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Bonding and structure

Electronic structure: shells and orbitals

Electronic structure: shells and orbitals

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Tutor: Sophie

Summary

Electronic structure: shells and orbitals

In a nutshell

Understanding electronic structure is crucial for understanding the chemistry of reactions. Here, we will review how to deduce electronic configurations of atoms and explain what is meant by the terms shells, subshells and orbitals.



Electron shells​

Electrons orbit the nucleus in electron shells, called quantum shells which are given principal quantum numbers. Each quantum shell contains different types of subshells. Each subshell contains a different number of orbitals. The four types of subshells and the number of electrons they can hold are summarised below.

​

subshell

number of orbitals

maximum electrons

​sss​​

​111​​

1 × 2 = 21\,\times\,2\,=\,21×2=2​​

​ppp​​

​333​​

3 × 2 = 63\,\times\,2\,=\,63×2=6​​

​ddd​​

​555​​

5 × 2 = 105\,\times\,2\,=\,105×2=10​​

​fff​​

​777​​

7 × 2 = 147\,\times\,2\,=\,147×2=14​​


And the subshells and electrons in the first four quantum shells are summarised below.


​​shell

subshells

number of electrons

​1st1^{st}1st​​
​1s1s1s​​
​222​​
​2nd2^{nd}2nd​​
​2s 2p2s\,2p2s2p​​
​2 + (3 × 2) = 82\,+\,(3\,\times\,2 )\,=\,82+(3×2)=8​​
​3rd3^{rd}3rd​​
​3s 3p 3d3s\,3p\,3d3s3p3d​​
​2 + (3 × 2) + (5 × 2) = 182\,+\,(3\,\times\,2)\,+\,(5\,\times\,2)\,=\,182+(3×2)+(5×2)=18​​
​4th4^{th}4th​​
​4s 4p 4d 4f4s\,4p\,4d\,4f4s4p4d4f​​
​2 + (3 × 2) + (5 × 2) + (7 × 2) = 322\,+\,(3\,\times\,2)\,+\,(5\,\times\,2)\,+\,(7\,\times\,2)\,=\,322+(3×2)+(5×2)+(7×2)=32​​

​​​

Orbitals

Each subshell contains different numbers of orbitals, which can hold up to two electrons. Orbitals have characteristic shapes.

  • sss​ orbitals are spherical.
  • ppp​ orbitals are shaped like dumbbells, of which there are three.  


Below is an sss-orbital.​


Chemistry; Atomic structure and the periodic table; KS5 Year 12; Electronic structure: shells and orbitals


Below is a ppp-orbital, of which there are three kinds; pzp_zpz​, pyp_ypy​ and pxp_xpx​.

​



Note: Electrons in each orbital spin in opposite directions, this is called 'spin-pairing'.



Deducing electronic configuration

A simple set of rules must be followed to work out electronic configuration. 


​​Rule

explanation

1. Arrows in boxes

Electrons in orbitals are represented by arrows in boxes. Each box represents one orbital and an arrow represents one electron.

Chemistry; Atomic structure and the periodic table; KS5 Year 12; Electronic structure: shells and orbitals

2. Lowest energy subshells first

Electrons fill up the lowest energy subshells first. From low to high, the order is: 1s 2s 2p 3s 3p 4s 3d 4p 4d 4f1s\,2s\,2p\,3s\,3p\,4s\,3d\,4p\,4d\,4f1s2s2p3s3p4s3d4p4d4f. Note, unusually, 4s4s4s​ is filled before 3d3d3d​ because it is lower in energy, even though it's principal quantum number is bigger; this is an exception you must remember. 

Below is the electronic configuration of calcium. 


3. Single before double

​​Electrons fill orbitals singularly before they start to pair up.

Chemistry; Atomic structure and the periodic table; KS5 Year 12; Electronic structure: shells and orbitals
1. Nitrogen's electron arrangement. 
2. Oxygen's electron arrangement.


To represent electronic configuration, you can either draw out the energy levels (as in the image for Rule 2. above) or use subshell notation. 


Example 

For potassium, the subshell notation is: 1s22s22p63s23p64s11s^22s^22p^63s^23p^64s^11s22s22p63s23p64s1. You can check this is correct by adding the total of the superscripts. Here, it equals 191919​, which tells you it is correct as that is the number of electrons in a KKK atom.​


Note: Noble gas symbols (i.e ArArAr or NeNeNe) are sometimes used for simplicity. For potassium, it is also correct to write the configuration as [ArArAr]4s14s^14s1, since the [ArArAr] corresponds to 1s22s22p63s23p61s^22s^22p^63s^23p^61s22s22p63s23p6.



Using the periodic table

The periodic table can be used as it is split into s-block, p-block and d-block elements.

Chemistry; Atomic structure and the periodic table; KS5 Year 12; Electronic structure: shells and orbitals
A. s-block elements
B. d-block elements
C. p-block elements
  • s-block elements have an outer shell configuration of s1s^1s1 or s2s^2s2​.
  • p-block elements have an outer shell configuration of p1p^1p1 to p6p^6p6.
  • d-block elements have an outer shell configuration of d1d^1d1 to d10d^{10}d10. 

Remember: 4s4s4s​ must be filled to 4s24s^24s2, and only afterwards 3d3d3d can be filled.


Exception to the rule

​CrCrCr and CuCuCu​ don't fit the usual rule. Their 4sss​ shell only ever fills to 4s14s^14s1 as they donate the second electron to the 3d3d3d​ subshell as it's more stable.


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Electronic configuration

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Electronic configuration

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Electronic structure: shells and orbitals

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Electronic structure: shells and orbitals

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FAQs - Frequently Asked Questions

What is meant by 'spin-pairing'?

Spin-pairing explains that electrons in each orbital spin in opposite directions.

How do you work out electronic configuration?

To deduce electronic configuration: 1. Draw arrows in boxes, 2. Fill lower energy subshells first, 3. Fill electrons singularly before pairing up.

What shapes do orbitals have?

s orbitals are spherical. p orbitals are shaped like dumbbells, of which there are three.

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