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Among the second period elements the actual ionization enthalpies are in the order Li lt B lt Be lt C lt O lt N lt F lt Ne. Explain why (i) Be has higherDelta_(i)H" and " B (ii) O has lower Delta_(i)Hthan N and Fthan N and F ? |
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Answer» Solution :Generally, when we go left to right in periodic table value of ionisation ethalpy increases. (i) `Delta_(i)H "" Be gt Delta_(i)H B `:Be has higher `Delta _(i)H`than B because Be has the electronic configuration `1 s^(2)2s^(2)` while B has the electronic configuration is `1s^(2)2s^(2)p^(1)`. The LARGE `Delta _(i)H` of Be in comparison to B is due to the fact that. The electronic configuration of Be is more stable (completely filled 2s orbitals) than that at B. In Be the electron to be removed during the ionization is an s-electron while the electron to be removed during ionization of B is a p-electron. The penetration of a 2s-electron to the nucleus is more than that of a 2p-electron and hence 2p-electron of B is more SHIELDED from the nucleus by the inner core of electrons than the 2s-electron of Be. As a result, 28-electron attracted to the nucleus more than 2p-electron. THEREFORE, it is difficult to remove a 2s-electron from Be than to remove the 2p electron from B. Thus, Be has higher ionization enthalpy than B. (ii) `DeltaH_(i)(1)O ltDelta_(i)H(1)N` : Oxygen has four electrons in 2p-orbitals and two of the four 2p-electrons must occupy the same 2porbital RESULTING in increased electronelectron repulsion. On the other hand, N has stable half filled configuration, while F has greater nuclear charge. Therefore, O has ionization enthalpy less than N as well of F. `N (Z = 7) [He] 2s^(2) 2p_(x)^(1) 2p_(y)^(1)2p_(z)^(1)` (Half filled subshell) `O (Z = 8) [He] 2s^(2) 2p_(x)^(2) 2p_(y)^(1) 2p_(z)^(1)` (one) 2p) has two electron) In nitrogen, unpair electrons present in outermost orbital. (According to Hund.s law) But, in oxygen having four electrons in 2p orbitals they PAIRED so electron-electron repulsion increases. So, electron of oxygen is easily removed than electron of nitrogen. So, ionisation enthalpy of nitrogen is higher than oxygen. |
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