<span>Although the nucleus of an atom is very important, it is the electrons of the atom that determine its chemical properties.
Number of electrons of an atoms (the number of electrons in the outermost level mostly) determine the type of the element (whether it is a metal, metalloid or non-metal). Based on this, the types of bonds that the atom can form (whether ionic or covalent) is determined, the type of reactions that the element can form is known and the elements that can react with it are also know.</span>
Biosphere is just a planet. Solar system contains planets and star. Galaxy holds multiple solar systems only. Universe hold everything, biospheres, solar systems, and galaxies. I hope this helps :D
