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Class 10 science CHP 5 periodic classification of elements notes
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CHAPTER – 5PERIODIC CLASSIFICATION OF
ELEMENTS
q Elements : Substances containing atoms of only one type. eg. Na, Au, Mg etc.
– There are around 118 elements known to us.
q Elements are classified to make the study easy.
q Dobereiner’s Traids : When the elements were written in order of increasing
atomic masses the atomic mass of the middle was the average of the atomic
mass of the other two elements.
eg. Elements Atomic Mass
Ca 40.1
Sr 87.6
Ba 136.3
Limitations : Only three triads were recognised from the elements known at
that time.
q Atomic mass of an element is the relative mass of its atom as compared widh
the mass of a Carbon-12 atom taken as 12 units
q Newland’s law of octaves :
– Based on increasing atomic mass of elements.
– When elements are arranged it was found that every eighth element had
properties similar to that of the first. eg properties of sodium and Lithium are
the same.
Limitations :
– Applicable only upto Calcium
– Properties of new elements couldn’t fit in it.
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, – It some cases properties of the elements were not same as defined by octave.
– Worked well only with lighter elements.
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 Table
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.
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