Introduction
There are more than hundred elements around us. But, we do not find these elements in their native form. This is because most of these elements are highly reactive. They interact with each other to form simple or complex substances. Only elements with the noble gas configuration have no tendency to react.
Chemical Combination
The force which holds together the ions, atoms and molecules in a chemical substance is called as chemical bond. It is the same force which makes substances combine with each other in many different ways to give newer products.
Types of Chemical Bonding
The type of chemical bond developed between the two combining atoms depends upon the way these atoms acquire a stable noble gas configuration.
Ionic (or electrovalent) Bond
When a complete transfer of one or more valence electrons from the atom of a metal to that of a non-metal takes place, an ionic (or electrovalent) bond is formed. As a result of this electron transfer following changes occur in the reacting atoms.
Factors Influencing the Formation of an Ionic Bond
Low ionisation energy
Ionisation energy is 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.
General Properties of Ionic Compounds
Physical state
Ionic compounds do not exist in the form of independent molecules but exist in the form of ions in crystalline solids, according to X-ray studies. These ions are arranged to form the crystal lattice in definite geometric pattern. For example, in NaCl crystal, each Na+ ion is surrounded by six Cl- ions and each Cl- ion is surrounded by six Na+ ions. The geometric arrangement of different ionic compounds differ, depending upon the size of the ions and the magnitude of the charges on the ions.
Born Haber Cycle
Based on Hess's law Born and Haber in 1919 developed a simplified and cyclic method to correlate lattice energies of ionic crystals to other thermodynamic data. For example, the net energy changes during the formation of sodium chloride from metallic sodium and chlorine gas can be represented by DHfยท
Covalent Bond
A covalent bond is defined 'as the force of attraction arising due to mutual sharing of electrons between the two atoms'. The combining atoms may share one, two or three pairs of electrons. The covalent bond is formed between two similar or dissimilar atoms by a mutual sharing of electrons, which are counted towards the stability of both the participating atoms.
General Properties of Covalent Compounds
State of existence
The covalent compounds do not exist as ions but exist as molecules. There are weak intermolecular forces between the molecules and hence they exist as liquids or gases at room temperature. However, a few compounds also exist in the solid state e.g. urea, sugar etc.
Multiple Covalent Bonds
Double covalent bond
The bond formed between two atoms due to the sharing of two electron-pairs is called a double covalent bond or simply a double bond. It is denoted by two small horizontal lines (=) drawn between the two atoms, e.g., O = O, O = C = O etc.
Formation of Molecules having Double Bond
Formation of oxygen (O2) molecule
Each oxygen atom has six electrons in its valence shell. Thus, it requires 2 more electrons to achieve the nearest noble gas configuration. This is achieved by sharing two pairs of electrons by the two oxygen atoms as shown below:
Comparison Between Single, Double and Triple Covalent Bonds
Single bond is formed by the sharing of one electron pair, (two electrons); a double bond is formed by the sharing of two electron pairs, (four electrons), whereas a triple bond involves sharing of three electron pairs, (six electrons).
Factors Favouring the Formation of a Covalent Bond
High ionisation energy
Elements that have higher ionisation energies cannot form ionic bonds, as they cannot lose electrons easily. Instead, they acquire stability by mutual sharing of electrons, and therefore form covalent bonds.
Coordinate Covalent Bond or Dative Bond
When the shared electron pair is provided by one of the combining atoms, a coordinate bond is formed. The atom, which provides the electron pair is termed as the donor atom, while the other atom, which accepts it, is termed as the acceptor atom. Such a bond is also known as dative bond. An arrow (g) pointing from donor towards the acceptor atom represents a coordinate bond. When a one sided sharing of electrons takes place, the coordinate bond so formed cannot be distinguished from a normal covalent bond.
Lewis Structures of Some Covalent Compounds
The electronic configuration of atom 'D' has seven electrons in the outermost shell. Each 'D' atom should share a pair of electrons with another 'D' atoms to form a substance containing only 'D' .i.e.,
Hence, the formula of the substance is D2. It is a covalent compound.
Exceptions to the Octet Rule
Where duplet is formed?
A hydrogen atom has only one electron in its valence shell. It needs one more electron to fill its valence shell. The completed shell has the electronic arrangement of the noble gas helium, yet an octet is not completed. However we still get a stable molecule.
Resonance
The phenomenon of resonance was put forward by Heisenberg to explain the properties of certain covalent molecules. Most covalent molecules have a unique Lewis formula (dot structure), which explains the bonding in that molecule. Some molecules lend themselves two or more dot structures. None of these structures can exactly describe all the properties of the molecule, but each structure contributes to the true structure of the molecule. Such different structures are called 'resonance structures' and the actual molecule is termed as the 'resonance hybrid' of all these structures. In every case the actual molecule is found to be energetically more stable than would be expected from any of its resonance structures.The energy difference between the most stable resonating structure and the resonance hybrid structure is termed as resonance energy.
Shapes of Molecules
The shape of a molecule affects its chemical and physical properties. For instance, some of the unique properties of water are due to the angular shape of its molecule (H2O). The stability and the self-replicating property of DNA is due to its double-strand structure. This shows that the geometry of a molecule affects its properties significantly.
Valence Shell Electron Pair Repulsion (VSEPR) Theory
The VSEPR theory, proposed by R.J.Gillespie and R.S. Nyholmm in 1957, is based on the repulsions between the electron-pairs in the valence-shell of the atoms in the molecule. It was developed to predict the shapes of the molecules in which the atoms are bonded together with single bonds only.
Quantum Theory of Covalent Bond
Although the Lewis structures give an idea of the valence electrons around the atom, in reality it does not precisely locate the position of the electrons in the chemical bond. The behaviour of electrons in molecules and compounds is made by the valence bond theory in terms of the quantum mechanical model.
Directional Property of the Covalent Bond
The sharing of orbitals between two atoms is possible only when the atoms are close enough to overlap their orbitals. Thus, orbital overlap is necessary for the sharing of electrons and bond formation. With the orbitals coming close together, there is an increase in the electron density between the two nuclei. This lowers the repulsion between the two nuclei and the two nuclei stay together to form a bond.
Types of Covalent Bonds
Sigma Bond (s)
When the overlap of orbitals of two atoms takes place along the line joining the two nuclei (orbital axis) then the covalent bond formed is called sigma (s) bond. These bonds can be formed due to 's-s', 's-p' or 'p-p' overlap along the orbital axis. Free rotation around a sigma bond is always possible.
Orbital Representation of Covalent Bonds
Covalent bonds are formed by the overlap of atomic orbitals. As orbitals have different shapes, it is difficult to represent the actual shape of the orbitals on the paper.
Representation of Bonding in Some Molecules
Hydrogen molecule
The bonding between hydrogen atoms to form a molecule of hydrogen can be represented as follows:
In this representation, half-filled '1s' orbital of one hydrogen atom overlaps (shown by 'dashed' lines) with that of the other having electron with opposite spin. The 's-s' overlap leads to a sigma bond formation.
Bonding Parameters
Covalent bonds are characterized by certain parameters such as bond length, bond angle and bond energy.
Polarity in Covalent Bonds
Non-polar covalent bond
When a covalent bond is formed between two atoms of the same element, the shared electron pair will lie exactly midway between the two atoms i.e. the electrons are equally shared by the atoms. The resulting molecule will be electrically symmetrical i.e., centre of the negative charge coincides with the centre of the positive charge. This type of covalent bond is described as a non-polar covalent bond. The bonds in the molecules H2, O2, Cl2 etc., are non-polar covalent bonds.
Dipole Moment - A Measure of Degree of Polarity
Molecules having two equal and opposite charges separated by certain distance are said to possess an electric dipole. In the case of such polar molecules, the centre of negative charge does not coincide with the centre of positive charge. The extent of polarity in such covalent molecules can be described by the term Dipole moment.
Hybridization
Hybridisation is the phenomenon of redistribution of energies of the orbitals of slightly different energies so as to give a new set of orbitals of equivalent energies. The new orbitals are called hybrid or hybridised orbitals. The number of hybridized orbitals formed is equal to the number of atomic orbitals taking part in hybridisation.This phenomenon is more predominant in carbon containing compounds and so to understand this concept, a study of the electronic structure of carbon is essential.
Types of Hybridization in Carbon Compounds
The most popular hybridisation in carbon containing compounds are sp, sp2 and sp3.
Hybridization in Beryllium and Boron compounds
Beryllium (Be) and Boron (B) form covalent compounds because of their high ionization energies and this can be explained on the basis of hybridisation.
Bonding in Solid State (Additional)
Solids are closely packed condensed states, with their constituent particles (atoms, ions or molecules) held together by strong physical or chemical bonds. These particles show a definite regular arrangement in a long range order. Solids may be broadly categorised into four types, depending upon the nature of bonds present in them.
Bonding in Ionic Solids
In ionic solids, the constituent particles are ions. Coulombic forces of attraction hold oppositely charged ions together, to form ionic bonds. Each ion is surrounded by a certain fixed number of the ions of the opposite charge (as its nearest neighbour) to attain the maximum stability and this regular arrangement extends continuously throughout the crystal.
Bonding in Covalent Solids
In covalent solids, the constituent particles are atoms of same or different kinds. Covalent bonds network, forming a continuous giant interlocking structure of atoms throughout the crystal that hold them together. They are also referred to as covalent network solids. Because, covalent bonds are strong and directional, these solids are very hard. Diamond, silicon carbide and graphite are the three important examples of covalent solids.
Bonding in Molecular Solids
Dipole-dipole interactions
These types of forces occur in molecular solids which have polar molecules. A polar molecule has separate centres of positive and negative charges. These forces arise due to electrostatic attraction of the other molecule and vice-versa (The positive end of one molecule attracts the negative end of the other)
Bonding in Metallic Solids
More than 80 elements in the periodic table are metals. Metals are solids at ordinary temperature and pressure, with the exception (of mercury and gallium).
Hydrogen Bonding
The electrostatic force of attraction between the hydrogen atom of one molecule and more electronegative atom of the same or another molecule, is called hydrogen bond. When hydrogen atom gets bonded to atoms of highly electronegative elements such as fluorine, oxygen or nitrogen,the formed molecule is polar.Hence the hydrogen atom forms a weak bond with the electronegative atom of the other molecule. This weak bond is called 'hydrogen bond'.
Typical Hydrogen Bonded Compounds
Hydrogen Fluoride (HF)
In the solid state, hydrogen fluoride consists of long zig-zag chains of molecules associated together through hydrogen bonds. Hydrogen fluoride molecule is represented as (HF)n. With fluorine, the hydrogen bond is so strong that HF molecules remain associated through H-bonding even through the gaseous state. In the liquid or gaseous state, the chain however, becomes linear.
Types of Hydrogen Bonds
Hydrogen bonds can be classified into two types; Intermolecular hydrogen bond and Intramolecular hydrogen bond.
Effects of Hydrogen Bonding on the Properties
Association
The weak electrostatic interactions of intermolecular hydrogen bonding, causes two or more molecules of a compound to exist as aggregates or associated molecules. This is seen in water molecules, which undergo molecular association. Alcohols and carboxylic acids (RCOOH) also exist as a group of molecules. Molecular association results in the increase in the size as well as in the molecular mass of the compound.
Hydrogen Bonding in Water and Ice
It is a common observation that density of ice is less than that of water because of which ice always floats over water. Moreover, it is a known fact that density of water is maximum at 4oC.
