ساينس الصف الثاني الاعدادي – الفصل الدراسي الاول ساينس الثاني اعدادي – ترم اول In This part we will study : By increasing the number of discovered elements, scientists classified them according to their properties in order to : – Facilitate their study. – Find a relationship between elements and their chemical and physical properties . 1. Mendeleev’s periodic table It is the first real periodic table for classifying elements . – At this time, only 67 elements were known . Advantages of Mendeleev’s table : 1. He left gaps (empty cells) in his table predicting the discovery of new elements. ‘ 2. He corrected the atomic weights of some elements which were estimated wrongly. Disadvantages of Mendeleev’s table : 1. He had to make a disturbance in the 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 as different elements due to the difference in their atomic weights (G.R) Mendeleev had to put more than one in one cell as iron, cobalt and nickel. Due to the similarity in their properties . لمتابعة مستر احمد الباشا وكل ما يخص الساينس : جروب الفيس بوك http://bit.ly/2Of1mcN صفحة الفيس بوك http://bit.ly/2Gq5xMo بالتوفيق مستر احمد الباشا