The main factor that influences the direction of any periodic trend across a period is . A.
the increasing number of electron shells.

B.
the increasing number of valence electrons.

C.
the increasing distance from the nucleus.

D.
the increasing number of protons in the nucleus.

The correct option is B

Explanation:

The number of valence electron(s) increases across the period and thus influences the direction of periodic trend of electronegativity, electron affinity and ionization energy across the period. Thus, making option B the answer.

It should be noted that option A is wrong because elements in the same period generally have the same number of electron shells and thus the distance of this shells from the nucleus remains the same throughout the same period and thus option C is also wrong. Option D is wrong because the periodic trends have to do with chemical reactions which actually involves electrons and not protons.

answer: atomic mass was originally used to arrange elements on the periodic table, but is no longer used to organize the modern version.

explanation:

originally atomic mass was used to arrange elements on the periodic table from left to right in a row. each column then shows similar properties of elements of a group.

in modern periodic table, the atoms are arranged in increasing atomic number. as atomic number represents the number of protons in an atom which is unique for every atom.