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Chemical Bonding

Question 6.7: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.

Question 6.8: 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.

Question 6.9: 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.

Question 6.10: 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. 

Question 6.11: 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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