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The final subatomic particle was not discovered until 1932. To account for the neutral charge of an atom as a whole, the number of positively charged protons and negatively charged electrons found within an atom must be equal. Therefore, any remaining subatomic particles must be discharged so as not to disturb this established charge balance. In fact,neutrons, which received this name because of their neutral charge, have no electrical properties. Consequently, these subatomic particles, which are symbolized by the notation "n0", were incredibly difficult to detect. Neutrons are also found in the nucleus of an atom, and the mass of a neutron has been found to befairslightly greater than the mass of a proton.
Each subatomic particle exists to serve a specific purpose. As indicated in the previous section, the number of valence electrons present in an atom determines the reactivity of that element. The number of protons found within an atom defines that atom's identity, and all of an atom's protons collectively attract surrounding electrons, holding the latter attached to the atom. Remember, however, that all the protons, each with a +1 charge, are close together in the central region of an atom. Therefore, every positively charged proton must be strongly repelled by every other proton in the nucleus, and furthermore, the combined force of these repulsion forces is substantial enough to fragment the nucleus. However, neutrons effectively act as "nuclear glue" and allow protons to exist in physical proximity to one another. In other words, neutrons are the subatomic particle responsible for maintaining the structural integrity of the nucleus.
Finally, remember that each atom of a given element must have a definite number of protons and electrons. Every carbon atom, C, that exists in the known universe isdefinedit must contain 6 protons, because its atomic number is 6, and it must also contain 6 electrons, in order for the atom to maintain an overall neutral charge. However, the number of neutrons within an atom of an element isNodefined by the atomic number of that element. In fact, the number of neutrons present in an element can vary from one atom to another. The "glue" analogy found in the previous paragraph can be extended to explain this phenomenon. While a minimal amount of glue is required to adhere one object to another, a small amount of excess glue will not prevent these objects from sticking together, but a large amount of excess glue can be problematic. Likewise, each element must contain a minimum number of neutrons to hold the nucleus together, but can contain a small number of additional neutrons without sacrificing the structural integrity of the nucleus. However, a nucleus that contains too many neutrons will become unstable and sufferradioactive decay, which will be discussed in Chapter 9 of this text.
mass number
Omass numberof an atom is equal to the total number of protons and neutrons contained in its nucleus. This definition can be represented in an equation, as shown below.
Mass number = # of protons + # of neutrons
The true mass of an atom is an incredibly small amount. To simplify the numerical values used, the mass of a single proton is given the value of 1atomic mass unit, oamor. Since the mass of a neutron is about the same as the mass of a proton, each neutron present is also assigned a value of 1 amu. Since the mass of an electron is 1/2000ºOf the mass of a proton, any contribution that electrons make to the total mass of an atom is negligible. Therefore, the number of electrons present in an atom is ignored when calculating the mass number of that atom.
Note that the mass number calculated in Example \(\PageIndex{1}\) does not match the number under the elemental symbol and name for hydrogen on the periodic table. This discrepancy can be explained by a subtle but incredibly important piece of information: The calculation performed on Example \(\PageIndex{1}\) was done tosingle atomof hydrogen. However, the periodic table purports to representallhydrogen atoms in the known universe. Fromallexisting hydrogen atom must contain 1 proton, the atomic number that is written above the elemental symbol for hydrogen actually representsallhydrogen atom.
However, remember that the number of neutrons contained in an element can vary from one atom to another. Changing the number of neutrons present in an atom will, in turn, cause those individual hydrogen atoms to have different calculated mass numbers. These individual "versions" of an element are calledisotopes, which are defined as atoms of an element that have the same atomic number and therefore contain the same number of protons but different mass numbers and therefore contain a different number of neutrons. Figure \(\PageIndex{1}\) models three isotopes of hydrogen. Most hydrogen atoms have one proton, one electron, and contain no neutrons, but less common isotopes of hydrogen may contain one or two neutrons. Hydrogen is unique in that its isotopes are given special names, which are also shown below in Figure \(\PageIndex{1}\).
For spatial reasons, it is not practical to list the mass numbers of all the isotopes of an element within a single box on the periodic table. Instead, a weighted average, calledaverage atomic mass, is calculated. A weighted average takes into account not only the mass number of each isotope, but also how frequent or common that isotope is in nature, relative to each of the other isotopes of that element. Therefore, an average atomic mass is a quantity that actually representsallisotopes of a given element, making it suitable for inclusion in the periodic table.
elemental symbolisms
In total, 252 stable isotopes of 80 different elements have been isolated. Taking into account the number of unstable isotopes that have been observed, the total number of known elemental isotopes increases substantially. Although each of the three most common isotopes of hydrogen has a unique name, it would be highly impractical to give them different names.allisotope ofallelement that has been shown to exist. Therefore, scientists use threeelemental symbolismsto refer to specific elemental isotopes. The first two symbolisms are very similar in that each includes the elemental name, or elemental symbol, of an element, followed by a hyphen and a numerical value, which corresponds to the mass number of a particular isotope of that element. In the third type of elemental symbolism, callednuclear symbol, the isotope's mass number is placed as a superscript before an element's symbol, and the element's atomic number is written directly below the mass number. It is important to note the difference between aisotopeit's aelemental symbolism. The figure \(\PageIndex{2}\) models these threedifferent elemental symbolisms, which represent allsameisotopebecause each one has aidenticalmass number
