Structure of Atom

How are Electrons Distributed in Different Orbits (Shells)?

By now, we know that, in an atom,  electrons revolve around the nucleus while protons and neutrons are inside the nucleus. The question is, how do electrons revolve? Do they move randomly? Or do they follow a specific route? How are they arranged? The answer to these questions is Electronic Configuration. Let’s learn about the arrangement of electrons around the nucleus.

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Distribution of Electrons in Different Orbits

Neils Bohr gave the planetary model of an atom. He was the first person to suggest the periodicity in the properties of the elements. “Bohr atomic model” forms the basis of the electronic structure of an atom. He was the person to describe the arrangement of electrons (electronic configuration) in different orbits/shells.

He proposed that electrons are distributed in circular electronic shells (orbits). These electrons revolve in the orbits around the nucleus from a fixed distance. In this topic, we will learn more about the electronic configuration of different elements.

Bohr Bury Schemes

The distribution of electrons in an atom is called as Electronic Configuration. Formula 2nhelps in the determination of the maximum number of electrons present in an orbit, here n= orbit number. The formula helps in determination of arrangement of electrons and is known as “Bohr Bury Schemes.”

Read more about Atomic models and Atomic numbers.

Electrons are negatively charged subatomic particles arranged like a cloud of negative charges outside the nucleus of an atom. The arrangement depends upon of their potential energies in different orbits. The different energy levels are known as 1, 2, 3, 4….. and the corresponding shells are known as K, L, M, N and so on.

For instance,

  • 1st energy level- K shell/orbit
  • 2nd energy level- L shell/orbit
  • 3rd energy level- M shell/orbit and so on.
Electronic configuration

(Source: schools.aglasem)

Learn about Isobars and Isotopes here.

The Arrangement of Electrons in Different Orbits

The shells begin from the centre and gradually move outwards. So K shell will always have minimum energy. Similarly, L shell is a little away from nucleus so it will have higher energy than K shell. The outermost shell will have maximum energy. Now it is important to understand the distribution and arrangement of electrons in the atoms of any elements in the different energy levels.

An atom of any element is most stable when it has minimum energy. An atom will first fill the lowest energy level so as to attain the state of minimum energy. Gradually, the electrons will fill the higher energy levels. Therefore, electrons will first fill K shell, then L shell, M shell, N shell, and so on.

Electronic configuration

(Source: siyavula)

Electronic Configuration of Elements

According to the postulate of Neils Bohr, “electrons revolve around the centre of an atom (nucleus) in a predictable pathway named orbits”. The representation of the orbits is done by letters and numbers such as K, L, M, N, O…. and 1,2,3,4…. respectively.  The arrangement and distribution of electrons in different orbits was given by Bohr and Bury.

The arrangement of electrons in different shells and sub-shells is known as the electronic configuration of a particular element. The electronic configuration diagram represents an element in its ground state or stable state. There are a set of rules to remember while distribution off electrons in different orbits.

  • Rule 1: The maximum number of electrons present in a particular shell is calculated by the formula 2n2, where “n” represents the shell number. For instance, K shell is the first shell and it can hold up to 2(1)2 = 2 electrons. Similarly, L shell is the second shell and it can hold up to 2(2)2 = 8 electrons. This formula helps to calculate the maximum number of electrons that an orbit can accommodate.
  • Rule 2: The maximum capacity to hold electrons in the outermost shell is 8.
  • Rule 3: The electrons will fill the inner shells before the outer shells. First electrons will fill the K-shell and then L shell and so on. Thus, electronic configuration of elements follows an ascending order.

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Examples of the Electronic Configuration

1) Helium

The atomic number of the element = 2. The total number of electrons present in Helium = 2. The maximum number of electrons in K shell (1st orbit) = 2. Therefore, shells needed = 1.

Electronic Configuration of Helium

Electronic Configuration of Helium

2) Lithium

The atomic number of the element = 3. Lithium has 3 electrons. We can apply rule number 3 to fill the electrons in different in different orbits. The maximum number of electrons accommodated in K shell (1st orbit) will be 2. The second orbit will accommodate rest of the electrons. Electronic configuration of Lithium= 2, 1. Therefore, the total number of shells required = 2.

Electronic Configuration of Lithium

Electronic Configuration of Lithium

3) Oxygen

The atomic number of the element = 8. Oxygen has 8 electrons. The maximum number of electrons accommodated in the K shell (1st orbit) will be 2. The second orbit will accommodate rest of the electrons left (6 electrons). Electronic configuration of Oxygen = 2, 6. Therefore, the total number of shells required = 2 (1st and 2nd shell/orbit).

Electronic Configuration of Oxygen

Electronic Configuration of Oxygen

4) Chlorine

The atomic number of the element= 17. Chlorine has 17 electrons. The maximum number of electrons accommodated in the K shell (1st orbit) will be 2. The second orbit will fill up to 8 electrons. Finally, The third orbit will accommodate rest of the electrons left. Electronic configuration of Chlorine = 2, 8, 7. Therefore, the total number of shells required = 3 (1st, 2nd, and 3rd shell).

Electronic Configuration of Chlorine

Electronic Configuration of Chlorine

5) Argon

The atomic number of the element= 18. Argon has 18 electrons. The maximum number of electrons accommodated in K shell is 2. The second orbit will fill up to 8 electrons. The third orbit can fill up to 18 electrons and it will accommodate left electrons of the element. Electronic configuration of Argon = 2, 8, 8. Therefore, the total number of shells = 3.

Electronic Configuration of Argon

Electronic Configuration of Argon

Uses of the Electronic Configuration

  • Electronic Configuration helps to understand the structure of periodic table with respect to each element.
  • It also helps in understanding and explanation of the chemical bonds between the atoms.
  • It explains the different properties and peculiar properties of certain elements. For example, electronic configuration explains the reason for the unique properties of lasers and semiconductors.

Importance of the Electronic Configuration

The electronic configuration is very important and basic part of understanding chemistry. It forms the basis of the periodic table. Additionally, the stability of any orbital will depend upon the electronic configuration of that element. It also helps us to understand the arrangement of elements in different periods and groups.

Electronic Distribution of First 18 Elements

Element Symbol Atomic Number No. of Electrons Electronic configuration Valency
Shells
K L M N
Hydrogen H 1 1 1 1
Helium He 2 2 2 0
Lithium Li 3 3 2 1 1
Beryllium Be 4 4 2 2 2
Boron B 5 5 2 3 3
Carbon C 6 6 2 4 4
Nitrogen N 7 7 2 5 3
Oxygen O 8 8 2 6 2
Fluorine F 9 9 2 7 1
Neon Ne 10 10 2 8 0
Sodium Na 11 11 2 8 1 1
Magnesium Mg 12 12 2 8 2 2
Aluminum Al 13 13 2 8 3 3
Silicon Si 14 14 2 8 4 4
Phosphorus P 15 15 2 8 5 3
Sulphur S 16 16 2 8 6 2
Chlorine Cl 17 17 2 8 7 1
Argon Ar 18 18 2 8 8 0

 

Learn about Charged Particles in Matter here.

 

A Solved Question for You

Q: Find the Electronic Configuration of Potassium (K).

  1. 2,8,8,4
  2. 2,8,8,1
  3. 2, 8, 7
  4. 2,8,8,3

Ans: The correct answer is option 2 (2, 8, 8, 1).

Solution: Atomic number of the potassium = 19. Potassium has 18 electrons. Applying rule number 3, the maximum number of electrons accommodated in K shell is 2. After filling the first orbit, the second orbit will consist of 8 electrons. Although it can fill up to 18 electrons the 3rd orbit will fill up to 8 electrons.

The reason behind the arrangement of electrons in such a manner is due to the presence of subshells. An atom always tends to remain in its stable state. Furthermore, it is necessary to arrange the electrons in the sub-shells in such a way that the element gains stability to attain the lowest energy level.

There are separate principles to fill the electrons in its subshells. Hence, the fourth orbit will accommodate the one electron left. Thus, the Electronic configuration of potassium = 2, 8, 8, 1

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