The answer is C. Elastic potential energy
The 4th energy level (row), 's' orbital block, 2nd group (column). The valence electrons are found in the highest energy level of the electron configuration in the 's' and 'p' orbitals. In the case of calcium this is 4s2 . This gives calcium an 's' orbital with a pair of electrons in its valence shell
Germanium is classified as a metalloid.
Mendeleev left gaps in the periodic table because, in the future, he knew that there would be more elements to be discovered because not all elements were discovered at the time.
