An element is defined as a substance composed entirely of one type of what?

An element is defined as a substance composed entirely of one type of atom. Each element is characterized by its unique number of protons, which defines its atomic number and identity. For example, all atoms of oxygen have eight protons, making up the element oxygen. Elements are the fundamental building blocks of matter and cannot be broken down into simpler substances by chemical means.

Molecules are formed when two or more atoms bond together chemically. Compounds consist of two or more different types of atoms that are bonded together in specific ratios. Ions are charged particles that can form when atoms gain or lose electrons, altering their electrical charge. While all these terms relate to atoms and elements at some level, only atoms are the singular, pure units that define elements.