What is atom
An atom is the smallest constituent unit or building block of an element or matter except for energy. We can say that anything that can be touched physically and occupies space or volume, called matter, comprises atoms such as a chair, table, laptop, plants, bird, dog, etc. So, except for energy, which does not fulfill the definition of matter, everything in the universe is matter and is made of atoms.
An atom is made of two regions that include the nucleus, which is located at the center of the atom, and the outer region around the nucleus. The nucleus contains protons and neutrons, whereas, the outer region holds electrons that are located in orbits (energy levels) around the nucleus as shown in the below image.