In the context of atomic structure and elements, "mass" and "atomic number" refer to two important properties of atoms:
Mass: The mass of an atom is the total mass of its protons, neutrons, and electrons. However, the mass of electrons is so small compared to protons and neutrons that it is typically negligible in calculations involving atomic mass. The mass of an atom is usually given in atomic mass units (amu) or in kilograms (kg). The atomic mass unit (1 amu) is defined as 1/12th the mass of a carbon-12 atom.
Atomic Number: The atomic number of an atom is the number of protons found in the nucleus of an atom. It is denoted by the symbol "Z" and uniquely identifies each element on the periodic table. The atomic number determines the chemical properties of an element. For example, all carbon atoms have an atomic number of 6, which means they have six protons in their nucleus. The atomic number defines an element's position in the periodic table and the number of electrons in a neutral atom (since atoms are electrically neutral and have the same number of protons and electrons).
In summary, the mass of an atom refers to the total mass of its protons, neutrons, and electrons, while the atomic number represents the number of protons in the nucleus of an atom and identifies the element.