It is the number of protons found in the nucleus of an atom.
It is conventionally represented by the symbol Z.
The atomic number uniquely identifies a chemical element. When an atom is in neutral charge, the atomic number is equals to the number of electrons.
Atoms of the same element having the same number of neutrons but different number of protons. Isotopes have different atomic number but same atomic mass.
e.g. 12C6 and 14C6 where 12 and 14 are the atomic number of Carbon Isotopes 6 is the Atomic mass number.
Atomic Weight or Atomic Mass number:
Atomic weight is an older term; usually it is called atomic mass now. Atomic mass is the most accurate mass that scientists can figure out for the weighted average of all isotopes. It is the weight of an isotope relative to the weight of isotope of carbon whose atomic mass number is 12.
The periodic table is a tabular arrangement of the chemical elements, organized on the basis of their atomic numbers, electron configurations (electron shell model) and recurring chemical properties. Elements are presented in order of increasing atomic number. The rows of the table are called periods; the columns are called groups. Russian Chemist, Dmitri Mendeleev is generally considered to have developed the periodic table in 1869. He developed his table to illustrate periodic trends in the properties of the then-known elements.
Organization of periodic table
The periodic law: the rule that the behavior of the elements may be predicted from their positions on the table is known as the periodic law.
Classes: the periodic table uses column to indicate classes of elements having similar properties.
Groups: the vertical columns of elements in the periodic table are termed as groups. According to IUPAC these groups are numbered from 1 to 18.
The periods: the horizontal rows of elements in the periodic table are called periods. There are 7 periods in a periodic table. The 1st period consists of 2 elements the subsequent consists of 8,8, 18, 18 and 32 elements. The seventh period is incomplete.