All Students Notes

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.  2. The probability (chances) of finding an electron never becomes zero even at large distance from the nucleus.  3. 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: ...

  Question 6.12:   Why is ionization measured when atom is gaseous state?  Answer:  Ionization energy is measured when an electron is removed from valence shell of an atom. In gaseous atom, the valence electron is under influence of single nucleus. On the other hand, when atom is closer to another atom of the same or different kind, then outer electron is under influence of more than one nucleus. Then it is impossible to measure exact force of attraction between valence electrons and nucleus within an atom.  Question 6.13:  Why the ionization energies decrease down the group although the nuclear charges increase? Answer:  There are two factors which cause decrease in the ionization energies from top to bottom in the periodic table.  Increase in shielding effect.  Addition a new shell in each element down the group.  These two factors decrease the force of attraction between the nucleus and oute...

Question 6.28: Why does a bond pair of electrons on an atom occupy lesser space than a lone pair? Answer: A bonding electron pair is attracted by both nuclei of atoms, while a lone pair (non-bonding electron pair) is attracted by only one nucleus. As lone pair of electrons experiences lesser nuclear attraction, their electronic charges spread out more in space and occupy more space than that for bond pair. Question 2,29: Why does the forces of repulsion between the electrons follow the following order? Lone pair –lone pair > lone pair–bond pair > bond pair –bond pair. Answer: A bonding electron pair is attracted by both nuclei of atoms,  whereas a non-bonding by only one nucleus. As non-bonding electron pair experience lesser nuclear attraction, their electronic charges spread out more in space and occupy more space than that for bond pair. As a result lone pair-lone pair repulsion is greater than lone pair – bond pair repulsion, which is in turn greater than bond pa...

Question 6.19:  Why the electron affinities of II–A elements are less than those of I–A group elements? Answer: This can be explained on the basis of electronic distribution in valence subshell. The II–A group elements have relatively stable electronic configuration due to completely filled s-sub shell. So extra electron has to be accommodated in the higher orbital. So, their electron affinities are lower. On the other hand, I–A group elements have partially filled s-sub shell, so incoming electron can be added to partially filled orbital. Thus, their electron affinities are higher. Question 6.20: First electron affinity for the most of the elements is negative, while the second electron affinity for all elements is positive. Answer: Energy is usually released when elements like halogens absorb an electron and E.A. is expressed in negative figures. However, when a second electron is added to a uni-negative ion, the incoming electron is repelled by already present negative....

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 th...

Contents 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 prof...