How do electrons populate atoms?

Electron population of atomic orbital lobes

In Part 1 of A Journey into Modern Physics, chapter 4 describes how electrons populate atoms in terms of a sequential order. Diagrams were used, like the one above, to indicate how atoms are made up with a maximum of 2 electrons (one spin-UP and one spin-DOWN) in orbital lobes that are full. Only outermost orbitals exist in an empty state or a partially populated state (one electron). There may even be empty orbitals in between populated orbitals, once the d3 orbitals are involved, as shown in the diagram.

This diagram, taken from chapter 4, shows how the orbitals are populated from hydrogen to calcium. A second diagram continued the process up to the noble gas krypton.

Each orbital is represented by a square box and electrons are represented as arrows, one showing a spin-UP alignment and another a spin-DOWN alignment. So, a given box, or orbital, can contain no electrons, one or two.

As you can see from the above diagram, things get out of sequence when the d3 orbitals are reached. This is because empty d3 orbitals correspond to a higher energy level than the s4 orbital. So, potassium's last electron skips all the d3 orbitals and plonks itself in the s4 orbital, which sits at a lower energy level as indicated in the inset to the above diagram. Calcium then fills the s4 orbital and the next element after calcium starts to populate the d3 levels, which now sit at a lower level to the s4 orbital. The next diagram shows how the process of filling the orbitals occurs up to the Nobel gas Krypton.

Once the s4 is full, electrons then start populating d3 levels as shown. Because of the way in which the energy levels fluctuate as more electrons arrive in the outermost orbitals, the s4 loses an electron to the d3 levels on occasion and gains it back on others.

The final result is that all d3 orbitals are filled at Copper.  Once krypton  has been reached, then all orbitals up to the p4 set are full and the atom becomes very stable.

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Both diagrams, showing the sequence of orbital population up to krypton, are now also available on the IMAGE WALL on the book's website. These are very informative representations and chapter 4 in Part 1 provides a good explanation of the populating of all the d3 orbitals to be filled at copper but only after depopulation and repopulation of the s4 lobe.

You can also see the two diagrams on the IMAGE WALL as follows;

go to the web site www.modern-physics-journey.com and select IMAGE WALL from the main menu. You'll find the diagrams on IMAGE WALL 3 and IMAGE WALL 4.

More images will be added to the IMAGE WALL in the future, so keep an eye on the blog as any additions will be announced with a blog.

© 2026 Kieron Conway - All rights reserved.

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modern-physics-journey.com 

where you can read all about an exciting new science series: A Journey into Modern Physics, available in three parts, from Amazon on-line shops. (Some of the above article may appear in this series).

Also, the web-site has an index of blog articles published to date, for easy access to an article that might interest you.

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