HARD
11th CBSE
IMPORTANT
Earn 100

The order of screening effect of electrons of s, p, d and f orbitals of a given shell of an atom on its outer shell electrons is:

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Important Questions on Classification of Elements and Periodicity in Properties

EASY
11th CBSE
IMPORTANT
The first ionisation enthalpies of Na, Mg, Al and Si are in the order:
HARD
11th CBSE
IMPORTANT
The electronic configuration of gadolinium (Atomic number 64) is
MEDIUM
11th CBSE
IMPORTANT
The statement that is not correct for periodic classification of elements is:
 
MEDIUM
11th CBSE
IMPORTANT
Among halogens, the correct order of the amount of energy released in electron gain (electron gain enthalpy) is: 
MEDIUM
11th CBSE
IMPORTANT
The period number in the long form of the periodic table is equal to
EASY
11th CBSE
IMPORTANT
The elements in which electrons are progressively filled in 4f -orbital are called
MEDIUM
11th CBSE
IMPORTANT
Which of the following is the correct order of the size of the given species: 
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11th CBSE
IMPORTANT

The formation of the oxide ion, O2- (g), from oxygen atom requires first an exothermic and then an endothermic step as shown below:

O (g) + e-O- (g) ;  H=-141 KJ mol-1

O- (g) + e-O2- (g) ;  H=+780 KJ mol-1

Thus, the process of formation of O2- In gas-phase Is unfavourable even though O2- Is isoelectronic with neon. It Is due to the fact that,