Write and interpret signs that depict the atomic number, fixed number, and charge of an atom or ionDefine the atom mass unit and also average atom massCalculate average atomic mass and also isotopic abundance

The advancement of contemporary atomic theory revealed much around the inner structure of atoms. It was learned the an atom consists of a very small nucleus written of positively charged protons and uncharged neutrons, surrounded by a much bigger volume of an are containing negatively fee electrons. The nucleus includes the bulk of an atom’s mass since protons and also neutrons are much heavier 보다 electrons, vice versa, electrons accounting almost every one of an atom’s volume. The diameter of an atom is top top the bespeak of 10−10 m, conversely, the diameter the the cell nucleus is about 10−15 m—about 100,000 time smaller. For a perspective around their relative sizes, think about this: If the nucleus were the dimension of a blueberry, the atom would certainly be about the dimension of a football stadium (Figure 1).

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Figure 1. If one atom might be broadened to the dimension of a football stadium, the nucleus would certainly be the dimension of a single blueberry. (credit middle: modification of occupational by “babyknight”/Wikimedia Commons; credit transaction right: alteration of occupational by Paxson Woelber)

Atoms—and the protons, neutrons, and also electrons that create them—are incredibly small. Because that example, a carbon atom weighs much less than 2 × 10−23 g, and an electron has actually a fee of much less than 2 × 10−19 C (coulomb). As soon as describing the properties of tiny objects such as atoms, we use appropriately little units the measure, such as the atomic fixed unit (amu) and also the fundamental unit of charge (e). The amu was initially defined based upon hydrogen, the lightest element, then later on in terms of oxygen. Because 1961, it has actually been defined with regard to the many abundant isotope the carbon, atom of which are assigned masses of specifically 12 amu. (This isotope is known as “carbon-12” as will certainly be questioned later in this module.) Thus, one amu is specifically frac112 that the mass of one carbon-12 atom: 1 amu = 1.6605 × 10−24 g. (The Dalton (Da) and also the unified atom mass unit (u) are alternative units that are tantamount to the amu.) The fundamental unit of fee (also called the elementary school charge) amounts to the size of the fee of one electron (e) v e = 1.602 × 10−19 C.

A proton has actually a mass of 1.0073 amu and a charge of 1+. A neutron is a contempt heavier fragment with a mass 1.0087 amu and a charge of zero; together its name suggests, it is neutral. The electron has a charge of 1− and is a much lighter fragment with a massive of around 0.00055 amu (it would certainly take about 1800 electrons to equal the fixed of one proton. The properties of these basic particles space summarized in Table 3. (An observant student might an alert that the amount of one atom’s subatomic particles does no equal the atom’s actual mass: The complete mass of 6 protons, six neutrons, and also six electrons is 12.0993 amu, slightly larger than 12.00 amu. This “missing” massive is well-known as the massive defect, and also you will certainly learn about it in the chapter on nuclear chemistry.)

NameLocationCharge (C)Unit ChargeMass (amu)Mass (g)
electronoutside nucleus−1.602 × 10−191−0.000550.00091 × 10−24
protonnucleus1.602 × 10−191+1.007271.67262 × 10−24
neutronnucleus001.008661.67493 × 10−24
Table 3. properties of Subatomic Particles

The variety of protons in the nucleus of an atom is that is atomic number (Z). This is the defining trait of an element: Its worth determines the identification of the atom. Because that example, any atom that consists of six protons is the aspect carbon and also has the atomic number 6, regardless of how numerous neutrons or electron it might have. A neutral atom have to contain the same number of positive and an unfavorable charges, for this reason the number of protons equates 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 dubbed its mass number (A). The variety of neutrons is as such the difference in between the mass number and the atomic number: A – Z = number of neutrons.

= l extatomic number (Z) & extnumber that protons \<1em> extmass number (A) & extnumber of proton + variety of neutrons \<1em> extA - Z & extnumber the neutrons endarray

Atoms space electrically neutral if lock contain the same variety of positively fee protons and negatively charged electrons. Once the number of these subatomic particles space not equal, the atom is electrically charged and is called an ion. The charge of an atom is identified as follows:

Atomic fee = number of protons − number of electrons

As will be debated in an ext detail later on in this chapter, atom (and molecules) generally acquire fee by obtaining or shedding electrons. One atom that gains one or an ext electrons will exhibit a an unfavorable charge and is referred to as an anion. Positively fee atoms referred to as cations are formed when an atom loser one or more electrons. For example, a neutral salt atom (Z = 11) has 11 electrons. If this atom loser one electron, it will end up being a cation v a 1+ charge (11 − 10 = 1+). A neutral oxygen atom (Z = 8) has eight electrons, and also if it gains 2 electrons the will end up being an anion with a 2− charge (8 − 10 = 2−).

Example 1

Composition of one AtomIodine is an important trace aspect in ours diet; that is essential to produce thyroid hormone. Inadequate iodine in the diet can lead come the advancement of a goiter, one enlargement the the thyroid gland (Figure 2).

Figure 2. (a) insufficient iodine in the diet can reason an enlargement of the thyroid gland called a goiter. (b) The addition of little amounts that iodine come salt, which prevents the development of goiters, has actually helped remove this issue in the us where salt usage is high. (credit a: alteration of occupational by “Almazi”/Wikimedia Commons; credit b: change of work-related by Mike Mozart)

The enhancement of small amounts the iodine come table salt (iodized salt) has essentially removed this health problem in the joined States, yet as much as 40% of the world’s population is still at danger of iodine deficiency. The iodine atoms are added as anions, and each has actually a 1− charge and a mass number of 127. Recognize the numbers of protons, neutrons, and also electrons in one of these iodine anions.


The atomic number of iodine (53) tells us that a neutral iodine atom consists of 53 proton in the nucleus and also 53 electrons external its nucleus. Because the sum of the numbers of protons and also neutrons equates to the massive number, 127, the variety of neutrons is 74 (127 − 53 = 74). Because the iodine is added as a 1− anion, the variety of electrons is 54 <53 – (1–) = 54>.

Check your Learning

An ion the platinum has a mass number of 195 and also contains 74 electrons. How numerous protons and also neutrons does the contain, and what is its charge?

Chemical Symbols

A chemical symbol is one abbreviation that we use to show an aspect or one atom of an element. For example, the symbol because that mercury is Hg (Figure 3). We use the exact same symbol to indicate one atom that mercury (microscopic domain) or to label a container of plenty of atoms of the facet mercury (macroscopic domain).

Figure 3. The price Hg represents the facet mercury regardless of the amount; it could represent one atom that mercury or a large amount the mercury.

The symbols for several usual elements and also their atoms are listed in Table 4. Some icons are derived from the typical name the the element; others space abbreviations the the name in one more language. Most symbols have actually one or 2 letters, yet three-letter symbols have been offered to describe some facets that have actually atomic numbers higher than 112. To stop confusion with other notations, just the an initial letter the a prize is capitalized. Because that example, Co is the symbol for the aspect cobalt, however CO is the notation for the link carbon monoxide, which contains atoms that the facets carbon (C) and also oxygen (O). All recognized elements and their symbols are in the routine table in number 2 in chapter 2.5 The periodic Table (also discovered in attachment A).

aluminumAlironFe (from ferrum)
bromineBrleadPb (from plumbum)
carbonCmercuryHg (from hydrargyrum)
cobaltCopotassiumK (from kalium)
copperCu (from cuprum)siliconSi
fluorineFsilverAg (from argentum)
goldAu (from aurum)sodiumNa (from natrium)
hydrogenHtinSn (from stannum)
Table 4. Some usual Elements and also Their Symbols

Traditionally, the discoverer (or discoverers) that a new element names the element. However, till the surname is recognized by the international Union the Pure and also Applied Chemistry (IUPAC), the recommended name of the new element is based on the Latin word(s) for its atom number. Because that example, element 106 was called unnilhexium (Unh), aspect 107 was referred to as unnilseptium (Uns), and element 108 was referred to as unniloctium (Uno) for number of years. These elements are now called after researchers (or periodically locations); because that example, facet 106 is now well-known as seaborgium (Sg) in respect of valley Seaborg, a Nobel prize winner who was energetic in the exploration of several heavy elements.

Visit this website to learn much more about IUPAC, the global Union of Pure and Applied Chemistry, and also explore its periodic table.


The symbol because that a specific isotope of any element is written by put the mass number as a superscript to the left the the aspect symbol (Figure 4). The atom number is occasionally written as a subscript preceding the symbol, but since this number specifies the element’s identity, together does that is symbol, the is regularly omitted. Because that example, magnesium exists as a mixture of three isotopes, each v an atomic number of 12 and also with mass number of 24, 25, and 26, respectively. This isotopes have the right to be determined as 24Mg, 25Mg, and 26Mg. These isotope icons are review as “element, mass number” and can it is in symbolized continuous with this reading. For instance, 24Mg is read as “magnesium 24,” and also can be written as “magnesium-24” or “Mg-24.” 25Mg is check out as “magnesium 25,” and can be written as “magnesium-25” or “Mg-25.” every magnesium atoms have actually 12 proton in your nucleus. They different only since a 24Mg atom has actually 12 neutron in that nucleus, a 25Mg atom has actually 13 neutrons, and a 26Mg has 14 neutrons.

Figure 4. The symbol for an atom indicates the element via its usual two-letter symbol, the fixed number as a left superscript, the atomic number as a left subscript (sometimes omitted), and the fee as a best superscript.

Information around the naturally occurring isotopes of facets with atomic numbers 1 with 10 is given in Table 5. Keep in mind that in addition to conventional names and also symbols, the isotopes of hydrogen are often referred come using usual names and accompanying symbols. Hydrogen-2, symbolized 2H, is likewise called deuterium and also sometimes symbolized D. Hydrogen-3, symbolized 3H, is also called tritium and sometimes symbolized T.

ElementSymbolAtomic NumberNumber the ProtonsNumber of NeutronsMass (amu)% organic Abundance
hydrogen_1^1 extH


1123.01605— (trace)
helium_2^3 extHe2213.016030.00013
_2^4 extHe2224.0026100
lithium_3^6 extLi3336.01517.59
_3^7 extLi3347.016092.41
beryllium_4^9 extBe4459.0122100
boron_5^10 extB55510.012919.9
_5^11 extB55611.009380.1
carbon_6^12 extC66612.000098.89
_6^13 extC66713.00341.11
_6^14 extC66814.0032— (trace)
nitrogen_7^14 extN77714.003199.63
_7^15 extN77815.00010.37
oxygen_8^16 extO88815.994999.757
_8^17 extO88916.99910.038
_8^18 extO881017.99920.205
fluorine_9^19 extF991018.9984100
neon_10^20 extNe10101019.992490.48
_10^21 extNe10101120.99380.27
_10^22 extNe10101221.99149.25
Table 5. Nuclear Compositions of atoms of the an extremely Light Elements

Use this build an Atom simulator to develop atoms that the first 10 elements, watch which isotope exist, examine nuclear stability, and gain experience with isotope symbols.

Atomic Mass

Because every proton and also each neutron contribute around one amu to the mass of an atom, and also each electron contributes much less, the atomic mass that a solitary atom is approximately equal come its mass number (a whole number). However, the typical masses of atoms of most facets are not whole numbers because most elements exist normally as mixture of 2 or more isotopes.

The mass of an aspect shown in a periodic table or noted in a table of atomic masses is a weighted, typical mass of all the isotopes current in a naturally arising sample of the element. This is equal to the sum of each individual isotope’s mass multiply by its spring abundance.

displaystyle extaverage mass = sum_i ( extfractional abundance imes extisotopic mass)_i

For example, the element boron is written of two isotopes: around 19.9% of all boron atoms space 10B through a massive of 10.0129 amu, and also the continuing to be 80.1% are 11B with a fixed of 11.0093 amu. The mean atomic mass because that boron is calculated come be:

= l} extboron typical mass & (0.199 imes 10.0129 ; extamu) + (0.801 imes 11.0093 ; extamu) \<1em> & 1.99 ; extamu + 8.82 ; extamu \<1em> & 10.81 ; extamu endarray

It is crucial to recognize that no solitary boron atom weighs specifically 10.8 amu; 10.8 amu is the median mass of all boron atoms, and individual boron atoms weigh either roughly 10 amu or 11 amu.

Example 2

Calculation of typical Atomic MassA meteorite uncovered in central Indiana includes traces of the noble gas neon choose up native the solar wind throughout the meteorite’s pilgrimage through the solar system. Evaluation of a sample the the gas verified that it included 91.84% 20Ne (mass 19.9924 amu), 0.47% 21Ne (mass 20.9940 amu), and also 7.69% 22Ne (mass 21.9914 amu). What is the mean mass that the neon in the solar wind?


= l} extaverage mass & (0.9184 imes 19.9924 ; extamu) + (0.0047 imes 20.9940 ; extamu)+(0.0769 imes 21.9914 ; extamu) \<1em> & (18.36+0.099+1.69) ; extamu \<1em> & 20.15 ; extamu endarray

The mean mass of a neon atom in the solar wind is 20.15 amu. (The average mass the a terrestrial neon atom is 20.1796 amu. This an outcome demonstrates that us may uncover slight distinctions in the natural abundance of isotopes, depending upon their origin.)

Check your LearningA sample of magnesium is found to save on computer 78.70% the 24Mg atoms (mass 23.98 amu), 10.13% the 25Mg atoms (mass 24.99 amu), and also 11.17% that 26Mg atoms (mass 25.98 amu). Calculate the typical mass of a Mg atom.

Example 3

Calculation of Percent AbundanceNaturally occurring chlorine is composed of 35Cl (mass 34.96885 amu) and also 37Cl (mass 36.96590 amu), through an average mass of 35.453 amu. What is the percent ingredient of Cl in terms of these two isotopes?

SolutionThe typical mass that chlorine is the portion that is 35Cl times the fixed of 35Cl to add the portion that is 37Cl time the massive of 37Cl.

extaverage mass = ( extfraction of ^35 extCl imes extmass of ^35 extCl) + ( extfraction of ^37 extCl imes extmass of ^37 extCl)

If us let x represent the fraction that is 35Cl, climate the fraction that is 37Cl is represented by 1.00 − x.

(The portion that is 35Cl + the portion that is 37Cl must include up to 1, for this reason the portion of 37Cl have to equal 1.00 − the fraction of 35Cl.)

Substituting this right into the average mass equation, we have:

= l}35.453 ; extamu & (x imes 34.96885 ; extamu) + <(1.00 - x) imes 36.96590; extamu> \<1em> 35.453 & 34.96885x + 36.96590 - 36.96590x \<1em> 1.99705x & 1.513 \<1em> x & frac1.5131.99705 = 0.7576 endarray

So solving yields: x = 0.7576, which method that 1.00 − 0.7576 = 0.2424. Therefore, chlorine is composed of 75.76% 35Cl and also 24.24% 37Cl.

Check your LearningNaturally arising copper consists of 63Cu (mass 62.9296 amu) and also 65Cu (mass 64.9278 amu), with an median mass the 63.546 amu. What is the percent composition of Cu in terms of these 2 isotopes?

Visit this website to do mixtures the the main isotopes that the first 18 elements, gain experience with typical atomic mass, and also check naturally developing isotope ratios utilizing the Isotopes and Atomic massive simulation.

The occurrence and also natural abundances the isotopes have the right to be experimentally determined using one instrument referred to as a mass spectrometer. Massive spectrometry (MS) is widely supplied in chemistry, forensics, medicine, environmental science, and also many other fields to analysis and aid identify the building materials in a sample that material. In a common mass spectrometer (Figure 5), the sample is vaporized and exposed come a high-energy electron beam that causes the sample’s atom (or molecules) to come to be electrically charged, commonly by losing one or much more electrons. These cations climate pass v a (variable) electrical or magnetic ar that deflects each cation’s path to an degree that counts on both the mass and charge (similar to how the route of a huge steel sphere bearing rolling past a magnet is deflected to a lesser degree that the of a little steel BB). The ions are detected, and a plot of the relative variety of ions produced versus your mass-to-charge ratios (a mass spectrum) is made. The elevation of every vertical attribute or optimal in a mass spectrum is proportional come the fraction of cations v the stated mass-to-charge ratio. Because its early stage use during the breakthrough of modern atomic theory, multiple sclerosis has evolved to become a an effective tool for chemical analysis in a wide variety of applications.

Figure 5. evaluation of zirconium in a massive spectrometer produce a mass spectrum through peaks showing the different isotopes of Zr.

See an computer animation that describes mass spectrometry. Watch this video from the Royal culture for Chemistry for a short description that the rudiments of massive spectrometry.

Key Concepts and Summary

An atom is composed of a small, positively charged nucleus surrounded by electrons. The nucleus consists of protons and neutrons; its diameter is about 100,000 times smaller than that of the atom. The mass of one atom is commonly expressed in atomic mass units (amu), which is referred to as the atom mass. An amu is defined as precisely frac112 that the mass of a carbon-12 atom and also is same to 1.6605 × 10−24 g.

Protons are reasonably heavy particles v a fee of 1+ and also a mass of 1.0073 amu. Neutron are fairly heavy particles v no charge and also a massive of 1.0087 amu. Electrons space light particles through a charge of 1− and also a mass of 0.00055 amu. The variety of protons in the cell core is referred to as the atomic number (Z) and also is the home that specifies an atom’s element identity. The amount of the number of protons and neutrons in the nucleus is dubbed the mass number and, expressed in amu, is around equal come the fixed of the atom. One atom is neutral when it contains equal number of electrons and also protons.

Isotopes that an facet are atoms with the same atomic number but different mass numbers; isotopes of one element, therefore, different from each other only in the variety of neutrons in ~ the nucleus. When a naturally arising element is written of several isotopes, the atomic mass that the element represents the median of the masses of the isotopes involved. A chemical symbol identifies the atoms in a substance making use of symbols, which space one-, two-, or three-letter abbreviation for the atoms.

Key Equationsdisplaystyle extaverage mass = sum_i ( extfractional abundance imes extisotopic mass)_i

Chemistry end of chapter Exercises

In what way are isotope of a offered element always different? In what way(s) space they always the same?Write the symbol because that each that the complying with ions:

(a) the ion v a 1+ charge, atomic number 55, and mass number 133

(b) the ion through 54 electrons, 53 protons, and 74 neutrons

(c) the ion v atomic number 15, mass number 31, and a 3− charge

(d) the ion with 24 electrons, 30 neutrons, and also a 3+ charge

Write the symbol for each the the following ions:

(a) the ion v a 3+ charge, 28 electrons, and also a mass number of 71

(b) the ion v 36 electrons, 35 protons, and 45 neutrons

(c) the ion through 86 electrons, 142 neutrons, and also a 4+ charge

(d) the ion v a 2+ charge, atom number 38, and also mass number 87

Determine the variety of protons, neutrons, and also electrons in the following isotopes that are used in clinical diagnoses:

(a) atomic number 9, mass number 18, fee of 1−

(b) atomic number 43, fixed number 99, fee of 7+

(c) atom number 53, atom mass number 131, charge of 1−

(d) atomic number 81, atomic mass number 201, fee of 1+

(e) name the aspects in parts (a), (b), (c), and (d).

The following are properties of isotope of two facets that are essential in our diet. Recognize the variety of protons, neutrons and also electrons in each and also name them.

(a) atom number 26, massive number 58, fee of 2+

(b) atomic number 53, massive number 127, charge of 1−

Give the variety of protons, electrons, and neutrons in neutral atom of every of the complying with isotopes:

(a) _5^10 extB

(b) _80^199 extHg

(c) _29^63 extCu

(d) _6^13 extC

(e) _34^77 extSe

Give the variety of protons, electrons, and neutrons in neutral atoms of each of the complying with isotopes:

(a) _3^7 extLi

(b) _52^125 extTe

(c) _47^109 extAg

(d) _7^15 extN

(e) _15^31 extP

An facet has the following natural abundances and isotopic masses: 90.92% abundance v 19.99 amu, 0.26% abundance v 20.99 amu, and also 8.82% abundance v 21.99 amu. Calculation the mean atomic fixed of this element.Average atomic masses noted by IUPAC are based upon a study of speculative results. Bromine has two isotopes 79Br and 81Br, who masses (78.9183 and also 80.9163 amu) and abundances (50.69% and 49.31%) were determined in earlier experiments. Calculate the mean atomic fixed of bromine based on these experiments.Variations in mean atomic mass may be it was observed for facets obtained from different sources. Lithium provides an instance of this. The isotopic composition of lithium from naturally arising minerals is 7.5% 6Li and also 92.5% 7Li, which have masses of 6.01512 amu and also 7.01600 amu, respectively. A commercial source of lithium, recycle from a army source, to be 3.75% 6Li (and the remainder 7Li). Calculation the mean atomic mass values for each of these 2 sources.The median atomic masses the some elements may vary, depending upon the sources of your ores. Naturally developing boron consists of two isotopes v accurately known masses (10B, 10.0129 amu and also 11B, 11.0931 amu). The actual atom mass of boron deserve to vary native 10.807 come 10.819, depending upon whether the mineral resource is native Turkey or the unified States. Calculation the percent abundances causing the two values that the average atomic masses of boron from these two countries.The 18O:16O abundance ratio in part meteorites is better than that supplied to calculate the average atomic mass of oxygen top top earth. Is the mean mass of an oxygen atom in these meteorites greater than, much less than, or equal to the of a terrestrial oxygen atom?


anionnegatively charged atom or molecule (contains much more electrons than protons)atomic massaverage mass of atoms of an element, to express in amuatomic massive unit (amu)(also, unified atomic fixed unit, u, or Dalton, Da) unit the mass same to frac112 of the massive of a 12C atomatomic number (Z)number of proton in the cell nucleus of an atomcationpositively charged atom or molecule (contains fewer electrons 보다 protons)chemical symbolone-, two-, or three-letter abbreviation provided to represent an element or that atomsDalton (Da)alternative unit identical to the atom mass unitfundamental unit of charge(also dubbed the elementary charge) amounts to the magnitude of the fee of an electron (e) v e = 1.602 × 10−19 Cionelectrically fee atom or molecule (contains unequal number of protons and electrons)mass number (A)sum of the number of neutrons and protons in the cell nucleus of one atomunified atom mass unit (u)alternative unit identical to the atomic mass unit


Answers to Chemistry finish of thing Exercises

2. (a) 133Cs+; (b) 127I−; (c) 31P3−; (d) 57Co3+

4. (a) Carbon-12, 12C; (b) This atom includes six protons and six neutrons. Over there are 6 electrons in a neutral 12C atom. The net charge of together a neutral atom is zero, and the massive number is 12. (c) The coming before answers room correct. (d) The atom will be stable because C-12 is a secure isotope the carbon. (e) The preceding answer is correct. Other answers for this practice are possible if a different element of isotope is chosen.

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6. (a) Lithium-6 includes three protons, three neutrons, and three electrons. The isotope prize is 6Li or _3^6 extLi. (b) 6Li+ or _3^6 extLi^+

8. (a) Iron, 26 protons, 24 electrons, and 32 neutrons; (b) iodine, 53 protons, 54 electrons, and also 74 neutrons

10. (a) 3 protons, 3 electrons, 4 neutrons; (b) 52 protons, 52 electrons, 73 neutrons; (c) 47 protons, 47 electrons, 62 neutrons; (d) 7 protons, 7 electrons, 8 neutrons; (e) 15 protons, 15 electrons, 16 neutrons

12. Let us use neon together an example. Since there are three isotopes, over there is no means to be certain to accurately predict the abundances to do the complete of 20.18 amu mean atomic mass. Let us guess the the abundances are 9% Ne-22, 91% Ne-20, and also only a map of Ne-21. The mean mass would be 20.18 amu. Checking the nature’s mix that isotopes reflects that the abundances room 90.48% Ne-20, 9.25% Ne-22, and 0.27% Ne-21, therefore our guessed quantities have come be slightly adjusted.

14. 79.904 amu

16. Turkey source: 0.2649 (of 10.0129 amu isotope); united state source: 0.2537 (of 10.0129 amu isotope)