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Atomic Mass and Diameter

Introducing Atomic Mass and Diameter

It is difficult sometimes to imagine the size of an atom, or its mass, because we cannot see an atom and also because we are not used to working with such small measurements.

How heavy is an atom?

It is possible to determine the mass of a single atom in kilograms. But to do this, you would need special instruments and the values you would get would be very clumsy and difficult to work with. The mass of a carbon atom, for example, is about \(\text{1.99} \times \text{10}^{-\text{26}}\) \(\text{kg}\), while the mass of an atom of hydrogen is about \(\text{1.67} \times \text{10}^{-\text{27}}\) \(\text{kg}\). Looking at these very small numbers makes it difficult to compare how much bigger the mass of one atom is when compared to another.

To make the situation simpler, scientists use a different unit of mass when they are describing the mass of an atom. This unit is called the atomic mass unit (\(\text{amu}\)). We can abbreviate (shorten) this unit to just \(\text{u}\). Scientists use the carbon standard to determine \(\text{amu}\). The carbon standard gives carbon an atomic mass of \(\text{12.0}\) \(\text{u}\). Compared to carbon the mass of a hydrogen atom will be \(\text{1}\) \(\text{u}\). Atomic mass units are therefore not giving us the actual mass of an atom, but rather its mass relative to the mass of one (carefully chosen) atom in the periodic table. In other words it is only a number in comparison to another number. The atomic masses of some elements are shown in the table below.

Table: The Atomic Mass Number of Some of the Elements


Atomic mass (\(\text{u}\))

Carbon (\(\text{C}\))


Nitrogen (\(\text{N}\))


Bromine (\(\text{Br}\))


Magnesium (\(\text{Mg}\))


Potassium (\(\text{K}\))


Calcium (\(\text{Ca}\))


Oxygen (\(\text{O}\))


The actual value of 1 atomic mass unit is \(\text{1.67} \times \text{10}^{-\text{24}}\) \(\text{g}\) or \(\text{1.67} \times \text{10}^{-\text{27}}\) \(\text{kg}\). This is a very tiny mass! If we write it out it looks like this: \(\text{0.000000000000000000000000167}\) \(\text{kg}\). An atom is therefore very very small.

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