just out of curiosity cause its a fact that i accepted but now i wanna find out the explanation behind it
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This may seem like a lame answer, but the question of 'why' is not strictly a scientific one, but a religious one, whose answer is because it pleases Almighty God. The scientific question would be how or by what process does something happen. This is my soapbox platform for better speech in science.
Anyway, If you assume that charge density is like a force field around an ion, you can then think that different ions have larger or smaller force fields depending on the initial size of the nucleus and the number of electrons around it. In polyatomic ions the argument is the same but there are more atoms in the ion and the force field is larger. So if a chloride ion is compared to a much larger nitrate ion both have a -1 charge, the density in the chloride ion would be higher since the charge, force field, is in a smaller area of space.
So, since a given water molecule has a relatively fixed size, the number of them that will fit around an ion has to do with how much space there is. Bigger ions have more space around them than smaller ions, but less force field to hold the water molecules in place.
A large ion, with a low charge density, results in a relatively minor disruption of water-water molecule interactions: therefore the hydration energy is relatively low. Note that hydration energy is the sum of a number of energy changes that take place in breaking the water-water hydrogen bonds and reforming the water-ion polar bonds. In large but low charge density ions only a single layer of structured water molecules can associate with the ion and even these exchange easily with the bulk water, so the hydration energy is relatively low per ion.
A small ion, with a high charge density, results in a relatively major disruption of water-water molecule interactions: therefore the hydration energy is relatively high. In small but high charge density ions, sometimes a multilayer of structured water molecules can associate with the ion which is calculated as a high change in bonds being broken and reformed, meaning higher hydration energy.
Anyway, If you assume that charge density is like a force field around an ion, you can then think that different ions have larger or smaller force fields depending on the initial size of the nucleus and the number of electrons around it. In polyatomic ions the argument is the same but there are more atoms in the ion and the force field is larger. So if a chloride ion is compared to a much larger nitrate ion both have a -1 charge, the density in the chloride ion would be higher since the charge, force field, is in a smaller area of space.
So, since a given water molecule has a relatively fixed size, the number of them that will fit around an ion has to do with how much space there is. Bigger ions have more space around them than smaller ions, but less force field to hold the water molecules in place.
A large ion, with a low charge density, results in a relatively minor disruption of water-water molecule interactions: therefore the hydration energy is relatively low. Note that hydration energy is the sum of a number of energy changes that take place in breaking the water-water hydrogen bonds and reforming the water-ion polar bonds. In large but low charge density ions only a single layer of structured water molecules can associate with the ion and even these exchange easily with the bulk water, so the hydration energy is relatively low per ion.
A small ion, with a high charge density, results in a relatively major disruption of water-water molecule interactions: therefore the hydration energy is relatively high. In small but high charge density ions, sometimes a multilayer of structured water molecules can associate with the ion which is calculated as a high change in bonds being broken and reformed, meaning higher hydration energy.
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the more the charge, the more energy req. for tear off electron