Periods 1 to 3 Atoms: Electron Configuration in Shells (energy levels) Chemistry Tutorial

Key Concepts

• An atom is represented as a nucleus surrounded by shells containing electrons

  (see Bohr model of the atom)

• Shells are given the symbols K, L, M and N
• Shells in order from closest to furthest from nucleus:

  K, L, M, N

• Electron shells correspond to energy levels:
  

  K is the 1^st energy level (lowest energy level)
  L is the 2^nd energy level (higher energy than K shell)
  M is the 3^rd energy level (higher energy than L shell)
  N is the 4^th energy level (higher energy than M shell)

• Electrons fill the lowest energy shells (energy levels) first.
• K shell (1^st energy level) can hold a maximum of 2 electrons.

  L shell (2^nd energy level) can hold a maximum of 8 electrons.

• Electron configurations (electronic configurations) are written with number of electrons in lowest energy level (shell) first, separated by a comma from the number of electrons in the next energy level (shell), etc

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Electron Shell Concept

The number of electrons around the nucleus of each atom is equal to the number of protons in its nucleus.
That is, for an atom to be neutral (no overall charge), the number of negative charges (electrons) must be equal to the number of positive charges (protons).
The number of protons in the nucleus of an atom is equal to its atomic number (Z).
Therefore, the number of electrons in an atom of an element is equal to the element's atomic number (Z):

For a neutral atom:
no. electrons = no. protons = atomic number (Z)

But the atomic number (Z) only tells us how many electrons in total there are around the nucleus of a neutral atom, it doesn't directly tell us how those electrons are arranged around the nucleus.

We can model the electronic structure of atoms as a series of concentric spheres, or shells, around the nucleus of an atom.
In the diagram below, the black circle represents the nucleus of the atom, and each circle represents the boundary of an electron shell. The shell closest to the nucleus is given the symbol K, the next shell is the L shell, which is followed by the M shell, then the N shell:

As the boundary of each shell gets further from the nucleus, the shell gets larger, so it makes sense that shells closer to the nucleus can contain fewer electrons than shells further away from the nucleus. Indeed, we find that the K shell can contain a maximum of only 2 electrons, but the next shell, the L shell, can contain a maximum of 8 electrons.
We could draw the electronic structure of each atom by placing a dot representing an electron in each shell, but that would take up a lot of space. So, scientists have come up with a short cut for representing the electronic structure of atoms, and this is based on an understanding of the energy of electrons in each shell.
Electrons closest to the nucleus, in the K shell, have less energy than electrons in the next shell out from the nucleus, the L shell.
Energy of electrons increases as they get further away from the nucleus. So instead of talking about "shells" we can also talk about energy levels:

And now we can draw the energy levels only, without drawing the rest of the circle to represent the shell:

And, we can now see an ever shorter way to represent the electronic structure of an atom, we can just write down how many electrons are present in each shell (energy level) separated by a comma, and this shorthand is referred to as the electron configuration or electronic configuration of the atom.

By assuming that an atom will exist in its lowest possible energy state, we can now add the electrons one by one to the energy levels (shells).
Electrons are shown as arrows (↑ and ↓) in the diagrams below.
The first electron goes into the 1^st energy level (K shell):

The electron configuration (electronic configuration) of the atom is 1

If an atom has 2 electrons, then both the first and second electron occupy the 1^st energy level (K shell):

The electron (electronic) configuration of the atom is 2

K shell is full because the K shell can only hold 2 electrons, so the next electron must be positioned in the next energy level (next shell).
So an atom with 3 electrons will have 2 electrons in the K shell and the third electron goes into the 2^nd energy level (L shell):

The electron configuration (electronic configuration) of the atom is 2,1
(2 electrons in the first energy level and 1 electron in the second energy level)

For atoms with more than 3 electrons, we can continue adding electrons to the second energy level or L shell until it reaches its maximum capacity of 8 electrons:
Fourth electron goes into the 2^nd energy level (L shell) ...
.... until the 2^nd energy level (L shell) is filled with 8 electrons.
Once both the 1^st energy level (K shell) and the second energy level (L shell) are full, we must begin filling the next energy level, the 3^rd energy level (M shell).

We can infer how the electrons are arranged around a nucleus by considering the position of an element in the periodic table.
The elements of the first three periods of the periodic table are shown below:

Electronic Configuration of Atoms of Period 1 Elements

There are only two elements in period 1 of the periodic table; hydrogen and helium:

An atom of hydrogen only has 1 electron, so this electron occupies the K shell (first energy level), and its shell electron configuration is simply 1
An atom of helium has 2 electrons, both of which can occupy the K shell (first energy level), so its shell electron configuration is simply 2

The electrons in the atoms of elements in Period 1 are filling the first energy level, or K shell.

Electronic Configuration of Atoms of Period 2 Elements

There are 8 elements in Period 2 of the periodic table:

The atoms of elements in Period 1 filled, or completed, the first energy level, or K shell.
The atoms of the elements of Period 2 elements all have the first energy level, K shell, full, that is, the electron configuration begins with a 2
The atoms of the elements in Period 2 must be filling the second energy level, or L shell.
The number of electrons in the second energy level (L shell) is separated from those in the first by a comma when writing the electron configuration (electronic configuration).
The beginning of the electron configuration (electronic configuration) of all period 2 elements will be 2,

The electron configuration (electronic configuration) of carbon is 2,4
This tells us that an atom of carbon has 2 electrons in the first energy level (K shell), and 4 electrons in the second energy level (L shell).
The total number of electrons in an atom of carbon is equal to the sum of all the electrons in all the energy levels (shells):

total number of electrons in a carbon atom = 2 + 4 = 6

The maximum number of electrons that can be placed in the second energy level (L shell) is 8.
This means that atoms of neon have a full K shell (first energy level) and a full L shell (second energy level).

Electronic Configuration of Atoms of Period 3 Elements

There are 8 elements making up period 3 of the periodic table:

Electrons of atoms of period 1 elements were filling the first energy level (K shell).
Electrons of atoms of period 2 elements had a completed first energy level (K shell) and were filling up the second energy level (L shell), which was completed by the last member of the period, neon.
Atoms of period 3 elements must:

• have full K shells (2 electrons in the first energy level)
• have full L shells (8 electrons in the second energy level)
• be filling the M shell (third energy level)

So the electron configuration (electronic configuration) of all period 3 elements must begin with 2,8

The electron configuration (electronic configuration) of an atom of sulfur is 2,8,6
An atom of sulfur has 2 electrons in the first energy level (K shell), 8 electrons in the second energy level (L shell) and 6 electrons in the third energy level (M shell).
The total number of electrons in an atom of sulfur = 2 + 8 + 6 = 16
which is the same as sulfur's atomic number (Z = 16), which means the number of protons in an atom of sulfur (16) equals the number of electrons in an atom of sulfur (16).

Electronic Configuration of Atoms of Periods 1 to 3 Elements Summary

The period of the periodic table an atom belongs to tells us which energy level or shell is being filled (previous shells or lower energy levels have been completed).

• Period 1 atoms are filling the 1^st energy level (K shell)
• Period 2 atoms are filling the 2^nd energy level (L shell)
• Period 3 atoms are filling the 3^rd energy level (M shell)

The Group of the periodic table an atom belongs to tells us how many electrons are in the highest energy level (shell furthest from the nucleus):

• Group 1: 1 electron in highest energy level
• Group 2: 2 electrons in highest energy level
• Group 13: 3 electrons in highest energy level
• Group 14: 4 electrons in highest energy level
• Group 15: 5 electrons in highest energy level
• Group 16: 6 electrons in highest energy level
• Group 17: 7 electrons in highest energy level
• Group 18: 8 electrons in highest energy level
  EXCEPT helium (He) in Group 18 which has only 2 electrons but these 2 electrons complete the K shell (1^st energy level)

All this information can be summarised as in the diagram below which shows a section of the periodic table:

Key to the section of the periodic table shown below: