Why
do the atoms of different elements differ from each other?
The numbers of protons present in the atom of an element
are different from those present in the atom of another
element. Thus, the number of protons present in the atom
of each element is fixed and is a characteristic property
of that element as you have already learnt in lesson 2.
This number is called the atomic number and is
denoted by Z .Hydrogen has one proton in its nucleus and
therefore, its atomic number is 1. Similarly, two protons
are present in the nucleus of helium atom and hence its
atomic number is 2. What about the number of electrons present
in hydrogen and helium? Since the atom is electrically neutral,
the number of electrons present in these atoms is 1 and
2 respectively.In addition to the protons, the helium atom
also has neutrons present in its nucleus. The total number
of protons and neutrons present in the nucleus of an atom
of an element is called its mass number. It is
denoted by A. Helium nucleus contains 2 protons and 2 neutrons;
hence, its mass number is 4.The atomic number and the mass
number of an element (X) can be denoted as follows:
AZX
Thus,
helium can be represented as 42H
Similarly, 126C means that the carbon atom has 6 protons
and hence 12–6 = 6 neutrons. But some carbon atoms
can have 7 or 8 neutrons also. The mass number of these
carbon atoms would be 6+7=13 or 6+8=14.Such atoms which
have the same atomic number but have different mass number
are called isotopes. Thus, carbon has three isotopes.
These isotopes can be represented as shown below:
126C,
136C, 146C
