An atom is the smallest building block of matter. Atoms consist of a nucleus surrounded by orbiting electrons. The nucleus is composed of protons and neutrons, which are individually referred to as nucleons. Protons carry a positive unit charge, electrons carry a negative unit charge, while neutrons possess a neutral charge. Protons and neutrons have approximately the same mass (the neutron is slightly more massive). An electron has approximately 1/1800 of the mass of a nucleon. An element is a substance that is in its simplest chemical form and has specific chemical properties. All atoms of a given element share these properties. The mass number of an element is the total number of protons and neutrons in the nucleus. The letter A is used to represent the mass number. Note that the mass number is sometimes referred to as atomic mass. The atomic number of an element is the number of protons in the nucleus. The atomic number is represented by the letter Z. Elements are identified by their atomic numbers. For example, an element of carbon always has 6 protons in its nucleus, and any element with 6 protons in its nucleus is a carbon atom. If a carbon atom were to lose a proton from its nucleus, it would no longer be identified as carbon. However, the number of neutrons in the nucleus may change without changing the element. Atoms with the same atomic number but with different mass numbers are called isotopes of each other. The elements are organized by their atomic numbers on the periodic table.
A molecule consists of two or more atoms bonded together. A compound is a
