MENDELEEV’S PERIODIC TABLE
- Dmitry Mendeleev a Russian chemist in 1869 gave Mendeleev’s Periodic Table.
- Till then 63 elements were known.
- Mendeleev arranged elements in increasing order of their atomic mass.
- He tried to put elements with similar properties in a group.
- Due to this we find empty boxes in his table.
- Mendeleev classified elements in the increasing order of their atomic masses and similarities in their properties.
- The formulae of the oxides and hydrides formed by the elements was also the basis for the classification of the elements.
- Mendeleev’s periodic table has 6 horizontal rows called periods and 8 vertical rows called groups.
- Groups 1 to 7 had two subgroups called A subgroup and B subgroup. Group 8 had 3 rows of elements.
- Elements having similar properties were placed in the same groups.
- There are some spaces left vacant in the table to accommodate the elements to be discovered in future.
PROPERTIES OF GROUPS STUDIED BY MENDELEEV
Formation of Oxides:
- Oxides are compounds of elements with oxygen.
- For example:
- Li2O , Na2O and K2O resembles to R2
- MgO, CaO, ZnO resembles to RO
Formation of Hydrides:
- Hydrides are compounds of elements with hydrogen.
- The horizontal rows present in the periodic table are called periods.
- The vertical columns present in it are called groups.
- There were total eight groups in Mendeleev’s periodic table, I to VIII.
- Properties of elements in a particular period show regular gradation (i.e. increase or decrease) from left to right.
- Groups I to VII are subdivided into A and B subgroups. Groups VIII don’t have any subgroups.
- All the elements in a particular group have similar properties. They show regular gradation in their physical properties and chemical reactivities.
Merits of Mendelev Periodic Table
- Elements were classified on a more fundamental basis of their atomic masses and properties.
- Spaces were left vacant to accommodate the elements to be discovered in future .
- It could predict the properties of the elements which helped in the discovery of new elements.
- he proposed the existence of some unknown elements
- Eka – boron → Scandium, Eka – aluminium → Gallium, Eka – silicon → Germanium
- The inert gas elements discovered later could be placed in a separate group without disturbing the table.
- Mendeleev’s periodic table could accommodate noble gases when they were discovered.
- When a whole new group of elements called noble gases were discovered, it got a place in the periodic table in the form of a separate group.
- It did not disturb the original arrangement of Mendeleev’s periodic table.
- The noble gases are placed in a separate group because they are chemically unreactive.
- In fact, the noble gases were discovered very late because they are very unreactive (or inert), and present in extremely low concentration in the atmosphere
Demerits of Mendeleev Periodic Table
- Some elements are not arranged in the increasing order of their atomic masses.
- Co is placed before Ni, Te is placed before I etc.
- Position of hydrogen is not clear because it shows properties similar to metals as well as non metals.
- The position of isotopes of elements is not clear.
Explanation of the Anomalies of Mendeleev’s Classification of Elements
- The explanation for the Position of Isotopes. All the isotopes of an element have the same number of protons, so their atomic number is also the same.
- Since all the isotopes of an element have the same atomic number, they can be put at one place in the same group of the periodic table.
- For example, both the isotopes of chlorine, Cl-35 and Cl-37, have the same atomic number of 17, so both of them can be put at one place in the same group of the periodic table.
- Explanation for the Position of Cobalt and Nickel. The atomic number of cobalt is 27 and that of nickel is 28.
- Now, according to modern periodic law, the elements are arranged in the order of increasing atomic numbers.
- So, cobalt with lower atomic number (27) should come first and nickel with a higher
atomic number (28) should come later, even if their atomic masses are in the wrong order.
- Besides gallium, which other elements have since been discovered for which gaps were left by Mendeleev in his periodic table ? (any two)
Ans. Scandium (Sc) and Germanium (Ge).
- What were the criteria used by Mendeleev in creating his periodic table?
Ans. Mendeleev used two criteria in creating his periodic table :
(i) increasing atomic masses, and
(ii) grouping together of elements having similar chemical properties.
Mendeleev took the formulae of the oxides and hydrides formed by the elements as the basic chemical properties of elements for their classification in the form of a periodic table.
- Why do you think the noble gases are placed in a separate group?
Ans. The noble gases are placed in a separate group because they are chemically very inert or unreactive (having completely filled outermost electron shells).
- (a) On what basis did Mendeleev arrange the elements in his periodic table?
(b) On what basis are they arranged now?
- State whether the following statements are true or false :
(a) Newlands divided the elements into horizontal rows of eight elements each.
- (b) According to Mendeleev’s periodic law, the properties of elements are a periodic function of their atomic numbers.
(c) The elements in a group have consecutive atomic numbers.
- Name the Russian chemist who said that the properties of elements are a periodic function of their atomic masses.