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Arrangement of Electrons in the Atoms

  • Last Updated : 22 Sep, 2021

The electron configuration of an element describes how electrons are distributed in their atomic orbitals. They follow a standard notation in which all electron-containing atomic subshells are placed in a sequence. This method was suggested by Bohr and Bury. The following rules are followed for writing the number of electrons in different energy levels or shells: 

  1. The maximum number of electrons in a shell is given by the formula 2n2, where n is the orbit number i.e. 1,2,3 and so on. Therefore, the maximum number of electrons in different shells are as follows-
    1. First orbit or K-shell: 2 × 12 = 2
    2. Second orbit or L-shell: 2 × 22 = 8
    3. Third orbit or M-shell: 2 × 32 = 18
    4. Fourth orbit or N-shell: 2 × 42 = 32
  2. The maximum number of electrons that can be accommodated in the outermost orbit is 8.
  3. Electrons are not accommodated in a given shell unless the inner shells are filled. That is, the shells are filled in a step-wise manner.

Orbit

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Maximum Number of Electrons



K

2

L

8

M

18

N

32

Electron Configurations are useful for:

  • Determining the valency of an element.
  • Predicting the properties of a group of elements.
  • Interpreting atomic spectra.

Electrons in a Shell

Electrons occupy different energy levels or shells. Each shell can hold a maximum number of electrons. Moving through the elements in the periodic table, each atom has one more electron than the last because the number of electrons is the same as the atomic number. Electrons occupy the shells in order, starting with the shell that is nearest the nucleus. They begin to occupy the next shell only when this shell becomes full.

Predicting an Electron Arrangement

The electron arrangement of an atom can be found out from its atomic number. E.g., the atomic number of sodium is 11 which means that the sodium atom has 11 protons and 11 electrons where:

  • 2 electrons occupy the first shell, 
  • 8 electrons occupy the second shell and
  • 1 electron occupies the third shell.

This electron arrangement can be written as 2, 8, 1. The electronic arrangement of the first 18 elements is listed below:

Name of ElementSymbolAtomic NumberKLMNValency

Hydrogen

H

1

1



1

Helium

He

2

2

2

Lithium

Li

3

2

1

1

Beryllium



Be

4

2

2

2

Boron

B

5

2

3

3

Carbon

C

6

2

4

4

Nitrogen

N

7

2

5



3

Oxygen

O

8

2

6

2

Fluorine

F

9

2

7

1

Neon

Ne

10

2

8

0

Sodium

Na

11

2

8

1

1

Magnesium

Mg

12

2

8

2



2

Aluminium

Al

13

2

8

3

3

Silicon

Si

14

2

8

4

4

Phosphorus

P

15

2

8

5

3 & 5

Sulphur

S

16

2

8

6

2

Chlorine

Cl

17

2

8

7

1

Argon

Ar

18

2

8

8

0

Valency

The electrons present in the outermost shell of an atom are known as the valence electrons. 



From the Bohr-Bury scheme, we find out the outermost shell of an atom can accommodate a maximum of 8 electrons and thus, It was observed that the elements, having a completely filled outermost shell (meaning zero valencies) show little chemical activity. Of these inert elements, the helium atom has two electrons in its outermost shell while the other elements have eight electrons in the outermost shell. An outermost shell, which had eight electrons possesses an octet and thus atoms would react to achieve an octet in the outermost shell. An octet can be achieved by sharing, gaining or losing electrons. The number of electrons gained, lost or shared to complete the octet of electrons in the outermost shell, gives us its valency.

Some Examples of Valency

  1. Hydrogen (H), lithium (Li), sodium (Na) atoms contain one electron each in their outermost shell which means that each one of them can lose one electron. Thus, all of them have a valency of one.
  2. Similarly, magnesium (Mg) and aluminium (Al) have their valency as two and three, respectively as they have two and three electrons in their outermost shell.
  3. If the number of electrons in the outermost shell of an atom is close to its full capacity, then valency is determined in a different way. E.g Fluorine (F) has 7 electrons in the outermost shell, and its valency could be 7 but it is easier for fluorine to gain one electron instead of losing seven electrons. So, its valency is determined by subtracting seven electrons from the octet which makes it valency as one.
  4. A similar procedure can be followed for oxygen (O) which will make its valency as two.

Sample Questions

Question 1: What are the rules for writing the number of electrons in different energy levels or shells?

Answer:

The following rules are followed for writing the number of electrons in different energy levels or shells-

  1. The maximum number of electrons in a shell is given by the formula 2n2, where n is the orbit number i.e. 1,2,3 and so on.
  2. The maximum number of electrons that can be accommodated in the outermost orbit is 8.
  3. Electrons are not accommodated in a given shell, unless the inner shells are filled. That is, the shells are filled in a step-wise manner.

Question 2: Why are electronic configurations useful?

Answer:

Electron Configurations are useful for:

  1. Determining the valency of an element.
  2. Predicting the properties of a group of elements.
  3. Interpreting atomic spectra.

Question 3: Predict the electronic arrangement of Magnesium.

Answer:



The electron arrangement of an atom can be found out from its atomic number. The atomic number of Magnesium is 12 which means that it has 12 protons and 12 electrons where-

  1. 2 electrons occupy the first shell
  2. 8 electrons occupy the second shell
  3. 2 electron occupies the third shell

This electron arrangement can be written as 2,8,2.

Question 4: Predict the electronic arrangement of the Sulphur.

Answer:

The electron arrangement of an atom can be found out from its atomic number. The atomic number of Sulphur is 16 which means that it has 16 protons and 16 electrons where-

  1. 2 electrons occupy the first shell
  2. 8 electrons occupy the second shell
  3. 6 electron occupies the third shell

This electron arrangement can be written as 2,8,6

Question 5: Write a short note on valence electrons and octets.

Answer:

The electrons present in the outermost shell of an atom are known as the valence electrons. 

From the Bohr-Bury scheme, we find out the outermost shell of an atom can accommodate a maximum of 8 electrons and thus, it was observed that the elements, having a completely filled outermost shell (meaning zero valency) show little chemical activity. Of these inert elements, the helium atom has two electrons in its outermost shell while the other elements have eight electrons in the outermost shell. An outermost-shell, which had eight electrons possesses an octet and thus atoms would react to achieve an octet in the outermost shell. An octet can be achieved by sharing, gaining or losing electrons.




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