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.
An element is uniquely determined by the number of proton s in the nuclei of its atom s. The number of protons in the nucleus of the atom of an element is known as the atomic number of that element. The number of electron s in the atom of a specific element can vary. If the number of electrons is the same as the number of protons, then the atom has no electric charge. However, if the number of electrons is different from the number of protons, the atom is called an ion. If there are more electrons than protons, it is a negative ion; if there are fewer electrons than protons, it is a positive ion. The number of neutron s in the atom of a specific element can also vary.