NEET Practice Questions (MCQs) with Answers & Solutions

The NCERT states, 'A unit vector is a vector of unit magnitude and points in a particular direction. It has no dimension and unit. It is used to specify a direction only.' In a rectangular coordinate system, $\hat{i}$, $\hat{j}$, and $\hat{k}$ specifically point along the x-, y-, and z-axes, respectively, to specify direction.

From the summary section, point 10, it is given that '$\tan \theta = \frac{A_y}{A_x}$'.

The NCERT text states: 'Since $\vec{A_1}$ is parallel to $\hat{i}$ and $\vec{A_2}$ is parallel to $\hat{j}$, we have : $\vec{A_1} = A_x \hat{i}$, $\vec{A_2} = A_y \hat{j}$ where $A_x$ and $A_y$ are real numbers.'

The contraction is, however, greater from element to element in this series resulting from poor shielding by 5f electrons. The statement that actinoid contraction is more significant than lanthanoid contraction due to more effective shielding by 5f electrons is incorrect. The context states that 'The contraction is, however, greater from element to element in this series resulting from poor shielding by 5f electrons.' This implies that poor shielding, not effective shielding, is the cause, and while the contraction per element is greater, the overall significance is often attributed to lanthanoid contraction's impact on subsequent elements, as stated: 'The lanthanoid contraction is more important because...'

The text states: 'The irregularities in the electronic configurations of the actinoids, like those in the lanthanoids are related to the stabilities of the $f^0$, $f^7$ and $f^{14}$ occupancies of the 5f orbitals. Thus, the configurations of Am and Cm are [Rn] $5f^7 7s^2$ and [Rn] $5f^7 6d^1 7s^2$.' Americium (Am) clearly shows a $5f^7$ configuration, indicating stability due to a half-filled orbital.

The text states: 'There is a greater range of oxidation states, which is in part attributed to the fact that the 5f, 6d and 7s levels are of comparable energies.' This allows for the participation of more electrons in bonding.

The text states: 'Hydrochloric acid attacks all metals but most are slightly affected by nitric acid owing to the formation of protective oxide layers; alkalies have no action.' Thus, hydrochloric acid attacks all actinoid metals.

The text states: 'The fourteen electrons are formally added to 5f, though not in thorium (Z = 90) but from Pa onwards the 5f orbitals are complete at element 103.' This clearly indicates that thorium (Th) does not have 5f orbital occupancy in its ground state configuration as explained here.

Referring to Table 4.11 and the accompanying text: '...the maximum oxidation state increases from +4 in Th to +5, +6 and +7 respectively in Pa, U and Np...' The highest oxidation state listed is +7 for Neptunium (Np).

The text states: 'The actinoids are radioactive elements and the earlier members have relatively long half-lives, the latter ones have half-life values ranging from a day to 3 minutes for lawrencium (Z =103). The latter members could be prepared only in nanogram quantities. These facts render their study more difficult.'