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    Why does the 4s orbital have a lower energy level than the 3d orbital ?
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    Because of overlapping of orbital yo


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    (Original post by Genericusername1)
    Because of overlapping of orbital yo


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    Thats a vague answer
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    (Original post by Lufthansa)
    Why does the 4s orbital have a lower energy level than the 3d orbital ?

    Warning this explanation contains undergrad level chemistry. Try to follow and you will impress your teacher loads!


    It's because of the effect of shielding....

    Now in a hydrogenic atom (only one electron), the orbitals with the same quantum number n (i.e. all the orbitals in the nth shell, e.g. the s,p,d etc) are all the same energy. So in-fact the 3d would be lower in energy than the 4s but this is where shielding steps in.
    Shielding raises the energy of all the orbitals but to differing extents. The 4s is less affected than the 3d because it is what we can more 'penetrating'. If you go look up the 'radial distribution function' (this is a probability density map as a function of radial distance away from the nucleus) for the orbitals you will see that a greater proportion of the 4s is located near the nucleus so it still experiences the full nuclear charge unlike the 3d which is well shielded. This accounts for why the 3d is higher in energy than the 4s for K and Ca.
    However the situation is different for the transition elements. By Sc, the increase in nuclear charge means that the 3d is now actually lower in energy than the 4s but we still see occupancy of the 4s...... This is because electrons in the 3d repel each other much more strongly than those in the 4s as the 4s is more diffusely distributed around the atom. So despite the absolute energy of the 3d being lower, the lowest energy configuration is to place 2 electrons into the 4s (filling it) and then adding electrons into the 3d.
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    (Original post by JMaydom)
    Warning this explanation contains undergrad level chemistry. Try to follow and you will impress your teacher loads!


    It's because of the effect of shielding....

    Now in a hydrogenic atom (only one electron), the orbitals with the same quantum number n (i.e. all the orbitals in the nth shell, e.g. the s,p,d etc) are all the same energy. So in-fact the 3d would be lower in energy than the 4s but this is where shielding steps in.
    Shielding raises the energy of all the orbitals but to differing extents. The 4s is less affected than the 3d because it is what we can more 'penetrating'. If you go look up the 'radial distribution function' (this is a probability density map as a function of radial distance away from the nucleus) for the orbitals you will see that a greater proportion of the 4s is located near the nucleus so it still experiences the full nuclear charge unlike the 3d which is well shielded. This accounts for why the 3d is higher in energy than the 4s for K and Ca.
    However the situation is different for the transition elements. By Sc, the increase in nuclear charge means that the 3d is now actually lower in energy than the 4s but we still see occupancy of the 4s...... This is because electrons in the 3d repel each other much more strongly than those in the 4s as the 4s is more diffusely distributed around the atom. So despite the absolute energy of the 3d being lower, the lowest energy configuration is to place 2 electrons into the 4s (filling it) and then adding electrons into the 3d.
    Thank you
    ah I see so to confirm with you,

    In general in some elements the 4s has a lower energy level than the 3d , this is because of shielding, the 4s experiences more shielding and so there is less nuclear attraction ? is this fine

    Why is transition so special in terms of the d and s? how come the electrons repel in 3d and not in other elements?
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    (Original post by Lufthansa)
    Thank you
    ah I see so to confirm with you,

    In general in some elements the 4s has a lower energy level than the 3d , this is because of shielding, the 4s experiences more shielding and so there is less nuclear attraction ? is this fine

    Why is transition so special in terms of the d and s? how come the electrons repel in 3d and not in other elements?
    There is nothing special about the transition metals, it just happens to be a crossing point for all these factors. Everything is still working the same way.
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    (Original post by JMaydom)
    There is nothing special about the transition metals, it just happens to be a crossing point for all these factors. Everything is still working the same way.
    ah okay, but is my thinking correct, the one i put above?

    Also would you know what hybridisation means in chemistry ? thanks
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    (Original post by Lufthansa)
    ah okay, but is my thinking correct, the one i put above?

    Also would you know what hybridisation means in chemistry ? thanks
    Well sort of no. The 4s is lower for the element prior to Sc (i'm not 100% certain on this but pretty sure) and higher for after that. For the 1st transition series the 4s is occupied to relieve e-e repulsions.

    Yes i do know about hybridisation of orbitals.
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    (Original post by JMaydom)
    Well sort of no. The 4s is lower for the element prior to Sc (i'm not 100% certain on this but pretty sure) and higher for after that. For the 1st transition series the 4s is occupied to relieve e-e repulsions.

    Yes i do know about hybridisation of orbitals.
    What is it then?
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    (Original post by Lufthansa)
    What is it then?
    You can read up on that. I'll answer specific questions not explain huge topics for you.
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    (Original post by Lufthansa)
    In general in some elements the 4s has a lower energy level than the 3d , this is because of shielding, the 4s experiences more shielding and so there is less nuclear attraction ? is this fine

    Why is transition so special in terms of the d and s? how come the electrons repel in 3d and not in other elements?
    You seem to be slightly confused. The ordering of 4s vs 3d depends very much on the atom in question. The 4s orbital has an inner node, which means that there is electron density close to the nucleus. This means the 4s is initially lower in energy (i.e. more stable) because it experiences higher nuclear attraction. However, as you go across the fourth period, the 3d energy falls much faster than the 4s — the inner node in 4s screens the increasing Zeff somewhat from the outer electron density, leading to reduced orbital contraction.

    So for the transition metals, the 3d orbital is lower in energy than 4s, but the greater electron repulsion associated with sticking electrons in the smaller 3d orbitals outweighs this and the 4s is filled first. It is not the case that electrons *only* repel in 3d and not the other orbitals. It just happens that the 4s/3d orbitals are the first pair that are close enough in energy for the repulsion to make a difference to the filling order.

    As for orbital hybridisation: when atoms bond together to form molecules, the bonding interactions are not actually simple overlaps of one orbital on each atom. (!) For example, in methane, all the hydrogen-carbon bonds have the same energy. If the bonding interactions were simple orbital overlaps, the three carbon(2p)-hydrogen(1s) bonds would be weaker than the carbon(2s)-hydrogen(1s) bond. But this is not the case — in fact the precise details are rather complex.

    So on the most basic level, we can understand that orbital hybridization is simply an attempt to create a more accurate bonding picture by more equally distributing the electron density from orbitals across the whole atom. For example, sp3 hybridization, as seen in methane, allows us to recast the low energy carbon(2s) orbital and higher-energy carbon(2p) orbitals as four equal-energy hybrid orbitals.

    If you want to learn more about hybridization, chemguide would be a good place to start.
 
 
 

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