4.3 CHAPTER 4. THE ATOM
How heavy is an atom? ESAAV
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 1 , 99 × 10 −^26 kg, while
the mass of an atom of hydrogen is about 1 , 67 × 10 −^27 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 describ-
ing the mass of an atom. This unit is called theatomic mass unit(amu). We can abbreviate
(shorten) this unit to just u. Scientists use thecarbon standardto determine amu. The
carbon standard gives carbon an atomic mass of 12 , 0 u. Compared to carbon the mass of
a hydrogen atom will be 1 u. Atomic mass units are therefore not giving us theactualmass
of an atom, but rather its massrelativeto 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 table 4.1.
Element Atomic mass (u)
Carbon (C) 12 , 0
Nitrogen (N) 14 , 0
Bromine (Br) 79 , 9
Magnesium (Mg) 24 , 3
Potassium (K) 39 , 1
Calcium (Ca) 40 , 1
Oxygen (O) 16 , 0Table 4.1: The atomic mass number of some of the elements.The actual value of 1 atomic mass unit is 1 , 67 × 10 −^24 g or 1 , 67 × 10 −^27 kg. This is a very
tiny mass! If we write it out it looks like this: 0 ,000 000 000 000 000 000 000 000 167kg.
An atom is therefore very very small.
68 Chemistry: Matter and Materials