What do an orgasm, a multiplication problem and a photo of a dead body have in common? Each induces a slight, irrepressible expansion of the pupils in our eyes, giving careful observers a subtle but meaningful signal that thoughts and feelings are afoot. For more than a century, scientists have known that our pupils respond to more than changes in light.
Review. 1  Atoms have their electrons arranged in layers called shells. 2  In order to maintain electrical balance the number of electrons is equal to the number of protons in most elements.
The number of protons determines the number of electrons. This, in turn, determines the chemical properties of the atom. Protons contribute to the mass of an atom and provide positive charge to the atom. The number of protons also determines the identity of the element. Electrons have a negative charge. Since the atom has no charge, the number of electrons must equal the number of protons. The electrons arrange themselves in energy levels around the nucleus.