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In This Part :
1. Mendeleev's periodic table:
It is the first real periodic table for classifying elements .
At this time, only 67 elements were known .
He arranged (classified) elements in an ascending order according to their atomic weights from left to right in horizontal rows which were known later as periods
He discovered that : The properties of elements were repeated periodically by the beginning of each new period .

2. Moseley's periodic table:
In 1913:
The New Zealand scientist Rutherford discovered that the nucleus of the atom contains positively charged protons
The English scientist Moseley discovered, that properties of elements are related to their atomic numbers and not to their atomic weights as Mendeleev believed.

The most important modifications of Moseley on Mendeleev's table :
1. He arranged elements in an ascending order according to their atomic numbers , where the atomic number of each element increases by one from the preceding element in the same period.
2. He added zero (0) group which includes inert (noble) gases to the table.

3. Modern periodic table:
Elements are classified in the modern periodic table in an ascending order according to :
• their atomic numbers.
• the way of filling the energy sublevels with electrons .

Description of the modern periodic table:
The modern periodic table consists of :
7 horizontal periods .
18 vertical groups.
The elements of modern periodic table are classified into 4 blocks (s, p, d and f).
Classification according to state of matter:
Solid state:
Most elements in modern periodic table are solids such as: Li , Na , C , P , Fe
Liquid state:
There are two elements: Hg (only liquid metal) , Br (Only liquid nonmetal)
Gas state:
The are (11) elements in Gas state:
6 gases are inert gases : He , Ne , Ar , kr , Xe , Rn
5 gases are nonmetals : H , N , O , F , Cl

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