When we say that an element has a fixed number of atoms, we are referring to the concept of the atomic structure and the composition of matter at the atomic level.
Elements are the building blocks of matter, and each element is composed of atoms that are chemically identical. Atoms are the smallest units of an element that retain its chemical properties. They consist of a nucleus containing protons and neutrons, and electrons that orbit the nucleus.
The number of atoms in an element is determined by its atomic number, which represents the number of protons in the nucleus. For example, hydrogen has an atomic number of 1, indicating that a hydrogen atom contains one proton. Oxygen has an atomic number of 8, indicating that an oxygen atom contains eight protons.
In a neutral atom, the number of electrons is equal to the number of protons, so the number of electrons in an atom of an element is also fixed. However, atoms can gain or lose electrons to form ions with a different number of electrons, resulting in charged particles.
When we say that an element has a fixed number of atoms, we are essentially referring to the fact that within a given sample or quantity of that element, the number of atoms remains constant. This concept is fundamental to the field of chemistry and underlies various calculations and interactions involving elements and compounds.