Atoms & Elements

Atoms contain a nucleus. This nucleus contains positively charged protons and neutral neutrons. It is surrounded by a smear of negatively charged electrons (e^-‘s). Atoms have volume and mass, which are both properties of matter. Mass measures the amount of matter present (not to be confused with weight, which measures the gravitational force on mass). Atoms have equal numbers of electrons and protons. Therefore, an atom is electrically neutral. Electrons determine the chemical properties of an atom. They swarm about the nucleus in concentric regions called electron shells or energy levels. Each electron shell holds a limited number of electrons. The one closest to the nucleus holds a maximum of 2 electrons, the second level can hold up to 8 electrons, and the third holds a maximum of 18 electrons. The electrons of the outermost shell, called the valence electrons, determine the binding properties of an atom. Atoms react to complete the outermost shell. Atoms with a single shell, react to fill this shell with 2 electrons. All other atoms react to produce eight electrons in the valence shell. This tendency is referred to as the octet rule.