The atomic number of an element is the number of protons found in the nucleus of an atom of that element. It is denoted by the symbol "Z" and is a fundamental property that distinguishes one element from another. Each element has a unique atomic number.
The mass number, on the other hand, refers to the total number of protons and neutrons present in the nucleus of an atom. It is denoted by the symbol "A." Since the mass number includes both protons and neutrons, it can vary for different atoms of the same element. Atoms of the same element with different mass numbers are called isotopes.
To summarize:
- Atomic number (Z): The number of protons in the nucleus of an atom.
- Mass number (A): The total number of protons and neutrons in the nucleus of an atom.
For example, consider the element carbon. The atomic number of carbon is 6 because it has 6 protons in its nucleus. However, carbon can have different isotopes with varying numbers of neutrons. Carbon-12 has a mass number of 12 (6 protons + 6 neutrons), while carbon-13 has a mass number of 13 (6 protons + 7 neutrons), and carbon-14 has a mass number of 14 (6 protons + 8 neutrons). Despite having different mass numbers, they are all carbon atoms because they have the same atomic number, which is 6.