Tag: mendeleev's periodic law and its limitations

Some gases are taking three times as long to effuse out as helium.

The rate of effusion is always inversely proportional to the square root molecular mass.

The rate of effusion is the ratio of the volume effused to the time taken as follows.

$ \dfrac{{V} _{1}}{{t} _{1}} \times \dfrac {{t} _{2}}{{V} _{2}}$  $=\sqrt {\dfrac{{M} _{{w} _{2}}}{{M} _{{w} _{1}}}}$

 Volume is same, so

$\dfrac{3}{1}= \sqrt{\dfrac{{M _{w2}}}{4}}$

So, $M _{w2} =  36\ unit.$

The molecular mass of the gas is 36 u.

So, the correct option is A.

Two earlier group number systems exist: CAS (Chemical Abstracts Service) and old IUPAC. Both use numerals (Arabic or Roman) and letters A and B. In 1988 the new system simply numbers the groups increasingly from left to right on the standard periodic table. 

Mendeleevs Periodic Table:
Elements are arranged in the periodic table in the increasing order of their relative atomic masses.
Mendeleev divided his periodic table in eight groups and seven periods.
Groups from I to VII are meant for normal elements and group VIII is for transition elements.
Groups from I to VII have been divided in two sub groups, while group VIII is meant for three elements.
Periods from 4th to 7th have been divided in two series: 1st series and 2nd series.
Elements having similar properties have been kept in the same group. For example; lithium, potassium, rubidium, etc. are in 1st group.

Mendeleeve is credited with the development of his periodic table to illustrate periodic trends of known elements and predicted properties of unknown elements also.

Mendeleev’s Periodic table says that the periodic function of their relative atomic masses. Mendeleev arranged all the 63 elements in it. These only were discovered in his time. These are in the order of their increasing relative atomic masses in a tabular form. It is the Periodic Table. He had the table with eight columns and seven rows. The columns are the groups whereas rows are the periods.

Hence, the correct option is A.

After the rejection of Newlands Octave Law, Mendeleev Periodic table came into existence. In Mendeleev’s periodic table, elements were placed based on the atomic mass, and chemical properties. During Mendeleev’s work, only 63 elements were found. After studying the properties of all elements, he found that the properties of elements were related to atomic mass.

Hence, the correct option is C.

In 1869, Dmitri Mendeleev and Lothar Meyer individually came up with their own periodic law "when the elements are arranged in order of increasing atomic mass, certain sets of properties recur periodically." 

The elements selected by Mendeleev were oxygen and hydrogen because most of the elements reacted with oxygen and hydrogen.