Why Does Neon Glow? - UCSB Science Line

A great question!

And I will tell you for the same reason that stars shine. How is that so?

Let's start with the idea that everything you see around you is made of very small particles known as atoms. The idea of atoms started very early with Democritus a little before 500 BC.

Democritus was a philosopher, and postulated that if he divided any object many times he would eventually come down to a unit that would be further indivisible, (which is in fact what the word "atom" means!) and called this unit an atom.

We know from later experiments that the atom is a little bit more complicated than what Democritus originally thought, but the name stuck. Several models since then have been proposed for what the structure of the atom looks like. The first idea was that the atom was just a jelly of positive charge with an even distribution of electrons. This is known as the Thompson Model.

We found out from clever experiments Rutherford experiment that there was actually some structure for the atom. The positive charge, aka nucleus, was actually concentrated in the middle of the atom. We call this the Rutherford Model. You can see the difference between the two here . And read more about it from Wikipedia atom (which is a great resource for anything you don't know and want to know more about).

Further throughout the years, our idea of what exactly the atom looks changed even more. Our current understanding is based off of quantum mechanics, and the basic idea is as follows.

Why do we need quantum mechanics to understand atomic structure? Because it tells us that electrons live at discrete energy levels. This is different from our regular intuition about energy. For instance, you would expect to be able to hold a ball at heights infinitesimally apart (e.g., 1 m versus 1.01 m versus 1.0001 m and so on). This is not the case for electrons. They live at different "heights" (i.e., different energy levels) away from the nucleus, and where these energy levels are is dictated by quantum mechanics. Amazingly, when quantum mechanics was first formulated, it exactly matched the energy levels seen for hydrogen.

So, why does neon glow? We've learned that electrons live at discrete energy levels, but they don't have to live at the same energy level all the time. They can be promoted to a higher energy (by absorbing energy) or demoted to a lower energy level (by emitting energy), as seen in the schematic here for hydrogen.

Each level is labeled with the variable n, the lowest one (i.e., closest to the nucleus) being n = 1. For example, an electron in the n = 1 energy level would need at least 10.2 eV to get to the second energy level. Conversely, an electron in the n=3 energy level would emit 1.9 eV of energy if it went to the n=2 energy level.

Note from moderator: symbol eV also written electron volt is a unit of energy.

This is essentially why neon glows, and why it glows certain colors. In a neon light, the voltage gives energy to the electrons, and promotes them to higher energy levels. Because this is not energetically favorable, the electrons fall back to lower energy levels and emit that energy in the form of light.

This is known as the emission spectrum (which is closely associated with the absorption spectrum), and is different for every element because every element has a different set of discrete energy levels. Some example emission spectrums are shown here

Because discrete energy levels exist, it is also why so many different colors can exist in the periodic table!

What does that mean for stars? Stars are made of the same kinds of atoms that behave in the same kind of way as they do on earth. In fact, it is from emission and absorption spectra that scientists are able to figure out what a star is made of and even how hot it is! And it's all from knowing the structure of the atom.

I'm assuming this means neon lights (like these neon lights ). These lights contain gasses in their tubes, usually neon gas (which is where they get their name). The neon lights up when you run electricity through the gas. This is because the electricity has a lot of energy, which excited electrons in the gas, and causes them to release photons, which we see as brilliant light. Many gasses can be used with different colors, but the common orange lights you see are filled with neon.

Neon only glows when it has electrical current flowing through it. It glows then because of the way that electrons move back and forth between different energy levels in the atom.

Neon lights are glass tubes with neon gas sealed inside. When you connect this tube to an electrical outlet, a voltage is applied across it so that electrons flow through the tube. As they flow through, they hit the neon atoms and provide them with excess energy. When the neon releases that energy, it glows with visible light.

This process is called fluorescence and lights that work this way are called fluorescent lights. Many gasses can be used for fluorescent lighting. What makes neon special is that it is a large enough molecule to emit mostly visible light, not invisible light like UV or infrared.

On its own, neon lights are orange but adding other gasses can change the color – add hydrogen for red, helium for yellow, carbon dioxide for white and mercury for blue. In fact, the lights you see in most buildings (offices, supermarkets, etc.) are fluorescent lights that use mercury and sodium to create white light, instead of neon.