# Why Energy of 4s is higher than 3d?

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• ImShiva
In summary: Lower energy states do indeed fill up before higher energy states. But, the Aufbau principle refers to how additional electrons fill up. An additional electron placed into the 3d state will have higher energy than an electron that is excited into the 3d state with the states below empty. That's because there is repulsion between the electrons. For helium, which has two electrons, there isn't too much interaction between an electron in 3d and in the 1s position, so, the energy isn't that high. Consider hydrogen, which has only one electron. Then the 3d state has the same energy as the 3p and 3s states. When you fill up the lower states, then the 3d state energy rises because it has

#### ImShiva

According to Aufbau principle energy of 3d should be greater than 4s but here 4s is having higher energy. Please anyone explain this.

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Hi ImShiva,

The principle you mention isn't about energy; it's about the order in which orbits are filled.

BvU said:
Hi ImShiva,

Tha principle you mention isn't about energy; it's about the order in which orbits are filled.
BvU said:
Hi ImShiva,

Tha principle you mention isn't about energy; it's about the order in which orbits are filled.
But isn't lower energy implying that orbital must be filled first?

Apparently not !

BvU said:
Apparently not !

Lower energy states do indeed fill up before higher energy states. But, the Aufbau principle refers to how additional electrons fill up. An additional electron placed into the 3d state will have higher energy than an electron that is excited into the 3d state with the states below empty. That's because there is repulsion between the electrons. For helium, which has two electrons, there isn't too much interaction between an electron in 3d and in the 1s position, so, the energy isn't that high. Consider hydrogen, which has only one electron. Then the 3d state has the same energy as the 3p and 3s states. When you fill up the lower states, then the 3d state energy rises because it has more interaction with the inner electrons than the 3s and 3p states.

ImShiva
Khashishi said:
Lower energy states do indeed fill up before higher energy states. But, the Aufbau principle refers to how additional electrons fill up. An additional electron placed into the 3d state will have higher energy than an electron that is excited into the 3d state with the states below empty. That's because there is repulsion between the electrons. For helium, which has two electrons, there isn't too much interaction between an electron in 3d and in the 1s position, so, the energy isn't that high. Consider hydrogen, which has only one electron. Then the 3d state has the same energy as the 3p and 3s states. When you fill up the lower states, then the 3d state energy rises because it has more interaction with the inner electrons than the 3s and 3p states.
So I guess (n+l) energy rule won't work here. What's the rule for energy of states then ?

ImShiva said:
So I guess (n+l) energy rule won't work here
The (n+l) rule is for the individual energy of electrons in a subshell denoted by the quantum number ##nl## to form a ground state of the atom, The energy ##E_{nl}## governed by that rule is the energy of each subshell in the ground state of an atom and not the total energy of the atom. In your diagram, it is the total energy of the atom (He atom) which is shown, in particular it shows energy levels including its ground state and the excited states of the form ##1s\, nl## where one electron stays in ##1s## subshell and the other in ##nl## subshell. Again, the (n+l) rule is irrelevant here because the diagram is not about the energy of each subshell in a He atom.
ImShiva said:
What's the rule for energy of states then ?
The only rule I know regarding the energy diagram of a ##1s\, nl## He atom is that the energy for orthohelium is always lower than for parahelium for the same ##nl##.

ImShiva
In Helium, the only orbital filled is 1s, so the aufbau principle is correct. The other orbitals are virtual orbitals and not filled, so they are irrelevant for the aufbau principle.

ImShiva and edguy99
For atoms with more than one electron, you need to go into lengthy calculations to come up with energy levels. There are various approximations, to varying degrees of accuracy (e.g. Hartree-Fock). But nothing so simple as the hydrogen atom.

ImShiva

## What is the explanation for the higher energy of 4s compared to 3d?

The explanation for this phenomenon is related to the filling order of orbitals. In the Aufbau principle, electrons fill orbitals in order of increasing energy, with the lowest energy orbitals filling first. However, in the case of 4s and 3d orbitals, the energy levels are very close, leading to a deviation from the expected order.

## Why is the 4s orbital filled before the 3d orbital?

The 4s orbital is filled before the 3d orbital because of its lower energy. Although the 3d orbital has a higher principal quantum number (n=3), the 4s orbital has a higher effective nuclear charge, which results in a lower energy level.

## How does the shielding effect play a role in the energy difference between 4s and 3d orbitals?

The shielding effect, also known as screening, refers to the repulsion between electrons in different orbitals. In the case of 4s and 3d orbitals, the 4s electrons experience more shielding from the nucleus due to the presence of the 3d electrons, leading to a slightly higher energy level for the 4s orbital.

## Is the energy difference between 4s and 3d orbitals consistent across all elements?

No, the energy difference between 4s and 3d orbitals can vary among elements due to factors such as atomic size and nuclear charge. However, in general, the 4s orbital is still filled before the 3d orbital in the Aufbau principle.

## How does the energy difference between 4s and 3d orbitals impact the chemical properties of elements?

The energy difference between 4s and 3d orbitals can affect the chemical properties of elements by influencing the stability of different electron configurations. For example, elements with a half-filled 4s orbital may exhibit unique properties due to the lower energy of this orbital compared to the 3d orbital.