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NEET Questions / Chemistry / Classification of Elements and Periodicity in Properties
Mendeleev classified elements based on atomic weight, while the modern periodic table uses atomic number. Which anomaly in Mendeleev's table was directly resolved by this change?
Placement of noble gases
Position of tellurium and iodine
Existence of isotopes
Lanthanide contraction
Which of the following pairs of elements would have been placed in the SAME group in Mendeleev's periodic table but DIFFERENT groups in the modern periodic table?
Tellurium (Te) and Iodine (I)
Sodium (Na) and Potassium (K)
Calcium (Ca) and Magnesium (Mg)
Iron (Fe) and Cobalt (Co)
In Mendeleev's periodic table, eka-silicon was predicted to have a density of 5.5 g/cm³. Germanium, later discovered to be eka-silicon, has a density of 5.323 g/cm³. This difference in predicted and observed density primarily highlights which limitation of Mendeleev's table?
Inability to predict the existence of noble gases
Placement of isotopes of the same element
Reliance on atomic weight as the primary organizing principle
Lack of understanding of the underlying electronic structure of atoms
If Mendeleev had known about the concept of isotopes, how would his periodic table have been different?
Isotopes of the same element would have been placed in different groups
Isotopes of the same element would have occupied the same position
The table would have been organized based on neutron number
The concept of periodicity would have been discarded
Which of the following properties generally decreases across a period from left to right, but shows an anomalous increase at Group 15 elements compared to Group 14 and 16 elements?
Ionization enthalpy
Atomic radius
Electronegativity
Electron gain enthalpy
Consider the following isoelectronic species: , , , , and . Which of these exhibits the highest second ionization energy?
S²⁻
Cl⁻
Ar
K⁺
Which of the following factors does NOT significantly influence the ionization energy of an element?
Effective Nuclear Charge
Atomic Radius
Electron Shielding
Nuclear Spin
The first ionization energy of Aluminum is lower than that of Magnesium. This anomaly is primarily due to:
Increased nuclear charge of Magnesium
Larger atomic radius of Aluminum
Shielding by filled inner orbitals and penetration of the 3p electron
Higher electronegativity of Magnesium
Across a period, the general trend for ionization energy is to increase. However, there are exceptions to this trend. Which of the following best explains these exceptions?
Increase in atomic radius
Decrease in nuclear charge
Changes in subshell energies and electron pairing effects
Shielding by inner d-electrons
Which of the following statements regarding the acidic strength of oxides of elements in the third period (Na to Cl) is INCORRECT?
The acidic strength increases linearly with increasing electronegativity of the element.
The oxides transition from basic to amphoteric to acidic across the period.
is the strongest acidic oxide in the period.
is the strongest basic oxide in the period.
