What are inner core electrons and how do they influence chemical and physical properties? There are six groups of p–block elements in the periodic
table numbering from 13 to 18. Boron, carbon, nitrogen,
oxygen, fluorine and helium head the groups. Their valence
shell electronic configuration is $\ce{ns^2 np^{1-6}}$(except for He).


  The inner core of the electronic configuration may,
  however, differ. The difference in inner core of elements
  greatly influences their physical properties (such as atomic
  and ionic radii, ionisation enthalpy, etc.) as well as chemical
  properties


The above lines are from my school's textbook.

How does inner core electron create such differences and and what are inner core electrons?
Inner core electrons do have a very strong affect on the chemistry of elements,for example inner d orbitals have a very poor shielding effect thus the outer electrons are bounded more strongly to the nucleus thus decreasing their reactivity and oxidation states.You will come across inert pair effect,stability of lower oxidation states down the group etc because of effect of inner core electrons.