Selenium(Se) Electron Configuration and Orbital Diagram

Selenium is the 34th element in the periodic table and its symbol is ‘Se’. In this article, I have discussed in detail how to easily write the complete electron configuration of selenium. I also discussed how to draw and write an orbital diagram of selenium. Hopefully, after reading this article, you will know more about this topic.

What is the electron configuration of selenium?

The total number of electrons in selenium is thirty-four. These electrons are arranged according to specific rules in different orbitals. The arrangement of electrons in selenium in specific rules in different orbits and orbitals is called the electron configuration of selenium.

The electron configuration of selenium is 1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p⁴, if the electron arrangement is through orbitals. Electron configuration can be done in two ways.

• Electron configuration through orbit (Bohr principle)
• Electron configuration through orbital (Aufbau principle)

Electron configuration through orbitals follows different principles. For example Aufbau principle, Hund’s principle, and Pauli’s exclusion principle.

Selenium atom electron configuration through orbit

Scientist Niels Bohr was the first to give an idea of the atom’s orbit. He provided a model of the atom in 1913. The complete idea of the orbit is given there.

The electrons of the atom revolve around the nucleus in a certain circular path. These circular paths are called orbit(shell). These orbits are expressed by n. [n = 1,2,3,4 . . . The serial number of the orbit]

K is the name of the first orbit, L is the second, M is the third, and N is the name of the fourth orbit. The electron holding capacity of each orbit is 2n².

Shell Number (n)	Shell Name	Electrons Holding Capacity (2n2)
1	K	2
2	L	8
3	M	18
4	N	32

For example,

1. n = 1 for K orbit.
   The maximum electron holding capacity in K orbit is 2n² = 2 × 1² = 2.
2. For L orbit, n = 2.
   The maximum electron holding capacity in L orbit is 2n² = 2 × 2² = 8.
3. n=3 for M orbit.
   The maximum electrons holding capacity in M orbit is 2n² = 2 × 3² = 18.
4. n=4 for N orbit.
   The maximum electrons holding capacity in N orbit is 2n² = 2 × 4² = 32.

Therefore, the maximum electron holding capacity in the first shell is two, the second shell is eight and the 3rd shell can have a maximum of eighteen electrons.

The atomic number is the number of electrons in that element. The atomic number of selenium is 34. That is, the number of electrons in selenium is thirty-four.

Therefore, the selenium atom will have two electrons in the first shell, eight in the 2nd orbit, eighteen electrons in the 3rd shell, and the remaining six electrons will be in the fourth shell.

Therefore, the order of the number of electrons in each shell of the selenium(Se) atom is 2, 8, 18, 6. Electrons can be arranged correctly through orbits from elements 1 to 18.

The electron configuration of an element with an atomic number greater than 18 cannot be properly determined according to the Bohr atomic model. The electron configuration of all the elements can be done through the orbital diagram.

Electron configuration of selenium through orbital

Atomic energy shells are subdivided into sub-energy levels. These sub-energy levels are also called orbital. The most probable region of electron rotation around the nucleus is called the orbital.

The sub-energy levels depend on the azimuthal quantum number. It is expressed by ‘l’. The value of ‘l’ is from 0 to (n – 1). The sub-energy levels are known as s, p, d, and f.

Orbit Number	Value of ‘l’	Number of subshells	Number of orbital	Subshell name	Electrons holding capacity	Electron configuration
1	0	1	1	1s	2	1s2
2	0 1	2	1 3	2s 2p	2 6	2s2 2p6
3	0 1 2	3	1 3 5	3s 3p 3d	2 6 10	3s2 3p6 3d10
4	0 1 2 3	4	1 3 5 7	4s 4p 4d 4f	2 6 10 14	4s2 4p6 4d10 4f14

For example,

• If n = 1,
  (n – 1) = (1–1) = 0
  Therefore, the value of ‘l’ is 0. So, the sub-energy level is 1s.
• If n = 2,
  (n – 1) = (2–1) = 1.
  Therefore, the value of ‘l’ is 0, 1. So, the sub-energy levels are 2s, and 2p.
• If n = 3,
  (n – 1) = (3–1) = 2.
  Therefore, the value of ‘l’ is 0, 1, 2. So, the sub-energy levels are 3s, 3p, and 3d.
• If n = 4,
  (n – 1) = (4–1) = 3
  Therefore, the value of ‘l’ is 0, 1, 2, 3. So, the sub-energy levels are 4s, 4p, 4d, and 4f.
• If n = 5,
  (n – 1) = (n – 5) = 4.

Therefore, l = 0,1,2,3,4. The number of sub-shells will be 5 but 4s, 4p, 4d, and 4f in these four subshells it is possible to arrange the electrons of all the elements of the periodic table.

Subshell name	Name source	Value of ‘l’	Value of ‘m’ (0 to ± l)	Number of orbital (2l+1)	Electrons holding capacity 2(2l+1)
s	Sharp	0	0	1	2
p	Principal	1	−1, 0, +1	3	6
d	Diffuse	2	−2, −1, 0, +1, +2	5	10
f	Fundamental	3	−3, −2, −1, 0, +1, +2, +3	7	14

The orbital number of the s-subshell is one, three in the p-subshell, five in the d-subshell and seven in the f-subshell. Each orbital can have a maximum of two electrons.

The sub-energy level ‘s’ can hold a maximum of two electrons, ‘p’ can hold a maximum of six electrons, ‘d’ can hold a maximum of ten electrons, and ‘f’ can hold a maximum of fourteen electrons.

Aufbau is a German word, which means building up. The main proponents of this principle are scientists Niels Bohr and Pauli. The Aufbau method is to do electron configuration through the sub-energy level.

The Aufbau principle is that the electrons present in the atom will first complete the lowest energy orbital and then gradually continue to complete the higher energy orbital.

The energy of an orbital is calculated from the value of the principal quantum number ‘n’ and the azimuthal quantum number ‘l’. The orbital for which the value of (n + l) is lower is the low energy orbital and the electron will enter that orbital first.

Orbital	Orbit (n)	Azimuthal quantum number (l)	Orbital energy (n + l)
3d	3	2	5
4s	4	0	4

Here, the energy of 4s orbital is less than that of 3d. So, the electron will enter the 4s orbital first and enter the 3d orbital when the 4s orbital is full. The method of entering electrons into orbitals through the Aufbau principle is 1s 2s 2p 3s 3p 4s 3d 4p 5s 4d 5p 6s 4f 5d 6p 7s 5f 6d.

The first two electrons of selenium enter the 1s orbital. The s-orbital can have a maximum of two electrons. Therefore, the next two electrons enter the 2s orbital.

The p-orbital can have a maximum of six electrons. So, the next six electrons enter the 2p orbital. The second orbit is now full. So, the remaining electrons will enter the third orbit.

Then two electrons will enter the 3s orbital and the next six electrons will be in the 3p orbital of the third orbit. The 3p orbital is now full. So, the next two electrons will enter the 4s orbital and ten electrons will enter the 3d orbital.

The 3d orbital is now full. So, the remaining four electrons enter the 4p orbital. Therefore, the selenium full electron configuration will be 1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p⁴.

Note: The short electron configuration of selenium is [Ar] 3d¹⁰ 4s² 4p⁴. When writing an electron configuration, you have to write serially.

How to write the orbital diagram for selenium?

To create an orbital diagram of an atom, you first need to know Hund’s principle and Pauli’s exclusion principle.

Hund’s principle is that electrons in different orbitals with the same energy would be positioned in such a way that they could be in the unpaired state of maximum number and the spin of the unpaired electrons will be one-way.

And Pauli’s exclusion principle is that the value of four quantum numbers of two electrons in an atom cannot be the same. To write the orbital diagram of selenium(Se), you have to do the electron configuration of selenium. Which has been discussed in detail above.

1s is the closest and lowest energy orbital to the nucleus. Therefore, the electron will first enter the 1s orbital. According to Hund’s principle, the first electron will enter in the clockwise direction and the next electron will enter the 1s orbital in the anti-clockwise direction.

The 1s orbital is now filled with two electrons. Then the next two electrons will enter the 2s orbital just like the 1s orbital. The next three electrons will enter the 2p orbital in the clockwise direction and the next three electrons will enter the 2p orbital in the anti-clockwise direction.

Then the next two electrons will enter the 3s orbital just like the 1s orbital and then the next six electrons will enter the 3p orbital just like the 2p orbital. The 3p orbital is now full. So, the next two electrons will enter the 4s orbital just like the 1s orbital.

The 4s orbital is now full. Therefore, the next five electrons will enter the 3d orbital in the clockwise direction and the next five electrons will enter the 3d orbital in the anti-clockwise direction.

The 3d orbital is now full. So, the next three electrons will enter the 4p orbital in the clockwise direction and the remaining one electron will enter the 4p orbital in the anti-clockwise direction. This is clearly shown in the figure of the orbital diagram of selenium.

Selenium excited state electron configuration

Atoms can jump from one orbital to another orbital in an excited state. This is called quantum jump.

The ground state electron configuration of selenium is 1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p⁴. In the selenium ground-state electron configuration, the last four electrons of the 4p orbital are located in the 4p_x(2), 4p_y and 4p_z orbitals.

The p-subshell has three orbitals. The orbitals are p_x, p_y, and p_z and each orbital can have a maximum of two electrons. Then the correct electron configuration of selenium in the ground state will be 1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p_x² 4p_y¹ 4p_z¹.

This electron configuration shows that the last shell of the selenium atom has two unpaired electrons. So in this case, the valency of selenium is 2.

When the selenium atom is excited, then the selenium atom absorbs energy. As a result, an electron in the 4p_x orbital jumps to the 4d_xy¹ orbital. We already know that the d-subshell has five orbitals.

The orbitals are d_xy, d_yz, d_zx, d_x²-y² and d_z² and each orbital can have a maximum of two electrons. Therefore, the electron configuration of selenium(Se*) in an excited state will be 1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p_x¹ 4p_y¹ 4p_z¹ 4d_xy¹.

The valency of the element is determined by electron configuration in the excited state. Here, selenium has four unpaired electrons. So, the valency of selenium is 4.

Selenium ion(Se^2-) electron configuration

The electron configuration of selenium shows that the last shell of selenium has six electrons. Therefore, the valence electrons of selenium are six.

The elements that have 5, 6, or 7 electrons in the last shell receive the electrons in the last shell during bond formation. The elements that receive electrons and form bonds are called anions.

During the formation of a bond, the last shell of selenium receives two electrons and turns into a selenium ion(Se^2-). That is, selenium is an anion element.

Se + 2e^– → Se^2-

The electron configuration of selenium ion(Se^2-) is 1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p⁶. This electron configuration shows that the selenium ion(Se^2-) acquired the electron configuration of krypton.

Selenium atoms exhibit -2, +2, +4, +6 oxidation states. The oxidation state of the element changes depending on the bond formation.

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