Answer:
in an atom or molecule, no two electrons can have the same four electronic quantum numbers. As an orbital can contain a maximum of only two electrons, the two electrons must have opposing spins.
Because they could have an explosive reaction and could possibly hurt you
Hept for 7, hence the number is seven.
Mass spectrometry was discovered by Francis Aston, a British Physicist, in 1919. This involves separating mixtures of substances into their components and transforming them into beams of particles and subsequently, bending them with the use of electricity and magnetism to come up with a spectrum that is inherently unique for every individual atoms for easy identification.