A table of elements obtained by arranging them in order of their increasing atomic number in which
elements having similar properties are placed in the same group is called Periodic Table.
The vertical column of elements in the periodic table are called Groups.
The horizontal rows of elements in the periodic table are called Periods.
The repetition of physical and chemical properties of elements periodically is called Periodicity of
Physical and chemical properties of elements are periodic function of their atomic masses.
Elements which are good conductors of heat and electricity are malleable and ductile and have a
metallic luster are called Metals like Sodium, Potassium, Gold, Copper etc.
Elements which are non or bad conductor of heat and electricity are neither malleable or ductile and
have no metallic luster are called Non-Metals like Carbon, Nitrogen, Chlorine etc.
Metalloids are semi metals have the properties which are intermediate between a metal and non-metal
like Boron, Silicon, Germanium, Arsenic, Antimony etc.
Law of Triads
A German Chemist, Dobereiner (1829), arranged chemically similar elements in groups of three on the
basis of their atomic masses called Triads and it was found that atomic mass of the middle element was
approximately equal to the average of atomic masses of other two elements. This is k