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The final subatomic particle was not discovered until 1932. In order 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 uncharged, so as to not upset this established charge balance. Indeed, neutrons, which were named as a result of their neutral charge, do not possess any electrical properties. Consequently, these subatomic particles, whichare symbolized using the notation "n0," wereincredibly difficult to detect. Neutrons are also located in thenucleus of an atom, and the mass of a neutron wasfound to bejustslightly greater than the massof a proton.
Each subatomic particle exists to serve a specific purpose. As stated in the previous section, the number of valence electrons present in an atom dictates the reactivity of that element. The number of protons found within an atom defines the identity of that atom, and all of an atom's protons collectively attract the surrounding electrons, keeping the latter bound tothe atom. Recall, however, that all protons, which each bear a +1 charge, are densely-packed into the central region of an atom. Therefore, each positively-charged protonmust be strongly repelled by every other proton in the nucleus, and, furthermore, the combined strength of these repulsive forces is substantial enough to splinter the nucleus. However, neutrons effectively act as "nuclear glue" and allow the protons to exist in close physical proximity to one another. In other words, neutrons are the subatomic particle responsible for maintainingthe structural integrity of the nucleus.
Finally, recall that every atom of a certain element must have a definednumber of protons and electrons. Every atom of carbon,C,that exists in the known universe isdefinedto contain 6 protons, because its atomic number is 6, and must also contain 6 electrons, in order for the atom to maintain an overall net neutral charge. However,the number of neutrons within an atom of an element is not defined by the atomic number of that element. In fact, the number of neutrons present in an element can vary from atom to atom. The "glue" analogy found within the previous paragraph can be extended to explain this phenomenon. While a minimum amount of glue is required to adhere one object to another, a small amount of excess glue will not prevent those objects from sticking together, but a large excess of glue could prove to be problematic. Likewise, each element must contain a minimum number of neutrons to hold the nucleus together, but could 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 undergoradioactive decay, which will be discussed in Chapter 9 of this text.
Mass Number
The mass number of 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 quantity. To simplify the numerical values being used, the mass of a single proton is assigned a value of 1 atomic mass unit, or amu. As the mass of a neutron is approximately the same as the mass of a proton, each neutron that is present is also given a value of 1 amu. Since the mass of an electron is 1/2,000thof the mass of a proton, any contribution that electrons make to the overall mass of an atom is negligible. Therefore, the number of electrons present in an atom are ignored when calculating the mass number of that atom.
Note that the mass number calculated in Example \(\PageIndex{1}\) does not match the number underneath 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 inExample \(\PageIndex{1}\) was done forasingle atomof hydrogen. However, the periodic table is intended to representallof the atoms of hydrogen in the known universe. Sinceeveryexisting atom of hydrogen must contain 1 proton, the atomic number that is written above hydrogen's elemental symbol truly does representeveryatom of hydrogen.
However, recall that the number of neutronscontained in an element can vary from atom to atom. Changing the number of neutrons present in an atom will,in turn, cause these individual atoms of hydrogen to have different calculated mass numbers. These individual "versions" of an element are called isotopes, which are defined as atoms of an element that have the same atomic numbersand, therefore, contain the same number of protons, butdifferent mass numbers, and, therefore, contain differingnumbers of neutrons. Three isotopes of hydrogen are modeled in Figure \(\PageIndex{1}\). Most hydrogen atoms have one proton,one electron, and do not contain anyneutrons, but less common isotopes of hydrogen can contain either one or two neutrons. Hydrogen is unique, in that its isotopes are given special names, which are also shown below inFigure \(\PageIndex{1}\).
For spatial reasons, listing the mass numbers for all of an element's isotopeswithin a single box on the periodic table is impractical. Instead, a weighted average, called anatomic mass average,is calculated. A weighted average takes into account not only the mass numberof each isotope, but also how prevalent, or common, that isotope is in nature, relative to each of that element's other isotopes. Therefore, an atomic mass average is a quantity that truly represents all isotopes of a given element, making it appropriate for inclusion on the periodic table.
Elemental Symbolisms
In total, 252 stable isotopes have been isolated for 80 different elements. Factoring in the number of unstable isotopes that have been observedcauses the total number of known elemental isotopes to increase substantially. While each of hydrogen's three most common isotopes has a unique name, it would ultimately be highly impractical to establish different names foreveryisotope ofeveryelement that has been shown to exist. Therefore, scientists utilize three different elemental symbolismstoreferto specificelemental isotopes.The first two symbolisms are very similar, in that each includes the elemental name, or elemental symbol, of an element, followed by a dash and a numerical value, which corresponds to themass number of a particular isotope of that element. In the third type of elemental symbolism, which is calledanuclear symbol,the mass number of the isotope is positioned as a superscript before anelemental symbol, and the atomic number of the element is written directly underneath the mass number. It is important to note the differencebetween an isotope and an elemental symbolism. Figure \(\PageIndex{2}\) models these threedifferent elemental symbolisms, which all represent thesame isotope, since each has an identical mass number.
