Class 10 Science notes

Question 1. Name two elements whose properties were predicted on the basis of their positions in Mendeleev’s periodic table. Answer: Gallium (Eka-aluminium) and germanium (Eka-silicon). Question 2. Write the formulae of chlorides of Eka-silicon and Eka-aluminium, the elements predicted by Mendeleev.  [NCERT Exemplar] Answer: Eka-aluminium represents gallium (Ga) with valency three and Eka-silicon is for germanium (Ge) with valency four. The formulae of their respective chlorides are GaCl 3  and GeCl 4 . Question 3. How many elements are there in period 2? Answer: Eight (8). Question 4. Name all the elements present in group-17 of the Modern Periodic Table. Answer: F, Cl, Br, I, At. Question 5. Name two elements whose atomic weight were corrected on the basis of their position in Mendeleev’s periodic table. Answer: Gold (Au) and platinum (Pt). Question 6. Arrange the following elements in the increasing order of their metallic character. Mg, Ca, K, Ge, Ga     [NC...

  Periodic Classification of Elements Chapter -5 Introduction to Periodic Classification of Elements Why Classification? Without the classification of elements, it would be extremely difficult and time-consuming to individually study the chemistry of all the elements.  Hence, to simplify and systematize the study of elements and their compounds, they are classified into groups and periods. Early Models of Periodic Table Dobereiner’s Triads Dobereiner arranged a group of three elements with similar properties in the order of increasing atomic masses and called it a triad. He showed that the atomic mass of the middle element is approximately the arithmetic mean of the other two. But, Dobereiner could identify only the following three triads from the elements known at that time. Newlands’ Law of Octaves sa (do) re (re) ga (mi)  ma (fa)  pa (so)  da (la) ni (ti) H Li Be B C  N  O F Na Mg Al Si  P  F Cl K Ca Cr Ti Mn Fe Co and ...