Electrons are thought to whirl around the nucleus in an unpredictable manner, now close to it, now farther away. Orbitals represent the places where electrons are most probably found. Isotopes Isotopes are atoms of the same element that contain the same number of protons but different numbers of neutrons. Isotopes, therefore, have different atomic mass numbers. The three isotopes of hydrogen contain zero, one, and two neutrons, respectively.
Elements usually occur in nature as mixtures of isotopes.
All isotopes of a given element have essentially the same chemical characteristics. Such isotopes are termed radioisotopes, since they emit high-energy radiation when they decay. Molecules Two or more atoms may combine chemically to form a molecule. When two atoms of oxygen combine, for example, a molecule of oxygen is formed. Different kinds of atoms can combine to form chemical compounds.