according to the periodic table, which two elements have an atomic mass less than twice their atomic number?

Hydrogen and Oxygen

Explanation:

Hydrogen atomic mass = 1.008  amu

atomic mass = 15.999  amu

b.

the accuracy of the data.

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.