How did moseley establish a more accurate periodic table?
-by arranging the elements according to atomic mass instead of atomic number
-by arranging the elements according to atomic number instead of atomic mass
-by arranging the elements into horizontal periods
-by arranging the elements into vertical groups

The correct answer is by arranging the elements according to atomic number instead of atomic mass.

Explanation:

Henry Moseley in 1913 studied the X-rays of each element and then identified the atomic number of each element which led to the more accurate distribution of elements in the periodic table.

He arranged elements on the basis of number of protons rather than their atomic weights. The number of protons directly led us to the atomic number of the elements.

Hence, the correct answer is by arranging the elements according to atomic number instead of atomic mass.

the seeing the electronic configuration we can say that the element is fluorine.

fluorine is the most electronegative element in the periodic table.

but by only seeing the configuration we cannot say about its reactivity because we don't know if its configuration after excitation or by addition of electrons.

but if not then, it is the lightest halogen and exists as a highly toxic pale yellow diatomic gas at standard conditions. as the most electronegative element, it is extremely reactive: almost all other elements, including some noble gases, form compounds with fluorine.

the answers are

c and c