Chemistry MCQs for NEET — Practice Questions with Answers

According to the NCERT text, 'The English chemist, John Alexander Newlands in 1865 profounded the law of octaves. He arranged the elements in increasing order of their atomic weights and noted that every eighth element had properties similar to the first element.'

The NCERT text specifies, 'Newlands’s Law of Octaves seemed to be true only for elements up to Calcium.'

The NCERT text states, 'The Periodic Law, as we know it today owes its development to the Russian chemist, Dmitri Mendeleev (1834-1907) and the German chemist, Lothar Meyer (1830-1895). Working independently, both the chemists in 1869 proposed that on arranging elements in the increasing order of their atomic weights, similarities appear in physical and chemical properties at regular intervals.'

The text mentions, 'Lothar Meyer plotted the physical properties such as atomic volume, melting point and boiling point against atomic weight and obtained a periodically repeated pattern.'

The NCERT text explicitly states Mendeleev's Periodic Law: 'The properties of the elements are a periodic function of their atomic weights.'

The text mentions, 'He ignored the order of atomic weights, thinking that the atomic measurements might be incorrect, and placed the elements with similar properties together. For example, iodine with lower atomic weight than that of tellurium (Group VI) was placed in Group VII along with fluorine, chlorine, bromine because of similarities in properties.'

The NCERT text states, 'At the same time, keeping his primary aim of arranging the elements of similar properties in the same group, he proposed that some of the elements were still undiscovered and, therefore, left several gaps.'

The NCERT text clearly states, 'In 1913, the English physicist, Henry Moseley observed regularities in the characteristic X-ray spectra of the elements. A plot of $\sqrt{\nu}$ (where $\nu$ is frequency of X-rays emitted) against atomic number (Z) gave a straight line and not the plot of $\sqrt{\nu}$ vs atomic mass. He thereby showed that the atomic number is a more fundamental property of an element than its atomic mass. Mendeleev’s Periodic law was, therefore, accordingly modified. This is known as the modern Periodic law and can be stated as: The physical and chemical properties of the elements are periodic functions of their atomic numbers.'

The NCERT text mentions, 'In fact, it is now recognized that the Periodic Law is essentially the consequence of the periodic variation in electronic configurations, which indeed determine the physical and chemical properties of elements and their compounds.'

The text notes, 'The Periodic law revealed important analogies among the 94 naturally occurring elements (neptunium and plutonium like actinium and protoactinium are also found in pitch blende – an ore of uranium).'