<P> Each atomic orbital corresponds to a particular energy level of the electron . The electron can change its state to a higher energy level by absorbing a photon with sufficient energy to boost it into the new quantum state . Likewise, through spontaneous emission, an electron in a higher energy state can drop to a lower energy state while radiating the excess energy as a photon . These characteristic energy values, defined by the differences in the energies of the quantum states, are responsible for atomic spectral lines . </P> <P> The amount of energy needed to remove or add an electron--the electron binding energy--is far less than the binding energy of nucleons . For example, it requires only 13.6 eV to strip a ground - state electron from a hydrogen atom, compared to 2.23 million eV for splitting a deuterium nucleus . Atoms are electrically neutral if they have an equal number of protons and electrons . Atoms that have either a deficit or a surplus of electrons are called ions . Electrons that are farthest from the nucleus may be transferred to other nearby atoms or shared between atoms . By this mechanism, atoms are able to bond into molecules and other types of chemical compounds like ionic and covalent network crystals . </P> <P> By definition, any two atoms with an identical number of protons in their nuclei belong to the same chemical element . Atoms with equal numbers of protons but a different number of neutrons are different isotopes of the same element . For example, all hydrogen atoms admit exactly one proton, but isotopes exist with no neutrons (hydrogen - 1, by far the most common form, also called protium), one neutron (deuterium), two neutrons (tritium) and more than two neutrons . The known elements form a set of atomic numbers, from the single proton element hydrogen up to the 118 - proton element oganesson . All known isotopes of elements with atomic numbers greater than 82 are radioactive, although the radioactivity of element 83 (bismuth) is so slight as to be practically negligible . </P> <P> About 339 nuclides occur naturally on Earth, of which 254 (about 75%) have not been observed to decay, and are referred to as "stable isotopes". However, only 90 of these nuclides are stable to all decay, even in theory . Another 164 (bringing the total to 254) have not been observed to decay, even though in theory it is energetically possible . These are also formally classified as "stable". An additional 34 radioactive nuclides have half - lives longer than 80 million years, and are long - lived enough to be present from the birth of the solar system . This collection of 288 nuclides are known as primordial nuclides . Finally, an additional 51 short - lived nuclides are known to occur naturally, as daughter products of primordial nuclide decay (such as radium from uranium), or else as products of natural energetic processes on Earth, such as cosmic ray bombardment (for example, carbon - 14). </P>

Which of the following atomic particles may vary for atoms of a given element