# Carbon atom larger than a water molecule? Measuring a molecule's size?

• am4th
In summary, the website says that the carbon atom is larger than the water molecule. It does not say how the water molecule's size (radius?) is measured.
am4th
So there's a website here:

https://learn.genetics.utah.edu/content/cells/scale/
That shows the scale of various entities (coffee bean, viruses, a carbon atom, a water molecule etc). It says that the carbon atom is larger than the water molecule. This is based on the 'van der waal radius'; however, it doesn't say how the water molecule's size (radius?) is measured.

From my understanding of what I've just read, the 'van der waal radius' is the distance between two atoms (carbon in this case) when they are at their most stable (so the potential energy between them is at its lowest?). Is that correct?

They are quoting a size of 275pm for the water molecule. Does anyone know how this value is reached?

OmCheeto
There is definitely something wrong. You can approximate atom as a sphere and sphere is indeed characterized by a single number (radius). That's not the case with water molecule, so the number given is meaningless.

The vdW radius is a non-covalent measure. In graphite, for example, the distance between adjacent graphene sheets is about 335 pm, so dividing this by two (~170 pm) would be a decent approximate value for vdW radius of carbon. The vdW radius of water is usually quoted at around 140-150pm. These values are usually ascertained by fitting a calculated water model to experimentally observed parameters.

am4th said:
They are quoting a size of 275pm for the water molecule. Does anyone know how this value is reached?
I found that number at one website, listed as 2.75 angstroms, but I don't understand the explanation; "The atomic diameter can be determined from interpolation of the effective ionic radii of the isoelectronic ions (from crystal data) of O2- (2.80 Å), OH- (2.74 Å) and H3O+ (2.76 Å) "

Good luck.

ps. Just bought my first microscope, and am very grateful for the cell size and scale site link.
pps. Just did a bunch of maths, and I don't get it either.

In my mind, real world water molecules, are bigger.

Density of diamond is what? Of graphite? Of amorphous carbon? And density of water is how many times less?

## 1. What is the size difference between a carbon atom and a water molecule?

The size difference between a carbon atom and a water molecule is significant. A carbon atom has a radius of approximately 70 picometers (pm), while a water molecule has a radius of about 275 pm. This means that a carbon atom is about four times smaller than a water molecule.

## 2. How is the size of a molecule measured?

The size of a molecule is typically measured using techniques such as X-ray crystallography, nuclear magnetic resonance (NMR) spectroscopy, or electron microscopy. These methods allow scientists to visualize the three-dimensional structure of a molecule and determine its size.

## 3. Can a carbon atom be larger than a water molecule?

No, a carbon atom cannot be larger than a water molecule. The size of an atom is determined by its atomic radius, which is a fundamental property of an element. The atomic radius of carbon is smaller than that of oxygen, which is the main component of a water molecule, so a carbon atom cannot be larger than a water molecule.

## 4. How does the size of a molecule affect its properties?

The size of a molecule can greatly affect its properties. For example, smaller molecules tend to have lower boiling points and are more volatile, while larger molecules tend to have higher boiling points and are more stable. The size of a molecule also affects its ability to interact with other molecules and can influence its chemical reactivity.

## 5. Why is it important to understand the size of molecules?

Understanding the size of molecules is important in many fields of science, including chemistry, biology, and materials science. The size of a molecule can provide valuable information about its structure, properties, and behavior. It can also help scientists design and develop new materials and drugs, and understand the mechanisms of chemical reactions and biological processes.

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