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Mendeleev's periodic law :– The properties of elements are the periodic function of their atomic mass.
Mendeleev's periodic table based on the chemical properties of elements.
Contain eight vertical columns called groups and seven horizontal rows called periods form Mendeleev’s peridic table.
Achievements of Mendeleev’s Periodic table
– Elements with similar properties could be grouped together
– Some gaps were left for the undiscovered elements.
– Noble gases could be placed without disturbing the existing order.
Limitations:
– No fixed position for hydrogen
– No place for isotopes
– No regular trend in atomic mass.

Modern Periodic Law : Properties of elements are a periodic function of their atomic number.
– Atomic Number – denoted by Z and equals to the no. of protons in the nucleus of an atom.
– Modern periodic table contains 18 vertical columns known as groups and 7 horizontal rows known as periods.
– Elements in a group have valence electrons
– No. of the shells increases as we go down the group.
– Elements in a period have same number of shells.
– Each period marks a new electronic shell getting filled.
– No. of elements placed in a particular period depends upon the fact that how electrons are filled into various shells.

– Maximum no. of electrons that can be accomodated in a shell depend on the formula 2n2 where n is the no. of the given shell.
eg. k shell – 2 x (1)2 = 2 elements in the first period L shell – 2 x (2)2 = 8 elements in the second period.
– Position of the element in the periodic table tells about its reactivity.

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