PERIODIC CLASSIFICATION OF ELEMENTS
Mendeleev’s Periodic Table
- To organize all the 63 elements then known, Mendeleev composed a table known as the periodic table.
- Mendeleev’s Periodic Table arranged elements in order of increasing atomic mass. The atomic mass is the mass of an atom. It is about equal to the mass of the protons plus the neutrons in an atom.
- He observed a periodic trend between the chemical properties of the elements and their compounds. Mendeleev formulated a periodic law state that: ‘The properties of elements are the periodic function of their atomic masses’.
- Mendeleev’s periodic table consists of vertical columns called ‘groups and horizontal rows called periods’. He arranged the elements with similar properties into each group. He left gaps for elements that is to be discovered.
- He was able to forecast the properties of these undiscovered elements based on the chemical properties and physical properties of the elements adjacent to the gaps. Mendeleev used the gaps in his periodic table to predict three elements, namely scandium, gallium, and germanium. These elements were known respectively as eka-boron, eka-aluminum, and eka-silicon.
- When Mendeleev developed the periodic table, he had to position some elements with a higher atomic mass before others with a lower mass. Atomic mass of iodine is lower than that of tellurium. Mendeleev’s tables should therefore put iodine before tellurium. Iodine, however, shares many properties with chlorine and bromine. Mendeleev switched tellurium and iodine positions in his table to make iodine line up with chlorine and bromine.
Demerits of Mendeleev’s classification
- In the periodic table, Hydrogen was not positioned appropriately.
- There was no regular trend in increasing of atomic mass when moved along the elements. This made difficult to one to find how many elements lies between two elements.
- The isotopes were found later which failed to comply with the periodic table.
The Modern Periodic Table
- In 1913, English physicist Henry Moseleyshowed the atomic number (Z) of an element is a more important property than its atomic mass. Later, Henry Moseley modified Mendeléev’s Periodic table with atomic numbers.
- Atomic number of an element is the same as the number of protons in its nucleus.
- Modern periodic table consists of 18 vertical columns called as groups, where the elements have similar properties and 7 horizontal rows known as periods.
- All elements in one group have the same number of valence electrons. Number of valence electron for Fluorine: 7 and that of chlorine: 7
- The number of shells increases as we go down the group.
- Number of valence shell electrons increase by one unit on moving from left to right in a period.
- Few trends to remember in Modern Periodic Table
- Valency– valency of the elements can be identified from the outermost shell of the atom.
- Atomic size– Distance between the centre of the nucleus and the outermost shell of an isolated atom.
- Metallic and non-metallic properties-a zig-zag line separates metals from non-metals with semimetals or metalloids at the borderline.