Atomic Number: Z:
The atomic number of an atom, denoted by Z, is the number of
protons present inside the nucleus of an atom.
Number of Electrons:
For a neutral atom, the number of electrons always equal the
number of protons (or atomic number; Z).
Mass Number: A:
The mass number of an atom, denoted by A, is the sum of the
number of protons and the number of neutrons present inside the nucleus of an
atom.
(Note: The mass of electrons is almost negligible as relative to
proton and/or neutron.)
Number of Neutrons:
For any case, the number of neutrons in an atom can be
calculated as:
Number of Neutrons = Mass Number - Atomic Number = A – Z
Isotopes:
Isotopes are atoms of the same element with same atomic numbers
but different mass numbers. This is because of the presence of different number
of neutrons in the nucleus.
Example: Cl - 35 and Cl - 37, are 2 isotopes of Chlorine. Both have 17 protons and 17
electrons but Cl - 37 has 20 neutrons, while Cl - 35 has 18 neutrons.
Isotopes &
Radioactivity:
Some isotopes of the same element are radioactive. For example, H - 3 (Tritium) is radioactive,
while H- 1 (Hydrogen) is not. The 2
neutrons in the Tritium nucleus make it unstable and radioactive in nature.
According to a thumb’s rule; an atom is unstable if the
neutron-proton ratio increases 1.5:1. In Tritium, the neutron-proton ratio is
2:1, thus it is unstable and radioactive.
Examples of Radioactive
Isotopes and their disintegration:
Radioactive decay of Uranium:
Uranium disintegrates to form Thorium and release alpha particles.
Radioactive decay of Thorium:
Thorium disintegrates to form Protactinium and release beta particles.
Note the atomic number of Thorium. It is 90. Number of neutrons
in Thorium are:
A – Z = 234 – 90 = 144.
Note the atomic number of Protactinium. It is 91. Number of
neutrons in Protactinium are:
A – Z = 234 – 91 = 143.
In the Radioactive decay of Thorium, 1 neutron decays or
breaks down to form a proton and an electron. Therefore, the atomic number of
the daughter element increases by 1 (from 90 to 91) while mass number remains
same. The resulting electron from this decay is very energetic and is released
from the nucleus as beta particle.
Radioactive decay of Protactinium:
Protactinium disintegrates to form Uranium and release beta particles and gamma radiations.
Note the atomic number of Protactinium. It is 91. Number of
neutrons in Protactinium are:
A – Z = 234 – 91 = 143.
Note the atomic number of Uranium. It is 92. Number of neutrons
in Uranium are:
A – Z = 234 – 92 = 142.
In the Radioactive decay of Protactinium, 1 neutron decays or is
broken down to form a proton and an electron. Therefore, the atomic number of
the daughter element increases by 1 (from 91 to 92) while mass number remains
same. The resulting electron from this decay is very energetic and is released
from the nucleus as beta particle. This
decay also yields gamma rays.
Details about radioactivity will be discussed in Chapter 26.
