It is dangerous to clean well remaining
closed for a long time, why?

Because of the accumulation of high percentage of carbon dioxide when a person enters into the closed well for a long time it becomes harder for the person to breathe in and hence may lead to death.

hope It helps

Because of the accumulation of high percentage of carbon dioxide when a person enters into the closed well for a long time it becomes harder for the person to breathe in and hence may lead to death.

"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."