The atomic sizes of `"_26Fe_(27)Co` and `"28Ni` are nearly same. Explain with a reason.

Why are the atomic radii of 4d and 5d elements nearly the same ?

The time taken for the combustion of one piece of coal in air is much greater than for the combustion of powdered coal with the same mass . Explain with reasons.

(a) At 25^(@)C , standard reaction enthalpy for the reaction 2 C(graphite, s) +O_(2)(g)to 2CO(g) is -221.0 kJ. Does this value indicate the standard enthalpy of formation of CO(g) ? If not, then what would be the value of enthalpy of formation of CO(g) at 25^(@)C ? (b) A gas is allowed to expand against zero external pressure. Explain with reason whether the procss is reversible or not.

Although Fe is placed above hydrogen in the electrochemical series, dihydrogen in not obtained by its reaction with nitric acid. Explain with reasons.

(1) The electronic configuration of C - atoms is: 1s^(2)2s^(2)2p^(2) , yet its valency is four - why? (2) The four C - H bonds of methane molecule are equivalent - explain with reasons.

Because of lanthanide contraction, which of the following pairs of elements have nearly same atomic radii (Number in the parenthesis are atomic numbers)-