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 in a nucleus is the same for all the atoms of a particular element. It corresponds to the atomic number of that element. For example, if a nucleus contains three protons, then it is part of an atom of lithium (atomic number 3). The neutron has a similar mass to the proton but has no charge. The neutrons help keep the nucleus together.