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 atom is always charge less i.e it is neutral. An atom always refers to a single element. The atomic number of an element refers to the number of protons or electrons in it. This is because an atom always has equal number of electrons and protons. Protons are positively charged while electrons are negatively charged. Atoms also contain neutrons which are neutral. Proton, neutron and electron are called sub atomic particles. Neutrons and protons are called nucleons since they are in the nucleus of the atom. Charged species are called ions.