2.3 Atomic structure and symbolism - Chemistry 2e | OpenStaxName (2023)

learning goals

At the end of this section you can:

  • Write and interpret symbols that represent the atomic number, mass number, and charge of an atom or ion
  • Define the atomic mass unit and average atomic mass
  • Calculate the average atomic mass and isotopic abundance

The development of modern atomic theory has revealed much about the internal structure of atoms. An atom was found to contain a very small nucleus, made up of positively charged protons and uncharged neutrons, surrounded by a much larger volume of space containing negatively charged electrons. The nucleus contains most of an atom's mass, since protons and neutrons are much heavier than electrons, while electrons take up almost the entire volume of an atom. The diameter of an atom is of the order of 10−10m, while the core diameter is about 10−15m: about 100,000 times smaller. To get an idea of ​​their relative sizes, consider this: if the nucleus were the size of a blueberry, the atom would be the size of a soccer stadium (Figure 2.11).

2.3 Atomic structure and symbolism - Chemistry 2e | OpenStaxName (1)

Figure2.11 If an atom could expand to the size of a soccer stadium, the nucleus would be the size of a single blueberry. (middle credits: modified work by "babyknight"/Wikimedia Commons; right credits: modified work by Paxson Woelber)

Atoms and the protons, neutrons and electrons that make them up are extremely small. For example, a carbon atom weighs less than 2××10−23g, and an electron has a charge less than 2××10−19C (coulomb). When we describe the properties of tiny objects like atoms, we use correspondingly small units of measurement, such asatomic mass unit (amu)it's himbasic unit of charge (e). Amu was originally defined in relation to hydrogen, the lightest element, and later in relation to oxygen. Since 1961 it has been defined in terms of the most abundant carbon isotope, whose atoms are assigned masses of exactly 12 amu. (This isotope is known as "carbon-12," as explained later in this module.) Therefore, an amu is accurate112112the mass of a carbon-12 atom: 1 amu = 1.6605××10−24grams. (HeDalton (Da)it's himuniform atomic mass unit (u)are alternative units corresponding to amu.) The basic unit of charge (also called elementary charge) is equal to the charge magnitude of an electron (e) with e = 1.602××10−19C.

A proton has a mass of 1.0073 amu and a charge of 1+. A neutron is a slightly heavier particle with a mass of 1.0087 amu and zero charge; As the name suggests, it is neutral. The electron has a charge of 1− and is a much lighter particle with a mass of about 0.00055 amu (it would take about 1800 electrons to reach the mass of a proton). The properties of these elementary particles are summarized inTable 2.2. (An attentive student may note that the sum of the subatomic particles of an atom does not equal the actual mass of the atom: the total mass of six protons, six neutrons, and six electrons is 12.0993 amu, just over 12.00 amu. missing "Mass is known as the mass defect, and you will learn more about it in the nuclear chemistry chapter.)

Properties of subatomic particles

NameLocationload (C)unit loadMasa (same)time (days)
Electronicouter core−1,602××10−191-0,000550,00091××10−24

Mesa 2.2

The number of protons in the nucleus of an atom is itsatomic number (Z). This is the defining characteristic of an element: its value determines the identity of the atom. For example, any atom containing six protons is the element carbon and has atomic number 6, regardless of how many neutrons or electrons it may have. A neutral atom must contain the same number of positive and negative charges, so the number of protons is equal to the number of electrons. Therefore, the atomic number also indicates the number of electrons in an atom. The total number of protons and neutrons in an atom is calledmass number (A). The neutron number is the difference between the mass number and the atomic number: A – Z = neutron number.

atomic number(Z)=number of protonsmass number(A)=number of protons+number of neutronsAZ=number of neutronsatomic number(Z)=number of protonsmass number(A)=number of protons+number of neutronsAZ=number of neutrons

Atoms are electrically neutral if they contain the same number of positively charged protons and negatively charged electrons. If the numbers of these subatomic particles areNOThe same atom is electrically charged and is calledrein. The charge on an atom is defined as follows:

Atomic charge = number of protons − number of electrons

As will be discussed in more detail later in this chapter, atoms (and molecules) normally gain charge by gaining or losing electrons. An atom that gains one or more electrons has a negative charge and is referred to as anAnion. called positively charged atomscationsThey are formed when an atom loses one or more electrons. For example, a neutral sodium atom (Z=11) has 11 electrons. When this atom loses an electron, it becomes a cation with a 1+ charge (11 − 10 = 1+). A neutral oxygen atom (Z = 8) has eight electrons, and if it gains two electrons it becomes an anion with a charge of 2− (8 − 10 = 2−).

Example 2.3

composition of an atom

Iodine is an essential trace element in our diet; It is needed to produce thyroid hormones. Too little iodine in the diet can lead to the development of goiter - an enlargement of the thyroid gland (Figure 2.12).

2.3 Atomic structure and symbolism - Chemistry 2e | OpenStaxName (2)

Figure2.12 (a) Too little iodine in the diet can cause an enlarged thyroid gland called goiter. (b) The addition of small amounts of iodine to salt, which prevents the formation of goiter, has helped address these concerns in the United States, where salt consumption is high. (Credit a: work modified by "Almazi"/Wikimedia Commons; Credit b: work modified by Mike Mozart)

The addition of small amounts of iodine to table salt (iodized salt) has essentially eliminated this health problem in the United States, yet up to 40% of the world's population is still at risk of iodine deficiency. Iodine atoms aggregate as anions with charge 1− and mass number 127. Find the number of protons, neutrons, and electrons in one of these iodine anions.


The atomic number of iodine (53) tells us that a neutral iodine atom has 53 protons in its nucleus and 53 electrons outside of its nucleus. Since the sum of the proton and neutron numbers equals the mass number 127, the neutron number is 74 (127 − 53 = 74). Since iodine is added as a 1− anion, the number of electrons is 54 [53 – (1–) = 54].

check your learning

A platinum ion has a mass number of 195 and contains 74 electrons. How many protons and neutrons does it contain and what is their charge?


78 protons; 117 neutrons; Load is 4+

chemical symbols

Achemical symbolis an abbreviation we use to indicate an element or an atom of an element. For example, the symbol for mercury is Hg (Figure 2.13). We use the same symbol to denote a mercury atom (microscopic range) or a container containing many mercury atoms (macroscopic range).

2.3 Atomic structure and symbolism - Chemistry 2e | OpenStaxName (3)

Figure2.13 The symbol Hg represents the element mercury regardless of quantity; it could represent a mercury atom or a large amount of mercury.

Symbols for several common elements and their atoms are listed belowTable 2.3. Some symbols are derived from the element's common name; others are abbreviations of the name in another language. Most symbols are one or two letters long, but three-letter symbols have been used to describe some elements with atomic numbers greater than 112. To avoid confusion with other notations, only the first letter of a symbol is capitalized. For example, Co is the symbol for the element cobalt, but CO is the name for the compound carbon monoxide, which contains atoms of the elements carbon (C) and oxygen (O). All known elements and their symbols are listed in the periodic tableFigure 2.26(also found inAnhang A).

Some common elements and their symbols

AluminiumAlabamaHierroFE (fromHierro)
BromBrotherto leadPb (deto lead)
MoneyCmercuryHg (abmercury)
ChlorKlof nitrogenNorte
coppercom (from)copper)siliconY
FluorFPlataAg (fromPlata)
GoldFor theaurora borealis)SodiumNa (deSodium)
hydrogenHlataSn (delata)

Mesa 2.3

Traditionally, the discoverer (or discoverers) of a new element names the element. However, until the name is recognized by the International Union of Pure and Applied Chemistry (IUPAC), the new element's recommended name is based on the Latin words for its atomic number. For example, element 106 was called Unnilhexium (Unh), element 107 was called Unnilseptium (Uns), and element 108 was called Unniloctium (One) for several years. These items are now named after scientists (or occasionally locals); for example, Article 106 is now known asseeborgio(Sg) in honor of Glenn Seaborg, a Nobel laureate who was involved in the discovery of several heavy elements. Element 109 is named after Lise Meitner, who discovered nuclear fission, a phenomenon that would have world-changing effects; Meitner also contributed to the discovery of some important isotopes, which are discussed below.

link to learning

visit this oneOrtto learn more about IUPAC, the International Union of Pure and Applied Chemistry, and explore their periodic table.


The symbol for a specific isotope of any element is written by placing the superscript mass number to the left of the element symbol (Figure 2.14). The atomic number is sometimes written as a subscript before the symbol, but since this number defines both the element's identity and its symbol, it is often omitted. For example, magnesium exists as a mixture of three isotopes, each with an atomic number of 12 and mass numbers of 24, 25, and 26, respectively. These isotopes can be identified as24mg,25mg, y26mg. These isotope symbols are read as "Element, Mass Number" and can be symbolized accordingly. For example,24Mg is read as "mg-24" and can be written as "mg-24" or "Mg-24".25Mg is read as "mg 25" and can be written as "mg-25" or "Mg-25". All magnesium atoms have 12 protons in their nucleus. They only differ by one thing24The Mg atom has 12 neutrons in its nucleus, one25The Mg atom has 13 neutrons and one26Mg has 14 neutrons.

2.3 Atomic structure and symbolism - Chemistry 2e | OpenStaxName (4)

Figure2.14 The symbol for an atom indicates the element by its usual two-letter symbol, the mass number as the left superscript, the atomic number as the left subscript (sometimes omitted), and the charge as the superscript to the right

For information on the natural isotopes of the elements with atomic numbers from 1 to 10, seeTable 2.4. Note that isotopes of hydrogen are often denoted by common names and accompanying symbols, in addition to the standard names and symbols. Hydrogen-2, symbolizes2H, also called deuterium and sometimes symbolized as D. Hydrogen-3, which is symbolized3H, also called tritium and sometimes symbolized as T.

Nuclear compositions of atoms of very light elements

ElementSymbolatomic numbernumber of protonsnumber of neutronsMasa (same)% Natural Abundance
1123.01605- (pursue)
614C614C66814.0032- (pursue)
of nitrogen714Norte714Norte77714.003199,63

Mesa 2.4

link to learning

one is thatBuild an atom simulatorto build atoms of the first 10 elements, see what isotopes exist, check nuclear stability and gain experience with isotope symbols.

Atomic mass

Since each proton and neutron contributes about one amu to the mass of an atom, and each electron contributes much less, theAtomic massof a single atom is approximately equal to its mass number (an integer). However, the average atomic masses of most elements are not whole numbers because most elements occur in nature as mixtures of two or more isotopes.

The mass of an element listed on a periodic table or atomic mass table is a weighted average mass of all isotopes present in a natural sample of that element. This equals the sum of the mass of each individual isotope multiplied by its fractional abundance.

medium mass=EU(fractional fullness×Isotopenmasse)EUmedium mass=EU(fractional fullness×Isotopenmasse)EU

For example, the element boron consists of two isotopes: about 19.9% ​​of all boron atoms are10B with a mass of 10.0129 amu, and the remaining 80.1% are11B with a mass of 11.0093 amu. The average atomic mass of boron is calculated as:

average mass of boron=(0,199×10.0129 identical)+(0,801×11.0093 identical)=1.99 identical+8.82 identical=10.81 sameaverage mass of boron=(0,199×10.0129 identical)+(0,801×11.0093 identical)=1.99 identical+8.82 identical=10.81 same

It is important to understand that no single boron atom weighs exactly 10.8 amu; 10.8 amu is the average mass of all boron atoms, and individual boron atoms weigh about 10 or 11 amu.

Example 2.4

Calculation of the average atomic mass

A meteorite found in central Indiana contains traces of the noble gas neon, picked up by the solar wind during its journey through the solar system. Analysis of a sample of the gas showed that it was 91.84% off20Ne (Mass 19.9924 amu), 0.47%21Ne (Mass 20.9940 amu), y 7.69%22Ne (mass 21.9914 amu). What is the average mass of neon in the solar wind?


medium mass=(0,9184×19.9924 identical)+(0,0047×20.9940 as)+(0,0769×21.9914 identical)=(18.36+0,099+1,69)like=20.15 the samemedium mass=(0,9184×19.9924 identical)+(0,0047×20.9940 as)+(0,0769×21.9914 identical)=(18.36+0,099+1,69)like=20.15 the same

The average mass of a neon atom in the solar wind is 20.15 amu. (The average mass of a neon atom on Earth is 20.1796 amu. This result shows that we can detect small differences in the natural abundance of isotopes depending on their origin.)

check your learning

A magnesium sample contains 78.70%24Mg atoms (mass 23.98 amu), 10.13%25Mg atoms (mass 24.99 amu) and 11.17%26Magnesium atoms (mass 25.98 amu). Calculate the average mass of a Mg atom.


24.31 same

We can also make variations on this type of calculation, as shown in the example below.

Example 2.5

Calculation of the fill percentage

Natural chlorine consists of35Cl (Masse 34,96885 amu) e37Cl (mass 36.96590 amu) with an average mass of 35.453 amu. What is the percentage composition of Cl with respect to these two isotopes?


The average mass of chlorine is the fraction that is35Cl peel too much35Cl plus the fraction that is37Cl peel too much37Kl.

medium mass=(a fraction of35Kl×Mesa35Kl)+(a fraction of37Kl×Mesa37Kl)medium mass=(a fraction of35Kl×Mesa35Kl)+(a fraction of37Kl×Mesa37Kl)

when we go outXrepresents the break that35Cl, then break that is37Cl is represented by 1.00-X.

(the fraction that35Cl + the fraction that is37Cl must add up to 1, which is the fraction of37Cl must equal 1.00 - the fraction of35Kl.)

Plugging this into the equation for average mass, we get:

35,453 identical=(X×34.96885 identical)+[(1,00X)×36.96590 identical]35.453=34.96885X+36.9659036.96590X1,99705X=1.513X=1.5131,99705=0,757635,453 identical=(X×34.96885 identical)+[(1,00X)×36.96590 identical]35.453=34.96885X+36.9659036.96590X1,99705X=1.513X=1.5131,99705=0,7576

If we then solve, we get:X= 0.7576, which means that 1.00 − 0.7576 = 0.2424. Therefore, chlorine is 75.76%35Cl y 24,24%37Kl.

check your learning

Natural copper consists of63With (mass 62.9296 amu)ySixty-fiveCu (mass 64.9278 amu) with an average mass of 63.546 amu. What is the percentage composition of Cu with respect to these two isotopes?


69.15% Cu-63 and 30.85% Cu-65

link to learning

visit this oneOrtto produce large isotope mixtures of the first 18 elements, gain experience with average atomic masses, and verify natural isotope ratios with isotope and atomic mass simulation.

As you will learn, isotopes are important in nature and especially to human understanding of science and medicine. Let's consider just one naturally occurring stable isotope - oxygen-18, which is given in the table above and is known to be one of the environmental isotopes. It's important in paleoclimatology, for example, because scientists can use the ratio of oxygen-18 to oxygen-16 in an ice core to determine precipitation temperature over time. Oxygen-18 has also been instrumental in discovering metabolic pathways and enzymatic mechanisms. Mildred Cohn pioneered the use of these isotopes as markers, allowing researchers to track their progress through reactions and better understand what is going on. One of his first discoveries provided information about the phosphorylation of glucose that takes place in mitochondria. And the methods of using isotopes for this research have contributed to entire areas of study.

The presence and natural abundance of isotopes can be determined experimentally using an instrument called a mass spectrometer. Mass spectrometry (MS) is widely used in chemistry, forensics, medicine, environmental science and many other fields to analyze and identify substances in a material sample. In a typical mass spectrometer (Figure 2.15) the sample is vaporized and exposed to a beam of high-energy electrons, which causes the atoms (or molecules) in the sample to become electrically charged, usually losing one or more electrons. These cations then pass through a (varying) electric or magnetic field that bends each cation's path by an amount dependent on both its mass and charge (much like the path a large steel ball rolling past a magnet bends to To a lesser extent jumps). than a small steel ball). Ions are detected and a plot of the relative number of ions produced versus their mass-to-charge ratio is displayed (amass spectrum) it is finished. The height of each vertical feature or peak in a mass spectrum is proportional to the proportion of cations with the specified mass-to-charge ratio. Since its initial use during the development of modern atomic theory, MS has evolved into a powerful tool for chemical analysis in a wide variety of applications.

2.3 Atomic structure and symbolism - Chemistry 2e | OpenStaxName (5)

Figure2.15 Analysis of zirconium in a mass spectrometer produces a mass spectrum with peaks representing the different isotopes of Zr.

link to learning

a... seeAnimationwhich explains mass spectrometry. look at thisVideofrom the Royal Society for Chemistry for a brief overview of the basics of mass spectrometry.

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