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 nucleus, that dense central core of the atom, contains both protons and neutrons. Electrons are outside the nucleus in energy levels. Protons have a positive charge, neutrons have no charge, and electrons have a negative charge. A neutral atom contains equal numbers of protons and electrons. But the number of neutrons within an atom of a particular element can vary. Atoms of the same element that have differing numbers of neutrons are called isotopes.