What Is Atomic mass unit?
The atomic mass unit is a very small unit of mass that scientists use to measure how heavy atoms and tiny particles are. It gives a simple number instead of long decimals in kilograms or grams.
Definition
The atomic mass unit, often called unified atomic mass unit, is defined in this way:
- 1 atomic mass unit is exactly one twelfth of the mass of one carbon 12 atom.
- In kilograms, 1 atomic mass unit is about 1.66 × 10−27 kg.
- In grams, 1 atomic mass unit is about 1.66 × 10−24 g.
Because atoms are extremely small, using atomic mass units makes their masses easier to write and compare. For example, a hydrogen atom has a mass close to 1 atomic mass unit, and a carbon 12 atom has a mass of exactly 12 atomic mass units.
History / Origin
The idea of an atomic mass unit came from chemists in the 1800s. They needed a way to compare the masses of different atoms without using very tiny fractions of grams.
- At first, some scientists based the scale on hydrogen, the lightest element.
- Others used oxygen as the reference because it was common in many compounds.
- This caused confusion because different groups used different reference atoms.
To fix this, in 1961 the international chemistry groups agreed on a single standard. They chose the carbon 12 atom as the base. They decided that 1 atomic mass unit would be exactly one twelfth of the mass of a carbon 12 atom.
Later, physics and chemistry groups agreed to call this the unified atomic mass unit. The symbol “u” and the name “dalton” were adopted so that everyone could use the same clear system worldwide.
Symbol & Abbreviation
The atomic mass unit is written in a few common ways:
- u – main symbol for the unified atomic mass unit.
- Da – dalton, another name for the same unit.
- amu – older style abbreviation that is still seen in some books.
Today, scientists prefer to use “u” or “Da”. They both mean exactly the same thing.
Current Use Around the World
The atomic mass unit is used everywhere in science when talking about atoms and molecules.
- Chemistry – to give the relative masses of atoms and to find molar masses of compounds.
- Physics – to describe the masses of protons, neutrons, electrons, and other tiny particles.
- Biology and biochemistry – to show the size of proteins, DNA, and other large molecules, often in kilodaltons (kDa).
- Medicine and drug design – to write the molecular masses of medicines and chemicals.
Although the official SI base unit of mass is the kilogram, the atomic mass unit is approved for use with SI in these scientific fields because it is much more convenient at the atomic scale.
Example Conversions
Here are some simple example conversions using rounded values:
- 1 u ≈ 1.66 × 10−27 kilograms
- 1 u ≈ 1.66 × 10−24 grams
| Mass in atomic mass units | Mass in kilograms (approximate) | Notes |
|---|---|---|
| 1 u | 1.66 × 10−27 kg | Basic unit |
| 12 u | 1.99 × 10−26 kg | Mass of one carbon 12 atom |
| 16 u | 2.66 × 10−26 kg | Close to mass of one oxygen 16 atom |
| 18 u | 2.99 × 10−26 kg | About mass of one water molecule |
Some quick comparison facts:
- Mass of a proton ≈ 1.007 u.
- Mass of a neutron ≈ 1.008 u.
- Mass of an electron ≈ 0.00055 u.
Related Units
Units that are closely related to the atomic mass unit include:
- Dalton (Da) – exactly the same as 1 u. Used very often in biology and biochemistry.
- Kilodalton (kDa) – 1 kDa = 1000 Da = 1000 u.
- Gram (g) – 1 g ≈ 6.02 × 1023 u.
- Kilogram (kg) – the SI base unit of mass. 1 kg ≈ 6.02 × 1026 u.
- Relative atomic mass – a number that compares the mass of an atom to 1 u. It uses atomic mass units as its scale.
FAQs
Is atomic mass unit the same as dalton?
Yes. 1 atomic mass unit is exactly equal to 1 dalton. The symbols u and Da both represent the same size of mass.
Why do scientists use atomic mass units instead of grams?
Atoms are incredibly light, so their masses in grams are very tiny decimal numbers. Using atomic mass units gives small, easy to read numbers like 1, 12, or 56 instead of long decimals.
What is the exact definition of 1 atomic mass unit?
1 atomic mass unit is exactly one twelfth of the mass of a neutral carbon 12 atom that is not moving and is in its lowest energy state.
Is atomic mass unit an SI unit?
No. The official SI unit of mass is the kilogram. However, the atomic mass unit is allowed for use with SI and is standard in chemistry and physics.
How do I convert from atomic mass units to grams?
To change u to grams, multiply the value in u by 1.66 × 10−24. For example, 18 u × 1.66 × 10−24 g per u ≈ 3.0 × 10−23 g.
What is the mass of a carbon 12 atom in atomic mass units?
By definition, a carbon 12 atom has a mass of exactly 12 atomic mass units. This definition is used to set the size of the unit.