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2.4: Chemical Elements

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  • Learning Objectives

    • To know the meaning of isotopes and atomic masses.

    To date, about 115 different elements have been discovered; by definition, each is chemically unique. To understand why they are unique, you need to understand the structure of the atom (the fundamental, individual particle of an element) and the characteristics of its components. In most cases, the symbols for the elements are derived directly from each element’s name, such as C for carbon, U for uranium, Ca for calcium, and Po for polonium. Elements have also been named for their properties [such as radium (Ra) for its radioactivity], for the native country of the scientist(s) who discovered them [polonium (Po) for Poland], for eminent scientists [curium (Cm) for the Curies], for gods and goddesses [selenium (Se) for the Greek goddess of the moon, Selene], and for other poetic or historical reasons. Some of the symbols used for elements that have been known since antiquity are derived from historical names that are no longer in use; only the symbols remain to indicate their origin. Examples are Fe for iron, from the Latin ferrum; Na for sodium, from the Latin natrium; and W for tungsten, from the German wolfram. Examples are in Table \(\PageIndex{1}\).

    Table \(\PageIndex{1}\): Element Symbols Based on Names No Longer in Use
    Element Symbol Derivation Meaning
    antimony Sb stibium Latin for “mark”
    copper Cu cuprum from Cyprium, Latin name for the island of Cyprus, the major source of copper ore in the Roman Empire
    gold Au aurum Latin for “gold”
    iron Fe ferrum Latin for “iron”
    lead Pb plumbum Latin for “heavy”
    mercury Hg hydrargyrum Latin for “liquid silver”
    potassium K kalium from the Arabic al-qili, “alkali”
    silver Ag argentum Latin for “silver”
    sodium Na natrium Latin for “sodium”
    tin Sn stannum Latin for “tin”
    tungsten W wolfram German for “wolf stone” because it interfered with the smelting of tin and was thought to devour the tin

    Atomic Number (Z)

    What single parameter uniquely characterizes the atom of a given element? It is not the atom's relative mass, as we will see in the section on isotopes below. It is, rather, the number of protons in the nucleus, which we call the atomic number and denote by the symbol Z. Each proton carries an electric charge of +1, so the atomic number also specifies the electric charge of the nucleus. In the neutral atom, the Z protons within the nucleus are balanced by Z electrons outside it.

    Henry Moseley

    Atomic numbers were first worked out in 1913 by Henry Moseley, a young member of Rutherford's research group in Manchester. Moseley searched for a measurable property of each element that increases linearly with atomic number. He found this in a class of X-rays emitted by an element when it is bombarded with electrons. The frequencies of these X-rays are unique to each element, and they increase uniformly in successive elements. Moseley found that the square roots of these frequencies give a straight line when plotted against Z; this enabled him to sort the elements in order of increasing atomic number.

    You can think of the atomic number as a kind of serial number of an element, commencing at 1 for hydrogen and increasing by one for each successive element. The chemical name of the element and its symbol are uniquely tied to the atomic number; thus the symbol "Sr" stands for strontium, whose atoms all have Z = 38.

    Mass number (A)

    This is just the sum of the numbers of protons and neutrons in the nucleus. It is sometimes represented by the symbol A, so

    \[A = Z + N \label{2.4.1}\]

    in which Z is the atomic number and N is the neutron number.

    Nuclides and their Symbols

    The term nuclide simply refers to any particular kind of nucleus. For example, a nucleus of atomic number 7 is a nuclide of nitrogen. Any nuclide is characterized by the pair of numbers (Z ,A). The element symbol depends on Z alone, so the symbol 26Mg is used to specify the mass-26 nuclide of manganese, whose name implies Z=12. A more explicit way of denoting a particular kind of nucleus is to add the atomic number as a subscript. Of course, this is somewhat redundant, since the symbol Mg always implies Z=12, but it is sometimes a convenience when discussing several nuclides.

    Figure \(\PageIndex{2}\): Formalism used for identifying specific nuclide (any particular kind of nucleus)

    The element carbon (C) has an atomic number of 6, which means that all neutral carbon atoms contain 6 protons and 6 electrons. In a typical sample of carbon-containing material, 98.89% of the carbon atoms also contain 6 neutrons, so each has a mass number of 12. An isotope of any element can be uniquely represented as \(^A_Z X\), where X is the atomic symbol of the element. The isotope of carbon that has 6 neutrons is therefore \(_6^{12} C\). The subscript indicating the atomic number is actually redundant because the atomic symbol already uniquely specifies Z. Consequently, \(_6^{12} C\) is more often written as 12C, which is read as “carbon-12.” Nevertheless, the value of Z is commonly included in the notation for nuclear reactions because these reactions involve changes in Z.


    Recall that the nuclei of most atoms contain neutrons as well as protons. Unlike protons, the number of neutrons is not absolutely fixed for most elements. Atoms that have the same number of protons, and hence the same atomic number, but different numbers of neutrons are called isotopes. All isotopes of an element have the same number of protons and electrons, which means they exhibit the same chemistry. The isotopes of an element differ only in their atomic mass, which is given by the mass number (A), the sum of the numbers of protons and neutrons.

    Two nuclides having the same atomic number but different mass numbers are known as isotopes. Most elements occur in nature as mixtures of isotopes, but twenty-three of them (including beryllium and fluorine, shown in the table) are monoisotopic. For example, there are three natural isotopes of magnesium: 24Mg (79% of all Mg atoms), 25Mg (10%), and 26Mg (11%); all three are present in all compounds of magnesium in about these same proportions.

    Approximately 290 isotopes occur in nature. The two heavy isotopes of hydrogen are especially important— so much so that they have names and symbols of their own:

    \[\underset{\text{protium}}{\ce{^1_1H} } \label{2.4.2a}\]

    \[\underset{\text{deuterium}}{\ce{^2_1H \equiv D}} \label{2.4.2b}\]

    \[\underset{\text{tritium}}{\ce{^2_1H \equiv T}} \label{2.4.2c}\]

    Deuterium accounts for only about 15 out of every one million atoms of hydrogen. Tritium, which is radioactive, is even less abundant. All the tritium on the earth is a by-product of the decay of other radioactive elements.

    For carbon, in addition to \(^{12}C\), a typical sample of carbon contains 1.11% \(_6^{13} C\) (13C), with 7 neutrons and 6 protons, and a trace of \(_6^{14} C\) (14C), with 8 neutrons and 6 protons. The nucleus of 14C is not stable, however, but undergoes a slow radioactive decay that is the basis of the carbon-14 dating technique used in archeology. Many elements other than carbon have more than one stable isotope; tin, for example, has 10 isotopes. The properties of some common isotopes are in Table \(\PageIndex{2}\).

    Table \(\PageIndex{2}\): Properties of Selected Isotopes
    Element Symbol Atomic Mass (amu) Isotope Mass Number Isotope Masses (amu) Percent Abundances (%)
    hydrogen H 1.0079 1 1.007825 99.9855
    2 2.014102 0.0115
    boron B 10.81 10 10.012937 19.91
    11 11.009305 80.09
    carbon C 12.011 12 12 (defined) 99.89
    13 13.003355 1.11
    oxygen O 15.9994 16 15.994915 99.757
    17 16.999132 0.0378
    18 17.999161 0.205
    iron Fe 55.845 54 53.939611 5.82
    56 55.934938 91.66
    57 56.935394 2.19
    58 57.933276 0.33
    uranium U 238.03 234 234.040952 0.0054
    235 235.043930 0.7204
    238 238.050788 99.274

    Sources of isotope data: G. Audi et al., Nuclear Physics A 729 (2003): 337–676; J. C. Kotz and K. F. Purcell, Chemistry and Chemical Reactivity, 2nd ed., 1991.

    Example \(\PageIndex{1}\)

    An element with three stable isotopes has 82 protons. The separate isotopes contain 124, 125, and 126 neutrons. Identify the element and write symbols for the isotopes.

    Given: number of protons and neutrons

    Asked for: element and atomic symbol


    1. Refer to the periodic table and use the number of protons to identify the element.
    2. Calculate the mass number of each isotope by adding together the numbers of protons and neutrons.
    3. Give the symbol of each isotope with the mass number as the superscript and the number of protons as the subscript, both written to the left of the symbol of the element.


    A The element with 82 protons (atomic number of 82) is lead: Pb.

    B For the first isotope, A = 82 protons + 124 neutrons = 206. Similarly, A = 82 + 125 = 207 and A = 82 + 126 = 208 for the second and third isotopes, respectively. The symbols for these isotopes are \(^{206}_{82}Pb\), \(^{207}_{82}Pb\), and \(^{208}_{82}Pb\), which are usually abbreviated as \(^{206}Pb\), \(^{207}Pb\), and \(^{208}Pb\).

    Exercise \(\PageIndex{1}\)

    Identify the element with 35 protons and write the symbols for its isotopes with 44 and 46 neutrons.

    Answer: \(\ce{^{79}_{35}Br}\) and \(\ce{^{81}_{35}Br}\) or, more commonly, \(\ce{^{79}Br}\) and \(\ce{^{81}Br}\).


    • The atom consists of discrete particles that govern its chemical and physical behavior.Contributors

    Each atom of an element contains the same number of protons, which is the atomic number (Z). Neutral atoms have the same number of electrons and protons. Atoms of an element that contain different numbers of neutrons are called isotopes. Each isotope of a given element has the same atomic number but a different mass number (A), which is the sum of the numbers of protons and neutrons. The relative masses of atoms are reported using the atomic mass unit (amu), which is defined as one-twelfth of the mass of one atom of carbon-12, with 6 protons, 6 neutrons, and 6 electrons.