Which of the following elements has eight valence electrons?
a. argon
b. beryllium
c. chromium
d. oxygen

Option (a) is the correct answer.

Explanation:

Atomic number of argon is 18 and its electronic configuration is  . Hence, there are eight electrons present in the outermost shell of argon.

Atomic number of beryllium is 4 and its electronic configuration is . Only 2 electrons are present in its outermost shell.

Atomic number of chromium is 24 with electronic configuration . There are only 5 valence electrons present in it.

Atomic number of oxygen is 8 and its electronic configuration is . Hence, there are only 6 electrons present in its valence shell.

Thus, we can conclude that out of the given options argon element has eight valence electrons.

"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."

the answer would be c