Explain the following: K 2 Cr 2 O 7 acts as a powerful oxidising agent in acidic medium.

An oxidizing agent (often referred to as an oxidizer or an oxidant) is a chemical species that tends to oxidize other substances, i.e. cause an increase in the oxidation state of the substance by making it lose electrons. Potassium dichromate is a good oxidizing agent, in the acidic medium the oxidation state of chromium changes by + 3 .

Explain the following: There is an increase in density of elements from titanium ( Z = 22 ) to copper ( Z = 29 ) in the 3 d - series of transition elements.

The decrease in metallic radius coupled with increase in atomic mass results in a general increase in the density of the elements from titanium ( Z = 22 ) to copper ( Z = 29 ) in the first series of transition elements.

Explain the transition metals and their compounds generally exhibit a paramagnetic behaviour.

Transition metals show paramagnetic behaviour. Paramagnetic property arises due to the presence of unpaired electrons, each such electron has a magnetic moment associated with it due to its spin angular momentum. Transition metals have in its ground state or ionised state have number of unpaired d-electrons which gives it a paramagnetic behaviour.

Why Zn 2 + ions are colourless while Ni 2 + ions are green and Cu 2 + ions are blue in colour?

The electronic configurations are given as Cu + 2 − [ Ar ] 3 d 9 and Zn + 2 − [ Ar ] 3 d 10 . Copper has an unpaired electron which can act as a F - centre and also allows electron transition in the visible region which importing colour while Zn + 2 is having paired electrons which do not involve in the transition and hence colourless. The nickel can also be involved in the transition due to its electronic configuration which has unpaired electrons. Thus, it imports green colour.