Relative Atomic Mass and Atomic Mass Unit
Q19: Discuss the concept of relative atomic mass and atomic mass unit. Also discuss relative atomic masses of some elements.
Relative Atomic Mass
Background
Atoms are super tiny, so measuring their actual mass is really hard. For example, a hydrogen atom weighs about 1.672 × 10⁻²⁴ grams, which is too small to measure with regular tools. To make things easier, scientists compare atomic masses to a standard atom. The standard is carbon-12, which has been given a mass of exactly 12 atomic mass units (amu). One atomic mass unit is 1/12 of the mass of a carbon-12 atom.
Definition
The relative atomic mass of an element is the average mass of its atoms compared to 1/12 of the mass of a carbon-12 atom.
Examples
- Hydrogen (H) = 1 amu
- Oxygen (O) = 16 amu
- Magnesium (Mg) = 24 amu
- Calcium (Ca) = 40 amu
- Sodium (Na) = 23 amu
Atomic Mass Scale
The atomic mass scale is based on carbon-12. Scientists measure the mass of other atoms by comparing them to carbon-12.
Atomic Mass Unit
One atomic mass unit (amu) is exactly 1/12 of the mass of a carbon-12 atom.
Relative Atomic Mass Table
| Element | Relative Atomic Mass |
|---|---|
| Hydrogen (H) | 1.008 amu |
| Aluminum (Al) | 26.9815 amu |
| Nitrogen (N) | 14.0067 amu |
| Sulfur (S) | 32.06 amu |
| Oxygen (O) | 15.9994 amu |
| Chlorine (Cl) | 35.453 amu |
| Sodium (Na) | 22.9898 amu |
| Iron (Fe) | 55.847 amu |
How can we determine the number of isotopes of a specific element, their masses, and also most abundant isotopes for a specific element?
Understanding Mass Spectrometry
If you have a solid or liquid sample, heat can turn it into gas. If it's already a gas, you can use it as is. Here, we use neon gas as a sample.
Electrons move quickly from a tungsten coil to an electron trapper. As they move, they hit neon gas atoms and give them a positive charge. The positive atoms move away from the positive plate and rush toward the negative plate. This makes them move fast into the magnetic field. This step is key in a mass spectrometer.
When these charged atoms enter the magnetic field, they bend based on their mass. Lighter atoms bend more than heavier ones. You can see this in the figure. Finally, the ions land on a detector. This detector counts them and measures their masses.
3.4 Relative Atomic Mass and Atomic Mass Unit
Historical Context
- Early chemists (Dalton, Gay Lussac, Lavoisier, Avogadro, Berzelius) made initial calculations based on element combination proportions in compounds
- Direct measurement of atomic masses not possible due to the extremely small size of atoms
Standardization of Atomic Mass
- 1961: Carbon-12 isotope chosen as the standard
- Carbon-12 assigned a value of exactly 12 atomic mass units (amu)
- All other atomic masses compared to this standard
Atomic Mass Unit (amu)
Definition: One-twelfth the mass of a carbon-12 atom
Used to express atomic mass (also known as atomic weight)
Relative Atomic Mass
The mass of an atom compared to the mass of the carbon-12 standard
Expressed in atomic mass units (amu)
Examples of Relative Atomic Masses
- Hydrogen: ~1.008 amu (about 8.40% as massive as carbon-12)
- Oxygen: 15.9994 amu
- Sodium: 22.9898 amu
- Aluminum: 26.9815 amu