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6 Cards in this Set
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. The stability of peroxide and superoxide of alkali metals increases as we go down thegroup. Explain giving reason. |
As we move down the alkali metal group, we observe that stability of peroxide increases.i.e. Li forms Li2O, Na forms peroxides Na2O2 and K, Rb and Cs forms superoxides KO2, RbO2 and CsO2 respectively.This can be explained as follows:The size of lithium ion is very small. As a result, the spread of negative charge towards another oxygen atom is prevented. Hence, Li2O can not combine with excess O2 to form peroxide. The size of potassium ion is larger than sodium atom. As a result, the positive field around potassium ion is comparatively weaker. The large potassium ion stabilises the large superoxide ion through lattice effects. This allows the peroxide ion to combine with another oxygen to form superoxide ion . |
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why does the m.p and b.p decrease in metals as we go down a grup and increase in non metals as we go down a group? |
In case of metals the attractive force of attraction exist between the atoms.On moving down a group, these attractive forces decreases because increasing size. Thus, melting and boiling point decreases.In case of non-metals, the van der Waals forces exist between the atoms. On moving down a group, the van der Waals increases, thus melting and boiling point increases. |
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what are the factors on which lattice enthalpy depends but not the hydration enthalpy so that as we go down the group both of them decreases but hydration enthalpy decreases more?? |
On moving down group of s block elements, the energy needed to break up the lattice falls as the positive ions get bigger because the bigger the ions, ( cation and anion) the more distance there is between their centres, and the weaker the forces holding them together.So, the lattice dissociation enthalpy is governed by the distance between the centres of the ions which is equal to the radius of the large anion plus the radius of the smaller positive ion.But the effect of the change in size of the positive ion is being diluted by the presence of the large anion, so the decrease in lattice enthalpy is less.On the other hand, hydration enthalpy depends on the size of the ion.It decreases with the increase in the size of the cation because bigger ions are not strongly attracted by water molecules.Thus, hydration enthalpy which depends on the size of cations decreases more on moving down the group as compared to lattice enthalpy. |
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Shielding effect |
In multi-electron atoms, the electrons present in the outermost shell do not experience the complete attraction by the nuclear charge because of the inner electrons. In other words, the force of attraction experienced by the valence electrons is less than that experienced by the inner electrons. Thus, the outermost electrons are shielded or screened from the nucleus by the inner electrons. This is known as shielding effect or screening effect.Poor shielding therefore means poor screening of nuclear charge. In other words, the nuclear charge is not effectively screened by electrons in question.- s orbitals have the largest screening effect for a given n value since s electrons are closer to the nucleus.- p orbital's have the next highest screening effect and then comes d and then f orbital's. Thus in simple terms the screening effect decreases ass orbitals > p orbitals> d orbitals> f orbitals. |
