The Last Electron of Potassium Enters into 4s Orbital rather than 3d

The electronic configuration is a very important and interesting subject in the chemistry field. The last electron of Potassium enters into 4s orbital but not in the 3d orbital. Yes, it is an exception when you think generally. Now, in this article, we will discuss the reason behind it as well as we will try our best to make it easy in simple terms. So not doing any delay let’s get started.

To understand this fact properly, we should have a clear concept about some topic. First, you should have a clear knowledge of quantum numbers. You can read the article below to get a clear view of it.

What is a quantum number what do quantum numbers determine?

Electronic configuration

So what is electronic configuration? we all know that every substance is made of atoms as well as the atoms consists of the electron, protons, and neutrons. The electrons remain outside of the center (nucleus) of the atom in some energy levels and sub-energy levels. The Electronic configuration is the arrangement of the electrons in the orbitals. For example, Na (Sodium) has eleven electrons. Therefore, the electronic configuration of this element is-

₁₁Na→ 1s² 2s² 2p⁶ 3s¹

Aufbau principle

The Aufbau principle for the electronic configuration of the elements states that the electrons contained in an atom must fulfill the lower energy orbitals first. That means at first, the electrons enter into the first energy level then the second energy level, the third and fourth energy level, and so on. Now, the 3d orbital is in the third energy level and the 4s orbital is in the fourth energy level. Therefore, according to the Aufbau principle, the 3d orbital should be fulfilled before the 4s orbital.

Unlike this observation, the case of the electronic configuration of the potassium-

The last electron of potassium enters into 4s orbital before 3d orbital.

The reason for this event lies in the method of calculation of the energy of an orbital in different energy levels.

The energy of the orbitals ( reason of Last Electron of Potassium Enters into 4s Orbital)

As we know from the Aufbau principle the electron should be entered into the lower energy-containing orbital first. But, which is lower containing and which orbital contains higher energy? The answer lies in the quantum number. To be more specific the energy of an orbital depends on the value of the principal quantum number (n) and the auxiliary quantum number (l). The greater the summation of the value of the principal quantum number and the auxiliary quantum number (n + l) of an orbital, the greater the energy of the orbital.

So the n+l value of 4d and 5s orbitals-

For 4d orbital- n=4 and l=d=2, therefore n+l = 6

For 5s orbital- n=5 and l=s=0, therefore n+l = 5

So the energy of 4s is lower than the energy of the 3d orbital. (5s<4d).

Example 2- the energy of 3d and 4p-

For 3d orbital- n=3 and l=d=2, therefore n+l = 5

For 4p orbital- n=4 and l=p=1, therefore n+l = 5

Now both orbital has the same n+l value. In such a case the orbital in the higher energy level is considered to be contained higher energy. The 4p orbital is in the fourth energy level, on the other hand, the 3d orbital is in the third energy level. Therefore, though both 3d and 4p have the same n+l value, the energy of 4p is higher than 3d (3d<4p).

The last electron of Potassium enters into 4s orbital – why?

Now Potassium has 19 electrons. The last electron of the potassium should enter into the 3d orbital.

₁₉K → 1s² 2s² 2p⁶ 3s¹ 3p⁶ 3d¹ (but it does not happen)

But as we consider the energy of the 4s and 3d orbital by calculating the value of n+l, we can clearly notice that the n+l value of the 4s is lower than the 3d. Since the 4s contains lower energy, the last electron of potassium which is the 19th electron must enter into the 4s orbital rather than the 3d orbital.

₁₉K → 1s² 2s² 2p⁶ 3s¹ 3p⁶ 4s¹(currect)

So according to the Aufbau principle and the n+l value, the sequence of the orbitals should be-

1s<2s<2p<3s<3p<4s<3d<4p<5s<4d<5p<6s<4f<5d<6p<7s<5f<6d<7p<8s – – –

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