Classification of elements and Periodicity in properties MCQs for NEET — Chemistry Questions with Answers

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, 'The anomalous behaviour is attributed to their small size, large charge/radius ratio and high electronegativity of the elements. In addition, the first member of group has only four valence orbitals (2s and 2p) available for bonding'. Second-period elements do not possess d-orbitals in their valence shell, which is a characteristic of later period elements. The absence of d-orbitals contributes to their anomalous behavior, not their presence.

The NCERT text states, 'For example, lithium unlike other alkali metals, and beryllium unlike other alkaline earth metals, form compounds with pronounced covalent character; the other members of these groups predominantly form ionic compounds.'

The NCERT text explicitly mentions, 'In fact the behaviour of lithium and beryllium is more similar with the second element of the following group i.e., magnesium and aluminium, respectively. This sort of similarity is commonly referred to as diagonal relationship in the periodic properties.'

The NCERT text explains, 'In addition, the first member of group has only four valence orbitals (2s and 2p) available for bonding, whereas the second member of the groups have nine valence...'. This is due to the absence of d-orbitals in the second period elements (n=2, so only 2s and 2p orbitals are available).