An atom is the smallest unit of an element that can exist. It is composed of a nucleus surrounded by electrons and is the building block of all matter. The nucleus of an atom is made up of protons and neutrons, and the electrons orbit around the nucleus. The number of protons and neutrons in an atom’s nucleus determines the atom’s atomic mass. Atoms are incredibly small and are not visible to the naked eye. In fact, it is estimated that a single atom is around one-tenth of a nanometer in size. Even though atoms are so small, they are the foundation for everything in the universe. All of the matter we can see and touch, from the human body to rocks, stars, and galaxies, is made up of atoms. The size of a nucleus is 10−15 m. The rest mass of an electron is 9.1 × 10−31 kg with a negative charge of 1.6 × 10−19 C. On the contrary, a proton has a rest mass of 1.673 × 10−27 kg with a positive charge of 1.6 × 10−19 C. The presence of protons inside the nucleus makes the nucleus positively charged and responsible for the electrons orbiting around the nucleus due to the coulomb force of attraction between the electrons and protons. An atom is usually represented by its atomic number and atomic mass. The atomic number (Z) represents the number of protons in an atom, and the mass number (A) is the summation of protons and neutrons. The difference between the two represents the total number of neutrons inside an atom.