User:Yasmine adel abdallah

Unit (1) – Lesson (1): Attempts of Elements Classification Mendeleev's periodic table: He arranged (67) elements of similar properties in vertical columns (groups), in an ascending order according to their atomic weights which increase by 1 from left to right in horizontal rows (periods). Advantages of Mendeleev's periodic table: 1-	He left gaps for discovery of new elements. 2-	He corrected wrong atomic weights of some elements. Disadvantages of Mendeleev's periodic table: 1-	He had to make a disturbance in ascending order of atomic weights of some elements to put them in groups that suit their properties. 2-	He had to deal with the isotopes of one element are different elements due to the difference in their atomic weights. So he had to put more than one element in one place. Rutherford: discover the protons inside the nucleus. Moseley's periodic table: 1-	He arranged elements in according to their atomic numbers which increases by 1 in the same period. 2-	He added (0) group which includes inert (noble) gases. 3-	He divided each group into (2) subgroups due to the differences between its elements properties. 4-	He specified a place below the table for lanthanides and actinides elements. Bohr: discovered the main 7 energy levels of the atom. Modern periodic table: -	Scientists discovered that each main level contains other levels (energy sublevels). -	Elements are classified in the Modern periodic table according to: 1-	Atomic number. 2- The way of filling the energy sublevels with electrons. Description of the modern periodic table: consists of (7) horizontal periods – (16) groups. -	The number of elements till now is (116), 92 of them are abundant, while the rest prepared artificially. -	The elements are classified into 4 blocks (s, p, d, f). Block	Location	Elements arrangement s	left	(2) groups: (1A), (2A) p	right	(6) groups: (3A), (4A), (5A), (6A), (7A) and (0) d	middle	(8) Groups: (3B), (4B), (5B), (6B), (7B), (8), (1B) and (2B). - Elements called "Transition elements". - Start from period (4). f	below	Lanthanides and actinides elements. Notes: 1-	The number of energy levels indicates the period number. 2-	The number of electrons in outermost energy level indicates the group number. Elements of the same group are: 1-	 Similar in chemical properties, because they have the same number of electrons in outermost energy. 2-	Different in the number of energy levels. Elements of the same period are: 1-	Different in chemical properties, because they don't have same number of electrons in outermost energy. 2-	Similar in the number of energy levels.