Clear Science!

We talked about how the red color in rubies is caused by unpaired electrons in chromium impurities interacting with light, making them fluoresce red. Rubies and sapphires are both kinds of corundum, which is a crystalline aluminum oxide, Al2O3.

Sapphires are any color of corundum besides red. The kind most associated with the word “sapphire,” however, is blue. The color in blue sapphires is not caused by chromium, but rather by iron and titanium impurities occurring together.

Iron and titanium atoms located near each other in the corundum crystal can transfer an electron between them. This exchange causes the blue color. Very small impurity levels can cause this: ~ 0.01%. The coloring in rubies requires more like 1% impurities.

We talked about gemstones, and about how rubies and sapphires are both made of the same thing: corundum, which is the crystalline form of Al2O3. So rubies and sapphires are mostly aluminum and oxygen.

In the case of rubies, the striking red color is caused by chromium atoms being substituted for a small fraction of the aluminum atoms. The octahedron above shows what the crystal structure in corundum looks like. (It works out to being 2 Al’s to every 3 O’s throughout the entire crystal.)

Pure corundum is colorless, much like a diamond. This is because if you look at the periodic table, you’ll see that when Al shares 3 electrons it then has no unpaired electrons.

However, when a chromium is sitting in for an aluminum, things are different. A Cr sharing 3 electrons still has unpaired electrons, which can interact with light. These electrons absorb violet and yellow-green light, and emit red light. This emission of light is called fluorescence.  

To understand why atoms can’t fall through each other—despite the huge amount of empty space in them—we first need to think about electrons. Remember, the nucleus has protons, which are positive. The electrons are negative and attracted to the protons, so they orbit the nucleus.

A hydrogen atom has one proton and one electron. However, remember we also said the physics for electrons were strange. Namely, they are quantum mechanical. What this means is that they are kind of everywhere around the nucleus, without being anywhere exactly. Weird, right?

What this means is they are kind of a blur, or a smear. Think of the space around the nucleus as being a cloud, all kind of negative.

The UP and DOWN arrows we wrote to represent the electrons in iron are called spins. Spins make magnetic moments. An UP and a DOWN spin together in an orbital cancel out, but unpaired electrons do not get canceled.

The spins of unpaired electrons add up to larger magnetic moments. The magnetic moments in a material will line up in larger domains. And if these domains can all be aligned by a magnetic field, that material is ferromagnetic.

Bring a magnet near the nails and all the blue arrows will jump into alignment, and the nails will stick to the magnet. So magnets work because of electrons. Specifically ones that are unpaired, or alone in their orbitals.

Atomic orbitals are where electrons “orbit” the nucleus. They look kind of like kidneys, pears, and bells. (Rather than like the orbits of planets around the sun.)

The shape of the periodic table tells you what kind of orbital an electron goes in. This is why the table looks like it does.

Orbitals hold 2 electrons each. The p-block is 6 elements wide, and p orbitals come in 3’s. (It’s like: if a car holds 2 people then 6 people have to fit in 3 cars.)

To talk about magnetic moments, we are first going to talk about electrons. And for that, we’re back to the periodic table.

Awhile back, we talked about the f-block on the periodic table. We can go further: all parts of the table go in a block. These blocks have to do with electrons, and it’s electrons that lead to magnetism (and many other things).

We’ve left the ferromagnetic elements on the table above. Can you see a pattern about where they fall?

Protons and electrons are equal in an atom’s natural state.

The atomic number tells how many protons an atom has. That determines its identity as one element or another. It will also have that many electrons in its orbitals, at least to start with.

The number of electrons does not determine an atom’s identity. Rather, this determines how elements will react and bond with each other.

Swapping and sharing electrons is how atoms interact. Water, which is not an element, is made up of hydrogen (Element 1) and oxygen (Element 8) sharing electrons. You probably already knew that.

Last week we introduced our Far Too Simple Model Of An Atom© so we could talk about protons.

A more complete view of the atom includes two other particles:

• neutrons, which hang out in the nucleus with protons, but don’t have + written on them
• electrons, which are smaller, have a - on them, and are in motion in the orbitals, rather than the nucleus

PS You know which element this atom is. (answer)