The slide are about the basics of chemistry. We all know that matter is composed of atoms. Few of these atoms have isotopes and with that we are able to calculate the atomic mass

2.  Isotopes are two atoms of the same elements
having the same atomic number but different
mass number. For example, there are two
atoms of an atom called magnesium below.
12
12
Mg
Mg
23
24

3.  The two atoms have the same atomic number
of 12 but the only different is the mass numbers
which are 23 and 24. This indicates that there
are 12 protons, electrons and neutron in an
atom which has the mass number of 24 and in
an atom with a mass number of 23 indicates
that there are 12 protons, electrons and 11
neutrons in an atom.