What is the Atomic Number?
The atomic number (Z) defines an element and is equal to the number of protons found in the nucleus of every atom of that element. Since protons carry a positive charge, the atomic number also equals the number of electrons in a neutral atom, ensuring electrical balance. This number is unique to each element, acting as its chemical identity on the periodic table.
What is the Mass Number?
The mass number (A), also known as atomic mass number, represents the total count of protons and neutrons in an atom's nucleus. Since protons and neutrons are the primary contributors to an atom's mass, the mass number provides an integer approximation of the atom's atomic mass. It does not identify the element but distinguishes between isotopes of the same element.
How They Relate to an Atom: An Example
Consider a carbon atom. All carbon atoms have 6 protons, so its atomic number (Z) is always 6. However, carbon can exist as different isotopes, such as Carbon-12 and Carbon-14. Carbon-12 has 6 protons and 6 neutrons (mass number = 12), while Carbon-14 has 6 protons and 8 neutrons (mass number = 14). Both are carbon because Z=6, but they have different mass numbers.
Why is This Distinction Important?
Understanding the difference between atomic number and mass number is crucial for identifying elements, distinguishing between their isotopes, and comprehending nuclear stability and reactions. The atomic number dictates an element's chemical properties, while the mass number helps determine its physical properties, especially its mass and nuclear behavior. This foundational knowledge is essential across chemistry and nuclear physics.