Periodic chemistry is a study of variation of properties of
elements with their atomic number. This brings out the hidden order and pattern
among elements. In 1869 Mendeleev a Russian Scientist stated that, the
properties of the element are a periodic function of their atomic number. This
periodic law means that if elements are arranged in increasing relative atomic
number or masses, elements of similar properties appear at regular intervals.
Mendeleev’s periodic table was incomplete because many element example noble
gases were not known at that time and it contain several errors which have
since been corrected. A modern form of the periodic table contains element in
order of their proton numbers or atomic numbers. The horizontal rows are periods
and the vertical columns are called groups.
Electrons and empty space surrounding the nucleus accounts
for almost for almost all the volume of an atom. The electrons in an atom can
move in seven different shells or energy level. Represented by. K ,L,M,N, O, P,
Q
The number of electrons in each shell depends on the atomic
number of element. The closer the shells to the nucleus, the lower the energy
of its electrons compared to the energy of the electrons further away.
The arrangement of electrons on atoms is known as electron
configuration .This can be represented by drawing or writing the number of
electrons in the shell.
Bohr’s assumption established the foundation of quantum
mechanics i.e. the study of laws of motion that governs the behavior of small
particles and also applied to large bodies. The basic principle is that only
specified energy levels are possible for electrons in an atom. These energy levels
are numbered starting with the lowest 1 then 2,3 etc.
The second principle of quantum mechanics is that the electron population of
any energy level is limited to 2n2 where
n is principle quantum number. This means that for the lowest energy
n=1. The maximum population is 2(1)2=2 be in K shell or the innermost orbit. These
electrons are the ones most tightly bound to the nucleus. For the second
energy level 2(2)2=8 and is L.
The higher energy levels are number n=3,4,5 etc. or lettered
M ,N, O. Higher energy levels are referred to as outer energy levels. Hence the
electrons in the outermost shell have the highest energy. They are called
valency electrons and the shell in which they are found is called valency shell.
The valency electron are least attracted to the protons and hence they can
easily be lost, shared or gained by other electrons. They are therefore
responsible for chemical properties of the element.
