When you first study Niels Bohr’s model of the atom in introductory physics, everything feels beautifully logical. You have the principal quantum number n = 1, 2, 3,,, , representing discrete, quantized energy levels.

But then, your textbook throws a curveball. It tells you that these exact same energy levels are also called the K, L, M, and N shells.

If physicists love order, why on earth did they start labeling the fundamental building blocks of atomic structure right in the dead center of the alphabet? Why not start with A, B, C, and D?

The answer isn’t a complex mathematical proof. Instead, it is a fascinating piece of historical detective work involving early X-ray experiments, a cautious British physicist, and a blank check left for future discoveries.

The Pre-Quantum Puzzle: X-Ray “Fluorescence”

To understand where these letters came from, we have to rewind the clock to the early 1900s—a chaotic, thrilling era when physicists knew atoms had internal parts but had no idea how they were arranged.

In 1911, a British physicist named Charles Glover Barkla was experimenting with a newly discovered phenomenon: X-rays. When he blasted various elements (like iron or copper) with a beam of primary X-rays, the elements absorbed that energy and scattered their own secondary X-rays. This is known as X-ray fluorescence.

Barkla noticed something crucial: the secondary X-rays coming out of the elements weren’t uniform. They fell into two distinct groups based on how easily they could penetrate a barrier, like a sheet of aluminum foil:

1. One group of X-rays was highly energetic and easily penetrated the foil.
2. The second group had lower energy and was easily blocked by the foil.

Enter the Alphabet: Why K and L?

Initially, Barkla did what any reasonable person would do: he labeled these two types of radiation Type A (for the high-energy, highly penetrating ones) and Type B (for the lower-energy ones).

But Barkla was a cautious experimentalist. He knew that physics was evolving at breakneck speed. He realized that if he used up “A,” and another scientist later discovered an even more powerful, highly penetrating type of X-ray, they would have nowhere to go in the alphabet.

So, in a classic footnote in his landmark 1911 paper published in Philosophical Magazine, Barkla officially abandoned A and B. He decided to anchor his terms right in the middle of the alphabet to leave a safety buffer in both directions.

He chose K for the highly penetrating radiation and L for the less penetrating radiation. If someone found a higher-energy line, they could use J, I, or H. If they found a lower-energy line, they could use M, N, and O.

Barkla’s gamble paid off quickly. Scientists soon discovered lower-energy series exactly where he predicted, and they naturally labeled them the M and N series.

How Bohr’s Model Inherited the Letters

Here is the twist: Barkla named these lines based purely on radiation spectrums, completely independent of atomic structure. He didn’t know why the atom produced K or L light; he just knew it did. (Barkla actually won the Nobel Prize in Physics in 1917 for this exact work).

Meanwhile, in 1913, Niels Bohr published his quantum model of the atom, defining orbits purely by integers ($n = 1, 2, 3$).

The bridge between Barkla’s letters and Bohr’s numbers was built in 1914 by German physicist Walther Kossel. Looking at Bohr’s equations, Kossel realized that Barkla’s X-ray data perfectly mapped onto Bohr’s quantum leaps:

• K-radiation happens when a high-energy collision knocks an electron completely out of the innermost, tightest orbit (n = 1). An electron from a higher orbit drops down to fill the vacancy, releasing a massive burst of energy (a K-quantum X-ray).
• L-radiation happens when an electron is knocked out of the second orbit (n = 2), and an outer electron drops down to fill that gap, releasing a slightly weaker chunk of energy.

Because of Kossel’s insight, the spectroscopic labels became permanently fused to the physical orbits of the Bohr model. The $n=1$ orbit became the K shell, $n=2$ became the L shell, $n=3$ became the M shell, and a piece of temporary experimental caution became permanent textbook terminology.

The Takeaway for Physics Students

The next time you are solving a quantum mechanics problem or working through electron configurations, remember that the K, L, M, N notation is a living artifact. It is a reminder that science isn’t delivered in a neat, perfectly designed package. It’s a relay race where one scientist invents a temporary tool to measure an unknown mystery, and the next scientist adopts it to map the universe.

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Tags: Bohr, hydrogen atom, Physics