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Chemical Bonding: Definition, examples

    What is chemical bond? 

    Answer: The attractive force which holds atoms together in a substance is called chemical bond. 

    Justify that bond distance is the compromise distance between two atoms.

    Answer: During bond formation when two atoms approach each other; attractive and repulsive forces operate simultaneously. The attractive forces tend to bring the two atoms close to each other and potential energy of system is decreased. On the other hand, repulsive forces tend to push the atom apart and potential energy is increased. Finally, at certain distance between two bonded atoms, the force of attraction predominates the repulsive forces. Here the potential energy is minimum and bond is said to be formed. This distance is called bond length or bond distance or compromise distance of two atoms. For example for hydrogen atoms compromise distance is 75.4 pm.

    Draw energy profile for the formation of H2 molecule.

    Answer:
    Figure: Energy profile of formation of  H2 molecule



    What is octet rule?

    Answer: The tendency of atoms to have eight electrons in their valence shell is called octet rule. The most atoms acquire the eight electrons in their valence shells when they form bond.

    Most of the elements of the periodic table attain the electronic configuration of inert gases during bond formation. Justify it. or What is cause of chemical combination of elements with each other?

    Answer: Noble gases do not react with each other due to their stable electronic configuration. However, all other elements react with one other. This is because they have not stable electronic configuration like noble gases. They get their electronic configuration of inert gases by bond formation during which they lose, gain or share the electrons. For example during the formation of NaCl, Na gains electronic configuration of Ne by losing an electron and Cl attain the stable electronic configurations of Ar by gaining an electron. 

    Why the atomic sizes decrease from left to the right in a period? 

    Answer: From left to right in a period, positive charge on the nucleus increases and outermost shell remains the same. The negatively charged electrons are pulled closer to the nucleus. Thus size of outermost shell becomes smaller so atomic size decreases from the left to right in the period. 
    Figure:Decreaing atomic raius actross the period(not to scale)

    Why atomic radii increase down the group? 

    Answer: From top to bottom in a group, the atomic radii increase due to addition of new shell. Although the positive charge on the nucleus increases down the group but its effect is neutralized by a corresponding shielding effect of inner shell electrons.

    Cationic radius is smaller then the parent atom, while the anionic radius is larger than parent atom. why? 

    Answer: When a neutral atom loses one or more electrons, it becomes a positive ion (cation). The radius of cation is smaller than the parent atom due to two reasons.

    1.The removal of one or more electrons from a neutral atom usually results in the lose of the                 outermost     shell. 
    2. The removal of electrons causes an imbalance in proton electron ratio. Due to greater attraction of        nuclear charge, the remaining electrons are drawn closer to the nucleus. For example, the radius of        Na is 157 pm and radius of Na+ is 95 pm.

    The atomic size of an atom cannot be determined precisely. Why?

    Answer: The atomic size of an atom cannot be determined precisely due to following reasons.

    1. There is no sharp boundary of an atom. The probability (chances) of finding an electron never becomes zero even at large distance from the nucleus. 
    2. Electron cloud is affected by neighboring atoms, therefore, size of atom may change from one compound to an other compound.

    Why the anionic radius is larger than parent atom?

    Answer: When a neutral atom gains one or more electrons, it becomes a negative ion. The radius of an anion is larger than the parent due to two reasons.
    An addition of one or more electrons in the shell of a neutral atom enhances repulsion between the electrons which causes the shell expansion. For example, radius of fluorine atom (F) is 72 pm and that of fluoride ion (F-) is 136 pm. 

    What is meant by ionization and ionization energy? What are units of ionization energy? What are factors which affect the ionization energy?

    Answer: The process in which a neutral atom is converted into an ion is called ionization energy. The ionization energy of an element may be defined as ‘’minimum energy required to remove an electron from gaseous atom (isolated atom) to form a cation’’. SI units: Kj/mol
    Example:     Na ----->Na+ e-    H = - 396 Kj/mol
    Factors: The ionization energy depends upon the following factors.

    1. Atomic size of atom (Greater the size of atom, smaller the ionization energy and vice versa). 
    2. Nuclear charge or proton number of an atom(Greater the nuclear charge, higher the ionization energy and vice versa). 
    3. Shielding effect of inner electrons (Greater inner shell electrons, greater the shielding effect and        lower the ionization energy). 
    4. Nature of orbital (for a given shell, decreasing order of ionization energy is s>p>d>f). 


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