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.

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Structure of Atom

 

Bohr Bury Schemes

The distribution of electrons in an atom is called 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 the determination of the 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 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, the L shell is a little away from the nucleus so it will have higher energy than the 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 the K shell, then the L shell, M shell, N shell, and so on.

Electronic configuration

(Source: siyavula)

What is Electronic Configuration?

Electronic configuration, also known as electronic structure, is the arrangement of electrons in energy levels surrounding an atomic nucleus. According to the older shell atomic model, electrons inhabit several levels ranging from the first shell, K, closest to the nucleus, to the seventh shell, Q, farthest from the nucleus. A more detailed quantum-mechanical model divides the K–Q shells into a set of orbitals, each of which can only be filled by a pair of electrons.

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.

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.

Representation of electronic configuration of an atom

shells begin from the centre and gradually move outwards. So K shell will always have minimum energy. Similarly, the L shell is a little away from the nucleus so it will have higher energy than the 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 the K shell, then the L shell, M shell, N shell, and so on.

electronic configuration

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 distributing 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, the K shell is the first shell and it can hold up to 2(1)2 = 2 electrons. Similarly, the 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, the electronic configuration of elements follows an ascending order.

Writing Electron Configurations

Shells

An electron shell in the region of an atom that surrounds the atomic nucleus. It is a collection of atomic orbitals that all have the same value of the primary quantum number n. Electron shells include one or more electron subshells, also known as sublevels.

Shell and ‘n’ value Max. Electrons in the Electron Configuration
K shell, n=1 2*12 = 2
L shell, n=2 2*22 = 8
M shell, n=3 2*32 = 18
N shell, n=4 2*42 = 32

Subshells

The arrangement of electrons in a shell is based on the form of the region of space they inhabit. Electrons are divided into orbitals inside each subshell, which are areas of space within an atom where the individual electrons are most likely to be located. Within a shell, a subshell is the collection of states described by the azimuthal quantum number, l. The numbers l = 0, 1, 2, 3 correspond to the subshells s, p, d, and f, respectively. 2(2l + 1) is the greatest number of electrons that may inhabit a subshell.

Notation

The distribution of electrons in an atom is called 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 the determination of the arrangement of electrons and is known as “Bohr Bury Schemes.” Electrons are negatively charged subatomic particles arranged like a cloud of negative charges outside the nucleus of an atom. The arrangement depends upon 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

 

Filling of atomic orbitals

Aufbau Principle

According to the Aufbau Principle, in the ground state of an atom, an electron with the lowest energy enters the orbital first, and following electrons are supplied in the sequence of increasing energies. In German, the phrase aufbau implies ‘building up.’

The ground-state electron configuration in atoms/ions with two or more electrons (1) minimises the total energy of the electrons, (2) obeys the Pauli exclusion principle, (3) obeys Hund’s rule of maximum multiplicity, and (4) addresses the exchange interaction.

Pauli Exclusion Principle

No two electrons in the same atom may have the same values for all four of their quantum numbers, according to Pauli’s Exclusion Principle. In other words, (1) no more than two electrons may occupy the same orbital at the same time, and (2) two electrons in the same orbital must have opposing spins. The exclusion principle is named after the fact that if one electron in an atom has the same specific values for the four quantum numbers, then all other electrons in that atom are precluded from having the same set of values.

Hund’s Rule

Any orbital in a subshell is singly occupied with one electron before it is doubly occupied, and all electrons in singly occupied orbitals have the same spin. Electrons always enter an empty orbital before they pair up. Electrons decrease repulsion by filling their own orbitals rather than sharing one with another.

Periodic Properties

Periodic characteristics of elements develop as a result of the recurrence of comparable electronic configurations with the same amount of electrons in the outermost orbit. The number of valence electrons in a given group remains constant. On the other hand, as we travel from left to right throughout a period, the number of valence electrons grows. The number of electrons in an element’s valence shell determines its chemical property.

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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 the 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 orbits. The maximum number of electrons accommodated in the K shell (1st orbit) will be 2. The second orbit will accommodate the 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 the 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 the 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 the 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 a 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
Aluminium 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 the 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 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 their 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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