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Uint one
Lesson (1)
Attempts of Elements Classification
* Many attempts are made by scientists for classification of elements:
1- To be easily studied
2- To find the relation between elements and their chemical and physical
properties.
Mendeleev's Periodic Table:
*It is the first real periodic table for classifying elements.
*He recorded on 67 single cards the names of elements, their atomic weights
and their important properties.
*He put the similar elements in vertical columns (groups).
*He arranged elements in an ascending order according to their atomic weights.
*He classified each main group into two subgroups (A,B) because he found
differences between their properties.
Advantages of Mendeleev's Table:
1-He left spaces in his table as he predicted the discovery of new elements.
2-He corrected the wrong estimated atomic weights of some elements.
Disadvantages of Mendeleev's Table:1-He had to make disorder in the
ascending arrangement of atomic weight for some elements because he put them
in groups which suit their properties.
2-He had to put more than one element in one cell in his table because he could
not deal with isotops which are forms of elements are different in their atomic
weights.
Rutherford
He discovered that the nucleus of the atom contains positively charged protons.
Moseley's Periodic Table:
*He discovered that the periodicity of elements properties is related to their
atomic number.
*He arranged the elements in an ascending order according to their atomic
number
*The atomic number of an element increases by one than the element before it in
the same period, and each period ends with inert gases in the (0) zero group.
*He allocated a place below his periodic table for lanthanides and actinides.
Bohr: He discovered the 7 main energy levels
Modern Periodic Table:
*Because of discovering energy sublevels ,118 elements are classified in a table
according to: 1-their atomic numbers
2- the way of filling the energy sublevels with electrons
*Description of the modern periodic table:
-The number of elements until now is 118 elements .92 elements are natural and
the rest are prepared artificially.
-It consists of 7 horizontal periods and 18 groups.
-The elements are classified into 4 blocks:
* S-block elements are located on the left side and consists of two groups.
*P-block elements are located on the right side and consist of six groups.
*D- block elements are located in the middle and consists of ten groups, they are
known as transition elements and they appear starting from period (4).
*f-block elements are located below the periodic table and they include
lanthanides and actinides.
How to locate the position of an element in the modern periodic table:
-The element period number=number of energy levels occupied by electrons in its
atom.
-The element group number=number of electrons in the outer most energy levels
in its atom.
Ex: 15P ,it lies in group (5) and in period (3).
How to determine the atomic number of an element by knowing its position in
the modern periodic table:
-Ex: element(y) exists in period(2) and in group(7),so its atomic number equals
9.