The size of an atom is typically measured using a parameter called the atomic radius, which represents the distance from the nucleus to the outermost electron orbit. The atomic radius is not simply the sum of the sizes of the individual particles (protons and neutrons) in the nucleus.
In the case of a carbon atom, the atomic radius is around 70-90 picometers (pm), not 340 pm as mentioned in your question. The atomic radius can vary depending on the specific definition used and the chemical bonding involved. The value you mentioned may be related to a measure of the covalent radius or an estimation considering the outermost extent of the electron cloud.
It is important to note that atoms are not perfectly spherical in shape. The electron cloud surrounding the nucleus is distributed in different orbitals and takes on complex shapes, resulting in an overall non-spherical distribution of electron density. However, the electron cloud is typically more concentrated around the nucleus.
To measure the size and structure of atoms, scientists use various experimental techniques. One common method is X-ray crystallography, which involves analyzing the diffraction pattern produced by X-rays passing through a crystalline sample. By interpreting the diffraction pattern, scientists can deduce the arrangement of atoms within the crystal, providing information about their distances and positions.
Other techniques such as electron microscopy, scanning probe microscopy, and spectroscopy also allow scientists to probe and visualize atomic structures and measure atomic dimensions. These techniques provide insights into the size, shape, and distribution of electrons around the nucleus, helping to determine the overall structure of atoms.
It's important to note that the measurements and visualization of atomic structures are indirect and rely on sophisticated instruments and mathematical models. Atoms are incredibly small, and direct observation with visible light or other common methods is not possible due to the limitations of the wavelengths involved. Therefore, scientists use specialized techniques and mathematical models to infer the structure and properties of atoms based on experimental data.