- 2913 - ATOM - can we see an atom? Well, that really depends on what we mean by “see.” We see something when light emitted or reflected from an object reaches our eyes and the signal is conducted to our brain.
--------------------- 2913 - ATOM - can we see an atom?
- So in this sense, no we can “not” see an atom. But, what if we use a magnifying glass? What if we use a microscope or a telescope? An electron microscope? Where do we draw the line of what it means to “see something“?
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- The naked human eye has a resolution of about 100 microns ( 100*10^-6 or 10^-4 meters). Atoms are separated by roughly a tenth of a nanometer ( 0.1*10^-9 or 10^-10 meters), and we can barely see a speck of dust that is a million atoms wide.
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- Visible frequency passive optics uses lenses and mirrors. If a microscope is converting light to electrical signals, or collecting data for longer than the human eye can, or the signal is being amplified digitally, or light is not being used at all, we are no longer seeing the object we are looking at, we are seeing what a computer screen is showing us.
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- With our naked eyes we cannot see atoms, and looking through any light microscope we also cannot, because even the best lenses and mirrors cannot beat the diffraction limit, which is roughly half a wavelength, about 200 nanometers, or 1000-2000 atoms (2*10^-7 meters)
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- If materials existed that could refract x-rays like glass refracts visible light, we could maybe make an x-ray microscope, but those materials don’t exist. Our eyes cannot see atoms, but what can?
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- The electron microscope images we usually see are from scanning electron microscopes (SEMs), which bounces a beam of electrons off a sample coated with metal. Although these do not have atomic resolution.
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- Transmission electron microscopes (TEMs) send a beam of electrons through a sample, and the electrons are detected on the other side. If SEMs are like a medical ultrasound, TEMs are like an x-ray.
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- A simple way to interpret a TEM image is that where there is something brighter in the image, there was less stuff blocking the electrons. So if the resolution is high enough, rather than seeing atoms in a plane, you are seeing the projected shadow of a column of atoms, rather than the shadow of an individual atom.
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- The bright spots on the image are regions where there are fewer atoms blocking the electrons. This is not quite how TEM imaging works, the electrons must be re-focused after going through the sample. The technology has improved over the years, and now it is possible perform TEM on single-layer graphene and see the atomic structure from a sheet of carbon.
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- An atomic force microscope (AFM) is not an electron microscope, it is an example of a scanning probe microscope. Basically, it is a little platform (often big enough to see, like a few millimeters), with a very very small tip on the end. The platform and tip are dragged across a surface and a laser beam is reflected off the surface of the platform, and deflections in the platform deflect the beam, which can be recorded.
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- There are two main ways that these operate. One is by having the platform vibrating up and down at the resonance frequency of the platform, called “tapping mode“. When something interacts with the tip, the frequency of the platform changes, and this is recorded.
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- The other just keeps the tip at a constant height, and measures the deflection of the platform as the tip is lifted by objects on the surface.
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- The resolution of an AFM is basically determined by how sharp the tip is, and with a tip that truncates with a single atom, one can have atomic resolution. So the way these images are made, is by scanning an extremely sharp tip across a surface and measuring at each point either the force exerted on the tip, or the height of the surface.
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- The measurements can be fine enough to detect surface deflections caused by single atoms. The tip generally scans in a straight line, then moves over slightly and scans again. On an atomically flat surface, each line looks like a zig-zag pattern, and putting these together you get a 2D image (which you can 3D render if you want). 2 nanometers square image of the heights of atoms across a plane.
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- This method requires not only a very sharp tip, but also hours and hours of extremely slow scanning.
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- Another example of a scanning probe microscope, the scanning tunneling microscope (STM) looks similar to an AFM except instead of measuring the deflection of the tip, it shoots electrons out of the tip. The electrons are bound to the atoms in the tip, but a voltage is applied between the tip and the surface, making the electrons likelier to quantum tunnel from the tip to the surface.
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- Instead of measuring height, the STM measures how likely at each point the electron is to tunnel (technically, the local density of states). STM images are the most likely to be excessively 3D rendered by their creators, making it look like someone is holding a camera right next to an atom and taking a picture with flash.
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- The tunneling probability depends on the electron density of the surface, so you can actually visualize the wavefunction of the unbound surface electrons, in the images.
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- One of the oldest techniques that could image with atomic resolution, field emission microscopy involves applying a massive voltage (thousands of volts) between a tungsten tip and a screen. The electrons start flying out of the tip, and follow the non-uniform electric field and hit the screen, where their position is noted and back-tracked to figure out where on the tip it came from. This can be used to ascertain the position of each atom in the tip.
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- A voltage is applied between the tip and the screen. Electrons fly off the tip, follow the electric field, and are back-tracked to determine the atomic organization of the tip.
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- There are other variants, including “field ion microscopy‘, where atoms are adsorbed onto the tip and then flung off (rather than electrons), and “atom probe tomography“, which is a destructive technique that involves applying sufficient voltage to a sample that the individual atoms start flying off.
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- Atomic imaging is now in 3D! These techniques generally rely on locating the positions of atoms in a plane and then rendering them so that people can see where the atoms are. There are a few methods, however, that can image the 3D positions of atoms.
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- With atom probe tomography each atom flies off the sample, its original position can ascertained, and a 3D image of each atom’s location can be built up. It can also distinguish different elements. However, it is generally limited to things that are small and shaped like a narrow cone.
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- For example with a Gallium/Indium cone combined TEM with computed tomography (CT) imaging produces a 2D TEM image of a nanoparticle at different angles, and combines all the data to obtain a 3D map of the 27,000 atoms in the particle.
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- We can’t see atoms with our eyes, or with anything that enhances our eyes, but we can figure out the positions of atoms and render those in a way that we can “see“.
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Source https://www.physicsforums.com/insights/can-see-atom/ Other reviews available:
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- 2867 - ATOM - and the electron cloud? The picture of the atom you were taught in high school is wrong, mainly because electrons aren’t point-like particles. Electrons are a‘fuzzy’ . They are tough to pin down due to their ‘Quantum Wave Function’, which is a complicated way of saying they exist as a field of “probability“, not as an individual particle.
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- 2709 - ATOMS - measuring how atoms work? - An atom can be viewed as a tiny electron orbiting a tiny nucleus at a certain radius. Let’s start with the hydrogen atom which is a single proton orbited by a single electron.
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- 2694 - ATOM - How can mathematics tell us how an atom works? It is 100 years of discovery. - It is how physicists were able to figure out the mathematics that defines the behavior of an atom. They are still figuring, but, we have come a long way.
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- 2685 - MOLECULE - how a molecule works? When there is more than one proton in the nucleus and more than one atom in orbit this classical physics math just becomes overwhelming. That is the reason the math of Quantum Mechanics was invented. When Quantum Mechanic’s math is used, the concept of the electron orbiting the proton completely disappears. The electron’s position around the nucleus becomes a probability distribution
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- 2452 -
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- 2377 - ATOM - defining the atom All the other elements in the periodic table above hydrogen and helium were created in the nuclear fusion of the stars The first stars formed with only hydrogen and helium. When they burned all their fuel and exploded as supernova they splattered the surrounding space with all the atoms in the higher level elements.
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- 2333 - Rainbows can tell us what the Universe is made of. Introduction to the science of spectroscopy.
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- 2318 - Brownian motion from atoms you can not see. My grandson, Michael, was 9 years old when he was looking at pond water under his microscope. He could see small plants and animals moving around in the water. But, he also saw all the little pieces of dust jiggling, almost vibrating, in a zigzag manner. He asked me what causes everything to move like that?
- 2315 - About how atoms were first discovered. How was the atom discovered, This review covers the first 100 years of discovery that started in 1808. John Dalton conclusively argued for the existence of the indivisible atom, and at the same time as Einstein was provided a way to directly measure those atoms, Thomson and Rutherford discovered that the atom wasn't indivisible at all. Instead, it was made of even tinier bits
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- 2307 - How small is the atom? An atom is very small. However, all atoms are about the same size, 10^-10 meters. Atoms of all the elements have different atomic weights but they still are about the same size in diameter.
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- 2255 - History of the atom.
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- 2256 - Atom’s stability and uncertainty?
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- 2147 - Rutherford’s atom. How the atom was discovered in 1911.
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- November 23, 2020 ATOM - 2913
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