Two forces act upon the particles within the nucleus to produce the nuclear structure.One, called the electrostatic force (or electromagnetic force), is the force that causesopposite electrical charges to attract each other and like charges to repel each other.The positively charged protons in the nucleus of an atom have an electrostatic forcepushing them apart. The other force within the nucleus, called the strong force, holdsnucleons (protons and neutrons) together.If one proton were to encounter another, the electrostatic force pushing them apartwould be greater than the strong force pulling them together, and the two protonswould fly in separate directions. Therefore, nuclei that contain more than one protonand no neutrons do not exist. Neutrons can be described as the nuclear glue thatallows protons to stay together in the nucleus. Because neutrons are uncharged, thereare no electrostatic repulsions between them and other particles. At the same time,each neutron in the nucleus of an atom is attracted to other neutrons and to protonsby the strong force. Therefore, adding neutrons to a nucleus increases the attractiveforces holding the particles of the nucleus together without increasing the amountof repulsion between those particles. As a result, although a nucleus that consists ofonly two protons is unstable, a helium nucleus that consists of two protons and twoneutrons is very stable. The increased stability is reflected in the significant amountof energy released when two protons and two neutrons combine to form a heliumnucleus.694 Chapter 16 Nuclear Chemistry+ + + + Energy2He p + p + n + n 4 2+ For many of the lighter elements, the possession of an equal number of protons andneutrons leads to stable atoms. For example, carbon-12 atoms, 6C12 , with six protonsand six neutrons, and oxygen-16 atoms, 8O16 , with eight protons and eight neutrons,are both very stable. Larger atoms with more protons in their nuclei require a higherratio of neutrons to protons to balance the increased electrostatic repulsion betweenprotons. Table 16.1 shows the steady increase in the neutron-to-proton ratios of themost abundant isotopes of the elements in group 15 on the periodic table.
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