The idea that a shared electron pair constitutes a covalent bond ignores any difficulty about the actual position and nature of the electrons in the combining atoms or in the resulting molecule. The idea that electrons are particles revolving in orbits or situated in ‘shells’ is inadequate when we desire to picture electrons in covalent bonds. It is, however, known that a beam of electrons can undergo diffraction, and that they therefore possess a wave-like nature like light waves. It has also been found that there is a simple relationship between the momentum of an electron (characteristic of its particle-nature) and the wavelength (characteristic of its wave-nature). But if we give a definite wavelength of amplitude to an electron, then its position in space becomes uncertain, i.e. it cannot be pin-pointed. Instead, the wave amplitude (strictly, the square of the amplitude) can be used to represent the probability of finding the electron at a given point in an atom or molecule. This amplitude is usually given the symbol ψ (psi) and is called a wave function. For hydrogen (or helium), with one (or two) electron in the K ‘shell’, ψ is found to depend only on the distance from the nucleus, diminishing as this distance increases.

The intensity of shading at any point represents the magnitude of ψ², i.e. the probability of finding the electron at that point. This may also be called a spherical ‘charge cloud’. In helium, with two electrons, the picture is same, but the two electrons must have opposite signs. (An electron can be imagined as spinning in one direction or another). These two electrons in helium are in a definite energy level and are said to occupy an orbital (derived from the earlier word ‘orbit’) in this case an atomic orbital. Now the combination of two hydrogen atoms to give a hydrogen molecule can be visualized.

When we look at elements with more electrons than helium, we find that if there are, say eight electrons in the outer shell, these can be placed in four orbitals, each containing two electrons of opposite spin; similarly, if there are eighteen, there are nine orbitals and so on.

Consider phosphorus, with five valency electrons; these can be placed either in four tetrahedral sp³ hybrid orbitals (with one orbital doubly occupied) or singly in five orbitals formed by hybridization of one 3s, and three 3p and one 3d (sp³ d) is found in the phosphine molecule PH₃, while sp³ d is found in the phosphorus pentafluoride molecule PF₅. Similarly with sulphur, sp³, mixing with two lone pairs is found in the H₂S molecule while sp³ d² mixing gives six octahedral orbitals as found in the SF6 molecule. It will now become apparent that all the common molecular shapes can be accounted for by assuming appropriate hybridization of the orbitals of the central atom – sp, linear, sp², trigonal planar, sp³, tetrahedral, sp³ d, trigonal bipyramidal and sp³ d², octahedral.

When the transition metals are reached, the first series being in Period 4, another aspect of valency behaviour becomes important. The ions of these metals, like the other metal cations, are hydrated in solution but the water molecules around transition metal cations can very readily be replaced by other molecules or ions (called ligands), when these are present in the solution. Thus addition of ammonia to a solution of a copper (II) salt forms the complex ion [Cu(NH₃)₄(H₂O)₂]²⁺, and addition of potassium cyanide, KCN, to a solution of a ferric salt forms the ion [Fe(CN)₆]^3-, and so on for many transition metal cations and ligands. These complex ions often possess great stability; also if each ligand is considered to form a coordinate link with the metal ion (i.e. to denote an electron pair), then it is often the case that the total number of electrons denoted by the ligands brings the configuration of the metal ion to a value equal to that of the next noble gas.

Notice that definite shapes are assumed by the complex ions, and by the nickel tetracarbonyl complex molecule, where a nickel atom receives electrons from four carbon monoxide molecules as ligands.

Services:- Covalent Bond Modern Theory Homework | Covalent Bond Modern Theory Homework Help | Covalent Bond Modern Theory Homework Help Services | Live Covalent Bond Modern Theory Homework Help | Covalent Bond Modern Theory Homework Tutors | Online Covalent Bond Modern Theory Homework Help | Covalent Bond Modern Theory Tutors | Online Covalent Bond Modern Theory Tutors | Covalent Bond Modern Theory Homework Services | Covalent Bond Modern Theory