In the early studies of chemistry, scientists used properties and changed to identify compounds. this is still done today. if you were given the following observation about salt water, how would you classify it? “when electricity flows through a flask of salt water, bubbles form in the water. these bubbles, when collected, will burn.”

The salt water is a mixture because the salt and the water can be separated from each other. The dissolution of salt inside water is a physical change.  At the same time, the salt water is an electrolyte, because it conducts electricity. The electricity that was pass through the water led to the decomposition of the water; those bubbles that form inside the water represent oxygen gas which is a product of decomposition of water, therefore the electricity that was passed through the salt water has caused chemical change to occur inside the salt water.

The problem here is that according to aufbau principle before 3d orbital 4s subshell comes.after filling 3p orbital with electrons you have 4 electrons left, you place two in 4s with opposite spin and the other two in 3d in any of its two orbitals of total five orbitals with the same spin.