The atomic number, mass number, and isotopes are all fundamental concepts in chemistry that describe the structure and properties of atoms, but they refer to different aspects.
Atomic Number
The atomic number defines an element. It's the number of protons found in the nucleus of every atom of that element. It is typically symbolized by Z. Because an atom is electrically neutral, the atomic number also equals the number of electrons in a neutral atom of that element.
- Importance: It uniquely identifies an element. Changing the number of protons changes the element itself. For example, all atoms with an atomic number of 6 are carbon atoms.
Mass Number
The mass number (also known as the nucleon number) is the total number of protons and neutrons present in the nucleus of an atom. It is usually represented by A.
- Calculation: Mass Number (A) = Number of Protons + Number of Neutrons
- Importance: It provides an approximate mass of an atom, since protons and neutrons each have a mass of approximately 1 atomic mass unit (amu). Note: this is not the same as atomic mass, which considers the weighted average of isotopes.
Isotopes
Isotopes are variants of a particular chemical element which differ in neutron number, and consequently in nucleon number. All isotopes of a given element have the same number of protons but different numbers of neutrons. Because isotopes have different numbers of neutrons, they also have different mass numbers.
- Example: Carbon-12, Carbon-13, and Carbon-14 are all isotopes of carbon. They all have 6 protons (atomic number = 6), but they have 6, 7, and 8 neutrons, respectively. Their mass numbers are therefore 12, 13, and 14.
- Chemical Properties: Isotopes of the same element have nearly identical chemical properties because these properties are determined by the number and arrangement of electrons, which are determined by the number of protons (atomic number).
- Physical Properties: Isotopes can have different physical properties such as mass, density, and radioactive behavior.
Table Summarizing the Differences
| Feature | Atomic Number | Mass Number | Isotopes |
|---|---|---|---|
| Definition | # of Protons | # of Protons + # Neutrons | Atoms of the same element (same # of protons) with different # of neutrons. |
| Symbol | Z | A | Represented as Element-Mass Number (e.g., Carbon-14) |
| Varies Within an Element | No | Yes | Yes |
| Determines | Element Identity | Approximate Atomic Mass | Different forms of the same element |
In summary, the atomic number defines what element an atom is, the mass number is the total number of protons and neutrons in the nucleus, and isotopes are different forms of the same element that have the same atomic number but different mass numbers due to differing numbers of neutrons.
