Bhabha was born on October 30th, 1909, in a wealthy Parsi Family. Even as a child he was interested in science. Infact he studied the special theory of relativity in his 16th year. By that time, no body could understand that clearly. Jawaharlal Nehru wanted to make India self sufficient in science and technology and he gave Babha a free hand to do what he wanted. In 1948 the Atomic Energy Commission was set up and Babha made its chairman. He was a man of refined taste and this was very evident whether it was the choice of design for the building of the Tata Institute of Fundamental Research or the site for the Ooty Radio Telescope.
He died in a plane crash at the age of 57. In 1967, the Atomic Energy Establishment, Trombay was renamed the Bhabha Atomic Research Centre as a tribute to his dedication and work.
| KEY WORDS | |
| * Electrons | * Noble gases |
| * Lewis dot structure | * Octet rule |
| * Chemical bond | * Ionic bond |
| * Covalent bond | * Anion |
| * Cation | * Electrostatic force |
| * Electrovalent | * Polar solvent |
| * Non-polar-solvent | * Formation of molecules |
| * Ionic compounds | * Covalant Compounds |
| * Electronegative character | * Electropositive character |
| * Bonded pair of electrons | * Polar bonds |
| * Bond length | * Lone pairs |
| * Shape of the molecule | * Bond energy |
| * Tetra hedral | * Linear |
| * Properties of ionic and covalent compounds | |
KEY WORDS - EXPLANATIONS
Electrons: These are negatively charged particles present outside the nucleus in the orbits of an atom.
Noble gases: Zero group elements are called noble gases. They are He, Ne, Ar, Kr, Xe and Rn.
Lewis dot structure: The valence electrons with the atom of an element is depicted in a short form by Lewis symbol or Lewis dot structure.
Octet rule: The atoms of elements tend to undergo chemical changes that help to leave their atoms with eight outer-shell electrons.
Chemical bond: The force of attraction between any two atoms or a group of atoms that results a stable entity is called a 'Chemical bond'.
Ionic bond: It is formed between atoms of two dissimilar elements due to transfer of electrons from the atom of one element to the other.
Covalent bond: A chemical bond that formed by sharing of valence - shell electrons between the atoms so that both of them can attain octet or duplet in their valence shell is called covalent bond.
Anion: Negatively charged ions are known as anions. Anions are formed by gaining of electron by the neutral atoms.
Cation: Positively charged ions are known as cations. Cations are formed by the loss of electron by neutral atoms.
Electrostatic force: The force of attraction between opposite charges or force of repulsion between the same charges is known is electro static force.
Electrovalent: The valence concept has been explained interms of electrons. It is also called the electrovalent.
Polar Solvent: The solutions of ionic compounds are called polar solvents. They have polar nature because of ions. The solvents are capable of dissolving ionic compounds or covalent compounds that ionise. e.g.: water, liquid, ammonia.
Non - Polar solvent: Solutions having non-polar nature are called non-polar-solvents. Ethene, Benzene are examples of non-polar solvents. These do not dissolve ionic compounds but will dissolve non-polar covalent compounds.
Formation of molecules: Atoms combine to form a molecule by transfer of electrons or sharing electrons.
Ionic compounds: The compounds formed by ionic bond are known ionic compounds.
Covalent compounds: The compounds formed due to sharing of electrons are called covelent compounds.
Electropositive character: The tendency to loose electrons by an element is called electropositive character.
Electronegative character: The tendency of an atom in a compound to attract the shared pair of electrons in a covalent bond towards itself is called as electronegativity.
Polar bonds: Polar covalent bonds mean that covalent bond which is formed by mutual sharing of electron pairs between two dissimilar atoms.
e.g.: H2O
Bonded pair of electrons: Shared electron pairs are called bonded pair of electrons.
Lone pairs: Unshared electron pairs are called lone pairs of electrons.
Bond length: Bond length is the distance between the nuclei of two covalently bonded atoms.
Bond energy: It is the energy needed to break a covalent bond between two atoms of covalent compound to give corresponding atoms in the gas phase.
Shape of molecule: The arrangement of different atoms in a molecule in definite positions gives the shape of the molecule.
Linear: If the angle between the atoms in a molecule is 180º, then its shape becomes linear.
Tetra hedral: If the atoms in a molecule are arranged at the corners of a tetra hedral then the molecule has tetra hedral shape. This will have four faces.
Properties of ionic and covalent compounds: Ionic compounds are crystalline solids, soluble in polar liquids like water. These are good conductors of heat and electricity.
Covalent compounds are amorphous solids, soluble in non-polar solvents like CCl4.
Synopsis
* The discovery of noble gases and the understanding of their electronic configurations helped us in explaining the formation of chemical bonds among the atoms of the elements.
* The valence electrons in the atom of an element is depicted in a short form by Lewis symbol or electron dot structure.
* Octet rule is still a rule not the law, because there are considerable exceptions for this rule.
* Any species with eight electrons in the valence shell is stable.
* Eight electrons in the outer most shell definitely gives stability to the ion or atom.
Octet rule: The atoms of elements tend to undergo chemical changes that help to leave their atoms with eight outer-shell electrons.
* The force of attraction between any two atoms or group of atoms that results a stable entity is called a 'Chemical bond'.
* Kossel proposed the ionic bond (electrostatic bond).
* Ionic bond is formed between atoms of two dissimilar elements due to transfer of electrons from the atom of one element to the other.
* Noble gases except Helium have eight electrons in the valence shells of their atoms. They are chemically inactive and stable.
* To attain eight electrons in the outer most shell similar to noble gases metal atoms that have one, two or three electrons in the valence shells generally lose those electrons and form stable positive ions called cations.
* To attain eight electrons in the outermost shell, non-metal atoms that have 5, 6 or 7 valence electrons gain 3, 2 or 1 electron respectively and form negative ions known as anions.
* The electrostatic attractive force that keeps cation and anion (Which are formed from metal atoms and non-metal atoms due to transfer of electrons) together to form a new electrically neutral compounds is called Ionic bond.
* When Sodium (Na) atom loses one electron to get octet electron configuration, it forms a cation (Na+) and gets electron configuration that of Neon (Ne) atom.
* Chlorine has shortage of one electron to get octet in its valence shell. So it gains the electron from Na atom to form anion and gets electron configuration as that of Argon (Ar).
* Electrostatic forces are non - directional. Therefore it is not possible for one Na+ to be attracted by one Cl- and vice - versa.
* Depending upon the size and the charge of a particular ion, number of oppositely charged ions get attracted by it, but, in a definite number.
* In sodium chloride crystal each Na+ is surrounded by 6 Cl -and each Cl - by six Na+ ions.
* Ionic compounds in the crystalline state consist of orderly arranged cations and anions held together by electrostatic forces of attraction in three dimensions.
* NaCl is said to possess face centred cubic lattice crystal structure.
* The number of ions of opposite charge, that surrounded a given ion of given charge is known as the coordination number of that given ion.
* For example, in sodium chloride crystal, the coordination number of Na+ is 6 and that of Cl - is also 6.
* Ionic bond is formed between atoms of elements with electronegativity difference equal to or greater than 1.9.
* The atoms of elements with low ionisation energy, low electron affinity, high atomic size and low electronegativity form cations.
* The atoms of elements with high ionisation potential, high electron affinity, small atomic size and high electronegativity form anions.
* The electrons shared between two atoms belong to both the atoms and the sharing of electrons between them leads to the formation of a chemical bond known as covalent bond.
* The chemical bond formed between two atoms by mutual sharing of a pair of valence shell electrons so that both of them can attain octet or duplet in their valence shell is called covalent bond.
* Take two fluorine atoms which form a stable molecule.
* Each flourine atom contributes one electron for bonding and the electron pair that is formed in this way is mutually shared by both the fluorine atoms.
* Each atom in F2 molecule has an octet of valence electrons.
* The electronic configuration of 8O is 2, 6. Oxygen atom has six electrons in its valence shell.
* Therefore oxygen atoms come close and each oxygen atom contributes two electrons for bonding.
* Therefore there exist two covalent bonds between two oxygen atoms in O2 molecule as there are two pairs of electrons shared between them.
* We can say that a doublet bond is formed between two oxygen atoms in O2 molecule.
* Similarly a triple bond is formed between two Nitrogen atoms in N2 molecule.
* In CH4, molecule, there are four C - H covalent bonds.
* The number of covalent bonds that an atom of an element forms is called its 'Covalency'.
* Bond length or bond distance is the equilibrium distance between the nuclei of two atoms which form a covalent bond.
* Bond length is generally given in nm (nano meter) or Aº (Angstrom unit).
* Bond energy or Bond dissociation energy is the energy needed to break a covalent bond between two atoms of a diatomic covalent compound in its gaseous state.
* This electronic theory of valence fails to explain the shapes of molecules.
* To explain the bond angles in the molecules V S E P R T (Valence - Shell - Electron - Pair - Repulsion - Theory) was proposed by Sidgwick and Powell (1940). It was further improved by Gillespie and Nyholm (1957).
* VSEPRT theory mainly fails in explaining the strengths of bonds. This theory still depends on the Lewis concept of covalent bond formation. It could not say anything extra about electronic nature of covalent bonds.
* To describe covalent bonding, a quantum mechanical model called valence bond theory was proposed by Linus Pauling (1954).
* The orbitals overlapping are different, the bond strengths of two Be - Cl (in Beryllium Chloride) must be different. But both bonds are of same strength and Cl 
Cl is 180º.
* To explain the discrepancies like this a phenomenon called 'hybridisation of atomic orbitals' was proposed by Linus Pauling (1931).
* Hybridisation is a phenomenon of intermixing of atomic orbitals of almost equal energy which are present in the outer shells of the atom and their reshuffling or redistribution into the same number of orbitals but with equal properties like energy and shape.
* The bond angles are thus explained.
* Valence bond theory already explained about 'σ' bond (stronger) and π (weaker) bonds.
Writer: C.V. Sarveswara Sarma
