prep 2

[مستر محمد طلعت]

Many attempts are made by scientists for classification of elements why?
1-to be easily studied
2-find the relation between elements and their physical and chemical properties.
Mendeleev`s table:
He discovered that the elements were arranged in an ascending order according to their atomic weights from left to right in the horizontal rows.
disadvanges of Mendeleev`s table:
1-He had to make a disturbance in the ascending order of atomic weights for some elements due to putting them in groups which suit their properties.
2-he also would have to deal with the isotopes of one element as different elements because they are different in their atomic weights.
Advantages:
1- Mendeleev predicted the ability of discovering new elements so he left spaces (empty cells) in his table.
2- He corrected the wrong estimated atomic weights of some elements.


Moseley `s table:
1- Moseley rearranged the elements in an ascending order according to their atomic number.
2- Added the inert gases in the (0) zero group.
3- classified each main group into two subgroups (A , where he found differences between their properties
4- he allocated a place below his periodic table for the two groups lanthanides and actinides

What is the scientific principle on which the classification of the elements based on?
• Mendeleev’s periodic table: .................................................. ..................................................
• Moseley’s periodic table............................................. .................................................. ..
Note;
1-Rutherford discovered that the nucleus of atom contains positively charged protons
2-Moseley discovered in the same year after studying the properties of X rays that the periodicity of elements properties is related to their atomic number not their atomic weights.
Modern periodic table:
1-Bohr discovered the main energy levels they are 7.
2-main energy level consists of a number of energy levels known as the energy sublevels.
Description:
1-Elements are classified in the modern periodic table according to
Their atomic numbers the way of filling the energy sublevels with electrons.