two atoms and the creation of covalent bonds.  Covalently bonded compounds typically have very different properties from ionic compounds, and they also involve combinations of different types of elements.

  The inference we draw from this is that the atom wants to obtain a filled shell, and this it achieves by forming bonds.  This can be done by either addition of electrons or removal of electrons. The noble gas atom already has a filled shell and does not need to indulge in bonding to achieve it. 

Elements on the left side of the table, metals, will lose electrons to form positive ions; elements on the right hand side of the table, non-metals, will gain electrons.  In both cases, a filled shell will result. Of course, we must recognize that the atom is now charged because the electron and proton counts are not equal.  Electron loss creates positive ions, and electron gain creates negative ions. In an ionic compound, a positive ion and a negative ion come together and form an ionic bond through the strong electrostatic interaction between the ions of opposing charge.

Predicting ion charges It is essential to be able to predict the charge on an ion in order to predict the composition of compounds formed containing it.  We can use the periodic table to assist us in this.

Factors affecting Ionic Bond Formation The conditions which favour the formation of an ionic bond are Two atoms must be different. Ionization potential of one atom must be small. Electron affinity of second atom must be high. Electro negativity of second atom must be high.

Low ionisation energy the amount of energy, which is required to remove the most loosely bound electron(s) from an isolated gaseous atom to form a positive ion. In forming an ionic bond, one atom must form a cation by losing one or more electrons. In general, elements having low ionisation energies have a more favourable chance to form a cation, thereby having a greater tendency to form ionic bonds. Thus, lower ionization energy of metallic elements favours the formation of an ionic bond. It is because of low ionization energy that the alkali and alkaline earth metals, form ionic compounds.

High electron affinity Electron affinity is the amount of energy released, when an isolated gaseous atom accepts an electron to form a negative ion. The other atom participating in the formation of an ionic compound must form an anion by gaining an electron (s). Higher electron affinity favours the formation of an anion. Therefore, generally, the elements having higher electron affinity favour the formation of an ionic bond. Halogens have high electron affinities, and therefore halogens generally form ionic compounds.

Large lattice energy When a cation, and an anion come closer to each other, they get attracted to each other due to the coulombic force of attraction. These electrostatic forces of attraction between oppositely charged ions release a certain amount of energy (when the ions come closer) and an ionic bond is formed. If the coulombic attractions are stronger, then more energy gets released and a more stable ionic bond is formed. Lattice energy 'is the energy released when one mole of an ionic compound in crystalline form is formed from the constituent ions'. Therefore, larger lattice energy would favour the formation of an ionic bond. Lattice energy thus is a measure of coulombic attraction between the combining ions. The lattice energy of an ionic compound depends directly on the product of the ionic charges, and inversely on the square of the distance between them.

Thus, small ions having higher ionic charge shall have larger lattice energy. Lattice energies of various sodium halides are: The minus sign of lattice energy indicates that the energy is released from ions in the gaseous state, during the formation of solid ionic compound.

An ionic bond is formed through the steps described above An ionic bond is formed through the steps described above. Now, if the total energy released is more than that which is absorbed, then the formation of ionic compound is favoured. The conditions that favour the formation of an ionic bond (or ionic compound) are summarized below:Low ionisation energy of the metallic element, which forms the cation. High electron affinity of the non-metallic element, which forms the anion. Large lattice energy i.e., the smaller size and higher charge of the ions.

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