An individual atom is generally not visible to the naked eye because of its extremely small size. Atoms are the basic building blocks of matter and are incredibly tiny, with diameters on the order of a few tenths of a nanometer (10^-10 meters). Visible light, which is the range of electromagnetic radiation that we can perceive with our eyes, has wavelengths on the order of hundreds of nanometers. This means that visible light is much larger than an individual atom and cannot interact with it directly.
However, there are techniques available that can indirectly visualize atoms. For example, in advanced microscopy techniques such as scanning tunneling microscopy (STM) and atomic force microscopy (AFM), it is possible to image individual atoms by scanning a sharp probe over the surface of a material. These techniques rely on detecting the interaction between the probe and the atoms to generate an image.
In summary, while atoms themselves are not visible to the naked eye, there are scientific methods that allow us to indirectly observe and visualize them.