The greater the distance the valence electrons are from the nucleus
(protons) the force of attraction between
positive and negative is less, and if the
distance is smaller the greater the force
of attraction.

Consider a culture medium on which only gram-positive organisms such as staphylococcus aureus colonies can grow due to an elevated nacl level. a yellow halo surrounds the growth, indicating the bacterium fermented a sugar in the medium, decreasing the ph as a result and changing the color of a ph indicator chemical. this type of medium would be referred to as a differential and enrichment culture.