01-Levels of Chemical Organization

The smallest unit of matter is the atom. At the core of the atom is the nucleus, consisting of positively charged particles called protons and uncharged particles called neutrons. The number of protons in an atom’s nucleus is called the atomic number, whereas the sum of the protons and neutrons is known as the atomic mass.

Negatively charged particles called electrons orbit the nucleus of an atom within specific regions associated with unique energy levels. Each energy level can contain up to a set amount of electrons. The number of electrons permitted in a particular level increases as the energy level increases. That is, the greater the distance from the nucleus, the greater the energy level and thus the more electrons in the energy level.

Atoms combine together to form substances. An element is a pure substance, which is made up of only one kind of atom. A molecule is a group of atoms bound together. A compound is a substance whose molecules have more than one kind of atom within them.