An atom is the smallest unit of matter that retains all of the chemical properties of an element. It consists of a central nucleus that is positively charged and contains one or more protons and neutrons, and one or more negatively charged electrons that orbit around the nucleus . Atoms cannot be broken down into anything smaller while still retaining the properties of the element. The word "atom" comes from the Greek word "atomos," meaning "indivisible".
For example, a hydrogen atom has one proton and one electron. The atomic number of an element is equal to the number of protons in its nucleus. The mass number of an atom is equal to the sum of its protons and neutrons.
