NEET Practice Questions (MCQs) with Answers & Solutions

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).'

The NCERT text describes, 'A modern version, the so-called 'long form' of the Periodic table of the elements (Fig. 3.2), is the most convenient and widely used. The horizontal rows (which Mendeleev called series) are called periods and the vertical columns, groups. Elements having similar outer electronic configurations in their atoms are arranged in vertical columns, referred to as groups or families.'

The context states, 'According to the recommendation of International Union of Pure and Applied Chemistry (IUPAC), the groups are numbered from 1 to 18 replacing the older notation of groups IA … VIIA, VIII, IB … VIIB and 0.'

According to the NCERT text, 'These reagents are obtained by the reaction of haloalkanes with magnesium metal in dry ether.' The presence of moisture would lead to the decomposition of the Grignard reagent.

The NCERT states, 'Grignard reagents are highly reactive and react with any source of proton to give hydrocarbons. Even water, alcohols, amines are sufficiently acidic to convert them to corresponding hydrocarbons.' Anhydrous diethyl ether is the solvent used for its preparation and does not contain active protons to react with the Grignard reagent.