JOHANN DOBEREINER’S TRIAD Arranged the elements in groups of three based on similarity in their properties Noted that if elements are arranged in order of increasing atomic mass, the average of the mass of the lightest and heaviest is very close to the mass of the middle member Element Atomic Mass Average Li 6.941 23.02 Na 22.99 K 39.10

JOHN NEWLAND’S LAW OF OCTAVES - if elements are arranged in terms of increasing atomic mass, every eight element had similar properties. - Compared this pattern with the musical scale 1 2 3 4 5 6 7 8 Li Be B C N O F Na Mg He Si P S Cl K do re mi fa sol la ti

                                                                              To Newlands, Li to Na is an octave of eight elements.

FEATURES OF THE PERIODIC TABLE PROPOSED BY DMITRI MENDELEEV 1. Arranged the elements in order of increasing atomic mass (Meyer did the same) 2. Gave attention to periodicity of chemical properties (Meyer did the same) 3. Left vacant spaces where an element should fit even when such an element was not yet known (Meyer did the same 4. Did not restrict the length of the periods to 8 elements but allowed the known properties to determine the length of the periods. 5. Allowed the chemical properties to prevail over the atomic mass. 6. Predicted the presence of missing elements based on the periodicity of properties.

PROPERTIES OF EKA-SILICON AND GERMANIUM PROPERTY Eka-silicon as predicted by Mendeleev Germanium (Ge) as actually determined later Atomic Mass 72 72.6 Physical Appearance Grey metal Density 5.5 g/ml 5.36 g/ml Melting Point High 958 oC Formation of Oxide EsO2 GeO2 Density of Oxide 4.7 4.70 Reaction with HCl slight None at 25 oC

THEORETICAL BASIS OF THE PERIODIC TABLE 1. Elements with the similar properties are placed in a vertical columns called groups, which are numbered from 1 to 18. 2. Elements in the same group have the same number of valence electrons and have similar electronic configurations. 3. A new period or row is started with the element which has one electron in its outermost main energy level.

Inner transition metals

PROPOSED VERSIONS OF THE PERIODIC TABLE

Same atomic number but different atomic mass number ISOTOPES Particles having the same number of protons but different numbers of neutrons. Same atomic number but different atomic mass number

Weighted average atomic mass Weighted average of the masses of the isotopes of an element Element Symbol Natural abundance Mass (amu) Weighted average atomic mass Oxygen 16 8 O 99.759 15.995 (0.99759 X 15.995) + (0.00037 X 16.995) + (0.00204 X 17.999) = 15.999 17 8 O 0.037 16.995 18 8 O 0.204 17.999