For example salt dissolving in water is usually considered to be a physical change, however the chemical species in salt solution (hydrated sodium and chlorine ions) are different from the species in solid salt. All chemical reactions are reversible although this can be difficult in practice.The reactant (sodium chloride, or NaCl) is different from the products (sodium cation and chlorine anion). Thus, any ionic compound that is soluble in water would experience a chemical change.
the number of electrons in a particular shell is given by the formula 2n^2 . However an electron when gets very exicited loose energy in the forms of radiation and comes in high energy level shell.
