Photo: Niels Bohr's research notes for his new atomic ... Bohr's theory that electrons existed in set orbits around the nucleus was the key to the periodic repetition of properties of the elements.

It is the principle strengthening element in carbon steels and low-alloy steels. Atomic number 6, atomic weight 12.01115. Manganese (Mn): Manganese is a brittle, metallic element that exists in ...