Coordination compound:

The compounds in which the central atom or group of atoms (example: VO), is surrounded via anions or neutral molecules termed ligands, are called as coordination compounds. In the coordination compounds, the central group acts as Lewis acids. The ligands are electron pair donors and function as the Lewis bases. The coordination compounds are, thus, Lewis adducts. The stream of chemistry under which properties of such compounds are studied is known as the coordination chemistry. There is no sharp line dividing between the covalent, coordination and ionic compounds. The only explanation of categorizing a compound as a coordination compound is that its behavior can be predicted suitably by considering a cationic central species Mⁿ⁺ surrounded through ligands L₁, L₂ and so on (the ligands might be similar or different) the charge on the resultant complex, is found out by the arithmetical summation of the changes on the central ion and the ligands attached to it.

The fundamental features of the coordination compounds were illustrated by the Danish Chemist S. M. Jorgensen (1887-1914) and the Swiss Chemist Alfred Werner (1866-1919). They synthesized thousands of coordination compounds to determine the manner in which the metal salts and the ligands united to make coordination compounds. As Werner was capable to give theory for such compounds which described and correlated a large number of observations, these compounds, are known as the Werner's Complexes.

Werner's Coordination Theory:

Werner's coordination theory is fundamentally very simples. This can be represented in the form of the given postulations:

1) Two kinds of valencies exist for an ion, and they are known as the primary (or ionisable) valences and secondary (or non-ionisable) valences. 

2) The number of secondary valences for an ion is fixed example: six for Rt⁴², Co³², Tl³², Fe³², four for nd²⁷, Rt²⁷, Cu²⁷, Ni²⁷and two for Cu⁺, Ag⁺, Au⁺ and Hg²¹.

3) The secondary valences should be satisfied via the anions or neutrals molecules having lone electron pair (example: halide, cyanide, ammonia, amine, water and so on).

4) In a compound, the secondary valences should be satisfied complexly. After which the primary valences are satisfied via the anions if the complex made is cationic or vice-versa.

5) The secondary valences are fixed in the space and have a definite geometric arrangement even in solution. Therefore for four secondary valences of nickel are tetrahedral, of Cu²⁺ are planar, and the six secondary valences of Co³⁺ or Cr³⁺ are octahedral, for illustration, the Werner's formula, for complex CoCl₃.6NH₃ or [Co(NH₃)₆]₃³⁺Cl^-.

Classification of Coordination Compounds:

In a coordination compound, the ligands work as the Lewis bases, while the central metal ion acts as the Lewis acid. The ligands which coordinate by the metal ion might be categorized as follows:

1) Monodentate ligands: Ligands donating only one pair of electrons to just one metal ion in a coordination compound are known as the Monodentate ligands example: halide ions, ammonia, water and PR₃.

2) Bidentate ligands: Ligands containing two donor atoms. As an outcome of the coordinate bond formation, a bidentate ligand yields in the formation of a ring structure incorporating the metal ions known as the chelate ring. The bidentate ligands might be neutral compounds (deamines, diphosphine, disulphides) or anions such as exalate, carboxylate, nitrate and glycinate ions.

3) Polydentate ligands: These ligands comprise having more than two donor atoms joined in the molecule, and can be known as the tri tetra, penta or hexadentate based on the number of the donor atoms present. 

It is not essential that a Polydentate ligand must for all time utilize all its donor atoms for the coordination purposes. Therefore sulphide or nitrate ion might act as a mono or a bidentate ligand based on the complexes concerned. Via OH^- or NH₂^- act as a Monodentate, they can as well function as bidentate for the bridging aims. 

On the basis of the nature of the coordinates bond formed, a ligand can be categorized as:

1) Ligands having no available π-electrons and no vacant orbitals in such a way that they can coordinate only via the bond, example: H^-, NH₃, SO₃^2- or RNH₂.

2) Ligands having two or three lone pair of electrons which might divide into one pair of lower energy and form a sigma bond, and others might become higher energy bonding electron pairs, example: N^3-, O^2-, F^-, Cl^-, Br^-, I^-, OH^-, S²-, NH^2-, H₂O, RS, RQ, NH₂^- and so on.

3) Ligands encompassing a sigma-bonding pair of electrons and low energy empty π-antibonding orbital, that can accept correctly oriented d orbital electrons from the metals (that is, back bonding), example: CO, R₃P, R₃As, Br^-, I^-, CN^-, Py and acac.

4) Ligands with no unshared lone pair, however having π-bonding electrons example: alkenes, alkynes, benzene, cyolopentadinyl anion.

5) Ligands which can form two sigma bonds by two separate metal atoms and thus can form bridges example: OH^-, cl^-, F^-, NH₂^-, O₂^2-, CO, SO₄^2- and O₂.

Most of the Polydentate can encompass their donor atoms similar or different and, thus, can't be categorized as any one of the kinds illustrated above.

On the basis of formation of complexes by various atoms, person has categorized the ligands and also the metals into hard acid that is metal ions having almost empty or fully filled d sub-shell that can't be employed for the formation of π bond like Group IA, IIA, Al, Ga, In, Sn, Pb and so on and soft bases, which are metals and ligands that form stronger complexes with this class of metals. These metal ions have almost filled d orbitals electrons that can form π-bonds with the ligands and can accept d orbital electrons in their d orbitals example: Cu(I), Hg(II), nd(II), Rt(II), PR₃ and so on. 

The coordination number is the number of ligands joined directly to the central metal ion in the coordination compound. Around 98% of the complexes belong to the coordination number (CN) 4 or 6, even although the coordination number from 2 to 12 are notice in the complexes. The coordination number of 3 and 7 are very rare and 5 is uncommon, present generally for stereo-chemically rigid ligand complexes.

Radioactivity:

Radioactivity is basically the property through which compounds emits radio action that could penetrate objects opaque to light. From scientific investigations, it is now acknowledged that there are elements; however some of them are weakly active.

Features of Radioactivity:

The radioactive substances spontaneously and continually emit radiation. The rate of which they emit radiation is not influenced by variation of ordinary experimental conditions, like temperature, pressure, chemical change, and gravitational, magnetic and electric fields. This radiation influences photographic plates, causes gases to ionize, initiates chemical reaction (that is, polymerisation) and makes some substances (example: crystalline ZnS) fluoreses. Radioactivity is always accompanied through the evolution of large amount energy. Radioactivity as well consists of physiological effects, some of them increasing with time. The fundamental effect of radiation on any living organism is the annihilation of cells.

Types of Radiation:

The crucial nature of radioactivity is the unstable state of the nucleus of the atoms of the radioactive substance. Therefore instability leads to a rearrangement of the nucleus by the discharge of energy in the form of α (alpha) or ₄He or β (beta) particles and γ (gamma) radiation. The nucleus that is formed after this rearrangement will be that of a different element and might be stable or unstable. The whole procedure is known as disintegration or radioactive decay.

 a) Alpha Rays (α-rays, ₂⁴He):

They are positively charged particles being a mass 4 times that of the hydrogen atom and baring two units of charge. They encompass very little penetrating power.

b) Beta Rays (β-rays):

These rays are fast moving streams of electrons. They might be negatively or positively charged. However generally the word β-rays refer to the negatively charged particles. They encompass a very high penetrating power however much less efficient in ionsing gases or matter is.

By beta-decay the mass number is unaltered; however there is a loss of one unit of negative charge example:

²³⁴₉₀Th → ²³⁴₉₁Pa (β^- decay)

¹¹₆C → ¹¹₅β (β⁺ decay)

c) Gamma Rays:

Gamma rays encompass no charge and are not influenced by the electric or magnetic fields. They are electromagnetic rays of the similar type as light or x-rays however encompass very short wavelengths and energies which differ from 0.01 to 3 Mev.

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