What trend does electronegativity follow, going down the periodic table?
a. electronegativity decreases because the size of the nucleus is
bigger.
b. electronegativity increases because there are more total electrons
in the atom.
c. electronegativity decreases because the valence electrons are
farther from the nucleus.
d. electronegativity increases because there are more valence
electrons in the atom.

a electronegativity decreases beacuse the size of nucleus is bigger.

"among the transition metal elements, manganese and zinc have a slight lowering of electronegativities. the reason behind this is that the nuclear charges of these elements are slightly weaker than the other transition metal elements due to the way their valence electrons are filled up. both manganese and zinc have their valence electrons filled up to the 4th shell with 1 pair of electrons (4s2) completely occupying it. although the electrons in the 3rd shell (3d) are yet to be paired since the outer most shell is filled up, the ability of the nucleus to attract electrons is lessened. hence,   the electronegativity (or the ability to attract electrons) is lessened."

when [oh-] is 10-5  m, and [h3o+] is 10-9  m, the poh is 5 and ph is 9. again in both cases the sum of ph and poh is 14. however, the hydrogen ion concentration is not always going to be equal to exactly 1 x 10 raised to a negative number. for example, we skipped over the value of 2.0 x 10-7.