If a metal changes color over time from exposure to air what has likely occurred

Which is the layer underground where all empty spaces are filled with a combination of air and water ?

The chemical bond connecting one nucleotide with the next one along the nucleic acid chain is called a

13. a covalent bond between two atoms is likely to be polar if: a. one of the atoms is much more electronegative than the other. b. the two atoms are equally electronegative. c. the two atoms are of the same element. d. the bond is part of a tetrahedrally shaped molecule. e. one atom is an anion.