Question

what were Mendeleeve's achievements

Answer 1

1). element with a slightly greater atomic mass were placed 
before an element with a slightly lower atomic mass with an inverted sequence so as to group the elements with similar properties. For example, cobalt with atomic mass 58.9 was placed before nickel with atomic mass 58.7.

2). He left some gaps in the Periodic Table with the prediction of 
existence of some elements that were not discovered at that time.

3). He named the future elements by prefixing a Sanskrit numeral, Eka (one) to the name of preceding element in the same group.

4). For instance, scandium, gallium and germanium, discovered later, have properties
similar to Eka–boron, Eka–aluminium and Eka–silicon, respectively.

5).Noble gases like helium (He), neon (Ne) and argon (Ar) were discovered
later as they are present in exceptionally low concentrations in the atmosphere due to their inertness.

Answer 2

Hey mate

here is your answer



 


(a) Achievements of Modem periodic table are:
(i) Prediction of properties of elements could be made with more precision since elements were arranged on the basis of increasing atomic number.
(ii) Metals are separated from non-metals by a zig-zag line. On this zig-zag line borderline elements-like Boron, Silicon, Germanium, Arsenic, Antimony, Tellurium and Polonium which are also known as metalloids are present.
(b) Limitations of Mendeleev’s classification are:
(i) Anomalous pairs : Mendeleev couldn’t
explain the presence of anomalous pairs in which an element with more atomic mass preceded the element with lower atomic mass, e.g., Co and Ni.
(ii) Isotopes could not be accommodated in the same slot since it was Based upon atomic masses.
© (i) The anomaly of higher atomic mass element
preceding the lower atomic mass element was easily removed when these elements were placed in the order of increasing atomic number.
(ii) Isotopes could be placed in the same slot since elements are arranged on the basis of their valence electrons (atomic number).

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