1. The variable valency is due to the following reason. The energy difference between inner ‘d’ level and outers level is so small. Therefore electrons from ‘d’ level as well as ‘s’ level can take part in reaction.
2. Sc to Cr – Atomic radius decreases due to increase in nuclear charge. Cr to Cu – Due to a balance between the increased nuclear charge and increased screening effect atomic radii become almost constant.
Cu to Zn – Repulsive interactions between the paired electrons in d-orbitals become very dominant towards the end of the period and cause the expansion of electron cloud and thus result in increased atomic size.
3. Lanthanoid, Contraction.