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How are the molecules? – Judge B., 6, Wimberley, Texas
A molecule is a group of atoms joined together. Molecules make up almost everything around you: your skin, your chair, even your food.
They vary in size, but are extremely small. An individual molecule cannot be seen with the eyes or even with a microscope. They are 100,000 times smaller than the width of a hair.
The smallest molecule is made up of two attached atoms, while a large molecule can be a combination of 100,000 atoms or more. A molecule can be a repetition of the same atom, like the oxygen molecules we breathe, or it can be made up of a variety of atoms, like a sugar molecule made of carbon, oxygen, and hydrogen.
But how are the molecules? It all starts with its building blocks: atoms.
The ends are attracted
The particles of matter that make up an atom are not all the same. They can have a positive charge, a negative charge or no charge. Scientists call them protons, electrons and neutrons.
A gold atom has a dense center of 79 protons and 118 neutrons, with a more extended cloud of 79 electrons around it. Illustration created by Galarza Creador.
Uncharged neutrons and positively charged protons form the heavy center of the atom. Negatively charged electrons surround this small center.
As atoms approach each other to join and form molecules, the negative electrons of one atom are attracted by the positive protons of the other, and vice versa. Both atoms fit together accordingly.
When an atom is alone, the negative electrons surrounding its center are symmetrical. As two atoms approach, the negative electrons of one atom move toward the positive center of the other atom. Christine Helms, CC BY-SA
You can compare it to trying to choose a seat in a classroom. There are some rules. For example, you have to stay in the classroom and you can’t sit on top of someone. By following these rules, you may try to sit next to your friends and away from your enemies. Finding the perfect position for everyone in the class to be happy is like finding the perfect position for the atoms of a molecule. Sometimes atoms cannot find a happy disposition and no molecule is formed.
Seeing the invisible
If the molecules are too small to see with your own eyes or even with a powerful microscope, how do scientists see them? The answer is that they have developed special tools to do so.
One tool uses X-rays, which you may know as doctors use them to see the bones of the body. X-rays are a type of light that human eyes cannot see, such as ultraviolet or infrared light.
When scientists shoot X-rays at molecules, some bounce. Scientists can record these bounce X-rays and use their patterns to find out what the individual molecules are like.
X-rays bouncing on the atoms of a protein molecule form the black dots in the image above. The location of these points tells scientists how the atoms in the molecule are arranged. Del45 / Wikimedia Commons, CC BY
In 1912, one of the first molecules seen in this way was salt (NaCl), the molecule that is the ingredient we all know and love about French fries.
Scientists have also invented other methods to see molecules. Just as electrons change their behavior when two atoms approach, the center of the atom can also change its behavior. A technique called nuclear magnetic resonance imaging detects these changes in the center of the atom and uses them as clues to determine which atoms are nearby.
An atomic force microscope works like a fragile trampoline that trembles when you walk and jump on it. But this trampoline is extremely small, so small that a negative charge at the end will bend it toward the positive center of an atom. Moving this trampoline and observing how it bends can show the location of atoms in a molecule.
An animation that shows how an atomic force microscope works.
One more technique that scientists have developed to see molecules is called ciroelectron microscopy. First, scientists freeze molecules at a much colder temperature than snow or ice. They then shoot electrons into the molecule and collect what passes through it to make an image. This technique won the Nobel Prize in Chemistry in 2017.
All shapes and sizes
So what are molecules like? They are a grouping of atoms, with the center containing most of the material, while the rest is largely empty space. Each atom has a specific position where it is happy, just like the students in this classroom.
Schemes of the atoms that make up the molecules benzene, left, and fullerene, right. Jynto (left) Benjah-bmm27 (right) / Wikimedia Commons
Every molecule is different, some are really different. For example, benzene is flat like a pancake, while fullerene is round like a ball. Penguinone can be drawn to look like a penguin, while other molecules look completely random. But the positions of atoms in a molecule are never random.
Although scientists know how many molecules there are, there are some that we are still trying to figure out. Knowing these answers can lead to the invention of new materials and drugs.
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