The atomic mass of an atom is essentially equal to its mass number, which is the sum of the number of protons and neutrons in its nucleus.
Understanding Atomic Mass
Atomic mass, often expressed in atomic mass units (amu) or Daltons (Da), is primarily determined by the particles located in the atom's nucleus: protons and neutrons. Electrons, while part of an atom, contribute negligibly to its mass due to their very small mass.
Components of Atomic Mass
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Protons: These positively charged particles have a mass of approximately 1 atomic mass unit (amu).
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Neutrons: These neutral particles also have a mass of approximately 1 amu.
The Mass Number
As highlighted in the provided reference:
Together, the number of protons and the number of neutrons determine an element's mass number: mass number = protons + neutrons.
The mass number is an integer value representing the total number of nucleons (protons and neutrons) in an atom's nucleus. This number closely approximates the atomic mass of an isotope in amu, but they are not identical. Atomic mass values account for the very slight mass difference within protons and neutrons, along with the binding energy of the nucleus.
Isotopes and Atomic Mass
It’s important to understand that most elements have isotopes — atoms of the same element with varying numbers of neutrons, leading to different mass numbers. The atomic mass displayed on the periodic table is not the mass number of any individual atom, but the weighted average of the masses of all the naturally occurring isotopes of an element, each multiplied by its relative abundance.
- Example: Chlorine (Cl) has two main isotopes: Cl-35 (75.77%) and Cl-37 (24.23%). Its average atomic mass is about 35.45 amu, not exactly 35 or 37, due to the weighted average.
Differences Between Mass Number and Atomic Mass
While the mass number gives you an integer approximation, the atomic mass provides a more precise, average value.
| Feature | Mass Number | Atomic Mass |
|---|---|---|
| Definition | Sum of protons and neutrons | Weighted average mass of all isotopes of an element (measured in amu or Da) |
| Type | Integer value | Decimal value |
| Usage | Defines a specific isotope | Used to describe an element and perform calculations involving mass or molar mass |
Summary
In summary, while the atomic mass is technically a more precise measurement and can vary slightly for each isotope, it is closely approximated by the mass number. The mass number is the sum of the protons and neutrons, which are the primary contributors to the mass of an atom.
