Summary
Here are the key differences between relative atomic mass and mass number:
| Feature | Relative Atomic Mass (Ar) | Mass Number (A) |
|---|---|---|
| Definition | The weighted average mass of an atom of an element, compared to 1/12th the mass of a carbon-12 atom. | The total number of protons and neutrons in the nucleus of an atom. |
| Symbol | Ar | A |
| Applies To | Elements (accounts for isotopes) | Individual atoms/isotopes |
| Decimal or Whole? | Usually a decimal (due to averaging isotopes). | Always a whole number. |
| Example (Chlorine) | 35.5 (due to Cl-35 & Cl-37 isotopes) | 35 or 37, depending on the isotope. |
In short: Mass number is specific to an isotope, while relative atomic mass is an average considering isotope abundance.
Mass Number
The mass number of an atom indicates how heavy it is. Since protons and neutrons make up most of an atom’s mass, their total number determines its overall mass. The mass of electrons is negligible in comparison and is usually ignored.
Thus, the mass number of an atom is the sum of its protons and neutrons:
Mass number = Number of protons + Number of neutrons
For example, an oxygen atom has 8 protons and 8 neutrons, giving it a mass number of 16. However, the number of protons is not always equal to the number of neutrons. For instance, a sodium atom has 11 protons and 12 neutrons, resulting in a mass number of 23.
Here are the mass numbers of the first 20 elements:
| Element | Symbol | Atomic Number | Protons | Electrons | Neutrons | Mass Number |
|---|---|---|---|---|---|---|
| Hydrogen | H | 1 | 1 | 1 | 0 | 1 |
| Helium | He | 2 | 2 | 2 | 2 | 4 |
| Lithium | Li | 3 | 3 | 3 | 4 | 7 |
| Beryllium | Be | 4 | 4 | 4 | 5 | 9 |
| Boron | B | 5 | 5 | 5 | 6 | 11 |
| Carbon | C | 6 | 6 | 6 | 6 | 12 |
| Nitrogen | N | 7 | 7 | 7 | 7 | 14 |
| Oxygen | O | 8 | 8 | 8 | 8 | 16 |
| Fluorine | F | 9 | 9 | 9 | 10 | 19 |
| Neon | Ne | 10 | 10 | 10 | 10 | 20 |
| Sodium | Na | 11 | 11 | 11 | 12 | 23 |
| Magnesium | Mg | 12 | 12 | 12 | 12 | 24 |
| Aluminium | Al | 13 | 13 | 13 | 14 | 27 |
| Silicon | Si | 14 | 14 | 14 | 14 | 28 |
| Phosphorus | P | 15 | 15 | 15 | 16 | 31 |
| Sulfur | S | 16 | 16 | 16 | 16 | 32 |
| Chlorine | Cl | 17 | 17 | 17 | 18 | 35 |
| Argon | Ar | 18 | 18 | 18 | 22 | 40 |
| Potassium | K | 19 | 19 | 19 | 20 | 39 |
| Calcium | Ca | 20 | 20 | 20 | 20 | 40 |
Relative Atomic Mass
The mass of an atom is extremely small, making it difficult to measure the mass of a single atom directly. To overcome this challenge, scientists found it more practical to compare the masses of different atoms instead. For example, how does the mass of a lead atom compare to that of an oxygen atom?
For convenience, chemists decided to compare atomic masses using a reference atom. Initially, hydrogen, the lightest element, was chosen as the standard and assigned an atomic mass of 1. The mass of any other atom relative to hydrogen became known as its relative atomic mass (RAM).
However, using hydrogen as the standard proved unreliable because it has two common isotopes, and their proportions vary depending on the source. Since this ratio isn’t constant, measurements based on hydrogen lacked the precision needed for accurate atomic mass comparisons.
Carbon-12 was ultimately chosen as the standard for measuring relative atomic masses for several reasons (see potential reasons below). Today, atomic masses are based on the carbon-12 scale instead of hydrogen.
On this scale, a carbon-12 atom, which contains 6 protons and 6 neutrons, has a mass of exactly 12 atomic mass units (amu). The relative atomic mass (RAM) of an element is therefore the average mass of its atoms relative to 1/12 of the mass of a carbon-12 atom.
Thus, RAM can also be precisely defined as 1/12 of the mass of a carbon-12 atom. Since relative atomic mass is a ratio, it has no units.
Looking to solve relative atomic mass calculations? Our calculator can help—give it a try! 😊
Why Chemists Decided To Settle On Carbon 12 as the Standard for RAM Instead Of Hydrogen
You might be wondering why chemists chose carbon-12 as the standard for relative atomic mass (RAM) instead of oxygen. Well, here are the reasons:
- Stability and Abundance – Carbon-12 is a very stable and naturally abundant isotope, making it a reliable reference.
- More Representative Mass – Hydrogen has an atomic mass close to 1, which means using it as a standard would result in many elements having non-integer atomic masses, making calculations less convenient.
- Less Affected by Isotopes – Hydrogen has a significant percentage of heavier isotopes (deuterium and tritium), which could cause inconsistencies in mass measurements. Carbon-12, on the other hand, is the most abundant carbon isotope, reducing variability.
- More Accurate Mass Measurements – Carbon-12 provides a better balance when measuring atomic masses of heavier elements, as its mass is closer to the average atomic masses of most elements.
- International Agreement – In 1961, the International Union of Pure and Applied Chemistry (IUPAC) officially adopted carbon-12 as the standard because it provided more precise and consistent atomic mass calculations.
- Ease of Use in Mass Spectrometry – Carbon-12 is widely used in mass spectrometry, a technique essential for determining atomic and molecular masses, making it a practical standard.
