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

The breakthrough of modern-day atomic theory revealed much around the inner structure of atoms. It was learned that an atom contains a very little nucleus composed of positively fee protons and also uncharged neutrons, surrounded by a much bigger volume of an are containing negatively charged electrons. The nucleus contains the bulk of one atom’s mass due to the fact that protons and also neutrons are lot heavier 보다 electrons, vice versa, electrons accounting almost every one of an atom’s volume. The diameter of one atom is ~ above the stimulate of 10−10 m, conversely, the diameter of the nucleus is about 10−15 m—about 100,000 times smaller. For a perspective around their loved one sizes, consider this: If the nucleus were the dimension of a blueberry, the atom would be about the dimension of a football stadium (Figure 1).

You are watching: The mass of 12 protons is approximately equal to

Figure 1. If an atom could be broadened to the dimension of a soccer stadium, the nucleus would be the dimension of a single blueberry. (credit middle: alteration of occupational by “babyknight”/Wikimedia Commons; credit right: alteration of work by Paxson Woelber)

Atoms—and the protons, neutrons, and electrons that create them—are very small. For example, a carbon atom weighs less than 2 × 10−23 g, and also an electron has a fee of less than 2 × 10−19 C (coulomb). As soon as describing the nature of small objects such together atoms, we usage appropriately small units of measure, such together the atomic mass unit (amu) and also the fundamental unit of charge (e). The amu was originally defined based upon hydrogen, the lightest element, then later on in regards to oxygen. Because 1961, it has actually been identified with regard to the most abundant isotope of carbon, atoms of which space assigned masses of exactly 12 amu. (This isotope is well-known as “carbon-12” as will be questioned later in this module.) Thus, one amu is precisely frac112 of the mass of one carbon-12 atom: 1 amu = 1.6605 × 10−24 g. (The Dalton (Da) and the unified atomic mass unit (u) are alternate units the are identical 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) with 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 slightly heavier fragment with a fixed 1.0087 amu and also a fee of zero; together its name suggests, that is neutral. The electron has actually a charge of 1− and also is a much lighter bit with a massive of about 0.00055 amu (it would certainly take around 1800 electrons to same the massive of one proton. The properties of these fundamental particles are summarized in Table 3. (An observant college student might notification that the sum of one atom’s subatomic corpuscle does not equal the atom’s actual mass: The total mass of 6 protons, 6 neutrons, and also six electrons is 12.0993 amu, slightly bigger than 12.00 amu. This “missing” mass is known as the massive defect, and you will 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. nature of Subatomic Particles

The number of protons in the cell core of one atom is its atomic number (Z). This is the defining trait of an element: Its value determines the identification of the atom. For example, any kind of atom that consists of six protons is the aspect carbon and also has the atomic number 6, regardless of how countless neutrons or electron it may have. A neutral atom need to contain the same variety of positive and an adverse charges, therefore the variety of protons equates to the number of electrons. Therefore, the atomic number likewise indicates the number of electrons in an atom. The total variety of protons and also neutrons in one atom is called its mass number (A). The variety of neutrons is therefore the difference in between the massive number and also the atom number: A – Z = variety of neutrons.

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

Atoms room electrically neutral if lock contain the same number of positively fee protons and negatively fee electrons. As soon as the number of this subatomic particles space not equal, the atom is electrically charged and also is dubbed an ion. The charge of one atom is defined as follows:

Atomic fee = variety of protons − number of electrons

As will certainly be discussed in an ext detail later in this chapter, atom (and molecules) typically acquire fee by obtaining or shedding electrons. One atom that gains one or more electrons will exhibit a an adverse charge and also is dubbed an anion. Positively charged atoms dubbed cations are formed when one atom loses one or more electrons. For example, a neutral sodium atom (Z = 11) has actually 11 electrons. If this atom loser one electron, the will become a cation with a 1+ charge (11 − 10 = 1+). A neutral oxygen atom (Z = 8) has eight electrons, and if the gains 2 electrons that will become an anion with a 2− charge (8 − 10 = 2−).

Example 1

Composition of an AtomIodine is an important trace aspect in our diet; that is needed to create thyroid hormone. Poor iodine in the diet can lead come the development of a goiter, an enlargement of the thyroid gland (Figure 2).

Figure 2. (a) poor iodine in the diet can cause an enlargement the the thyroid gland called a goiter. (b) The addition of small amounts that iodine come salt, which avoids the formation of goiters, has helped get rid of this problem in the us where salt usage is high. (credit a: alteration of occupational by “Almazi”/Wikimedia Commons; credit transaction b: modification of work by Mike Mozart)

The enhancement of little amounts of iodine to table salt (iodized salt) has essentially eliminated this health concern in the joined States, but as lot as 40% the the world’s populace is tho at risk of iodine deficiency. The iodine atom are included as anions, and each has actually a 1− charge and a mass variety of 127. Identify the numbers of protons, neutrons, and also electrons in one of these iodine anions.


The atomic number of iodine (53) tells united state that a neutral iodine atom contains 53 protons in that is nucleus and 53 electrons external its nucleus. Due to the fact that the amount of the number of protons and neutrons amounts to the massive number, 127, the number of neutrons is 74 (127 − 53 = 74). Due to the fact that the iodine is added as a 1− anion, the variety of electrons is 54 <53 – (1–) = 54>.

Check your Learning

An ion of platinum has actually a mass variety of 195 and also contains 74 electrons. How many protons and also neutrons does it contain, and also what is that charge?

Chemical Symbols

A chemical symbol is an abbreviation the we usage to indicate an element or one atom of one element. Because that example, the symbol for mercury is Hg (Figure 3). We use the same symbol to suggest one atom of mercury (microscopic domain) or to label a container of numerous atoms of the facet mercury (macroscopic domain).

Figure 3. The symbol Hg represents the facet mercury nevertheless of the amount; it might represent one atom that mercury or a big amount that mercury.

The signs for several usual elements and their atoms are listed in Table 4. Some icons are acquired from the usual name the the element; others are abbreviations of the name in another language. Many symbols have one or two letters, but three-letter symbols have been offered to describe some elements that have atomic numbers greater than 112. To avoid confusion with other notations, only the very first letter of a prize is capitalized. For example, Co is the symbol because that the facet cobalt, yet CO is the notation for the compound carbon monoxide, which consists of atoms the the facets carbon (C) and also oxygen (O). All recognized elements and also their symbols space in the routine table in number 2 in chapter 2.5 The periodic Table (also found 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 typical Elements and also Their Symbols

Traditionally, the discoverer (or discoverers) the a brand-new element names the element. However, till the name is recognized by the worldwide Union of Pure and Applied Chemistry (IUPAC), the recommended surname of the brand-new element is based upon the Latin word(s) because that its atom number. For example, element 106 was called unnilhexium (Unh), facet 107 was dubbed unnilseptium (Uns), and element 108 was referred to as unniloctium (Uno) for several years. These elements are now named after scientists (or occasionally locations); because that example, aspect 106 is now recognized as seaborgium (Sg) in respect of valley Seaborg, a Nobel prize winner who was active in the discovery of several hefty elements.

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


The symbol because that a details isotope of any kind of element is composed by placing the mass number together a superscript to the left that the aspect symbol (Figure 4). The atom number is occasionally written as a subscript coming before the symbol, but due to the fact that this number defines the element’s identity, together does the symbol, it is often omitted. Because that example, magnesium exists as a mixture of three isotopes, each v an atomic variety of 12 and with mass number of 24, 25, and 26, respectively. These isotopes can be established as 24Mg, 25Mg, and 26Mg. These isotope signs are review as “element, massive number” and can be 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 read as “magnesium 25,” and also can be composed as “magnesium-25” or “Mg-25.” every magnesium atoms have actually 12 protons in their nucleus. They different only because a 24Mg atom has actually 12 neutrons in the nucleus, a 25Mg atom has 13 neutrons, and also a 26Mg has 14 neutrons.

Figure 4. The symbol for an atom suggests the aspect via its usual two-letter symbol, the mass number together a left superscript, the atom number as a left subscript (sometimes omitted), and also the fee as a best superscript.

Information around the naturally developing isotopes of elements with atomic numbers 1 with 10 is offered in Table 5. Keep in mind that in addition to standard names and symbols, the isotopes of hydrogen are frequently referred come using typical names and also accompanying symbols. Hydrogen-2, symbolized 2H, is additionally called deuterium and sometimes symbolized D. Hydrogen-3, symbolized 3H, is likewise called tritium and sometimes symbolized T.

ElementSymbolAtomic NumberNumber that ProtonsNumber that 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 really Light Elements

Use this build an Atom simulator to construct atoms of the very first 10 elements, check out which isotopes exist, inspect nuclear stability, and gain endure with isotope symbols.

Atomic Mass

Because each proton and each ghost contribute about one amu come the mass of one atom, and also each electron contributes much less, the atomic mass the a solitary atom is approximately equal come its fixed number (a whole number). However, the median masses of atom of most facets are not whole numbers since most elements exist naturally as mixtures of two or much more isotopes.

The mass of an element shown in a routine table or detailed in a table of atom masses is a weighted, mean mass of all the isotopes present in a naturally arising sample of that element. This is equal to the amount of each individual isotope’s mass multiplied by its fountain abundance.

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

For example, the facet boron is written of two isotopes: about 19.9% of every boron atoms are 10B v a massive of 10.0129 amu, and the continuing to be 80.1% room 11B v a fixed of 11.0093 amu. The typical atomic mass for boron is calculated come be:

= l} extboron median 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 know that no solitary boron atom weighs specifically 10.8 amu; 10.8 amu is the average mass of all boron atoms, and individual boron atoms weigh either around 10 amu or 11 amu.

Example 2

Calculation of mean Atomic MassA meteorite found in central Indiana has traces that the noble gas neon choose up native the solar wind throughout the meteorite’s expedition through the solar system. Evaluation of a sample of the gas showed that it consisted of 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 median 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 median mass the a neon atom in the solar wind is 20.15 amu. (The typical mass the a terrestrial neon atom is 20.1796 amu. This result demonstrates that we may discover slight differences in the organic abundance that isotopes, depending on their origin.)

Check your LearningA sample of magnesium is uncovered to contain 78.70% of 24Mg atom (mass 23.98 amu), 10.13% the 25Mg atom (mass 24.99 amu), and also 11.17% the 26Mg atom (mass 25.98 amu). Calculate the average mass that a Mg atom.

Example 3

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

SolutionThe median mass of chlorine is the fraction that is 35Cl times the massive of 35Cl to add the portion that is 37Cl times the fixed 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 stand for the fraction that is 35Cl, climate the fraction that is 37Cl is represented by 1.00 − x.

(The fraction that is 35Cl + the portion that is 37Cl must add up come 1, so the fraction of 37Cl should equal 1.00 − the fraction of 35Cl.)

Substituting this 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 resolving yields: x = 0.7576, which way that 1.00 − 0.7576 = 0.2424. Therefore, chlorine consists of 75.76% 35Cl and 24.24% 37Cl.

Check your LearningNaturally occurring copper is composed of 63Cu (mass 62.9296 amu) and 65Cu (mass 64.9278 amu), v an typical mass the 63.546 amu. What is the percent ingredient of Cu in terms of these 2 isotopes?

Visit this site to do mixtures of the main isotopes of the very first 18 elements, obtain experience with typical atomic mass, and check naturally occurring isotope ratios utilizing the Isotopes and Atomic massive simulation.

The occurrence and also natural abundances that isotopes deserve to be experimentally established using one instrument referred to as a massive spectrometer. Massive spectrometry (MS) is widely offered in chemistry, forensics, medicine, environmental science, and many other fields to analyze and aid identify the building materials in a sample the material. In a typical mass spectrometer (Figure 5), the sample is vaporized and also exposed come a high-energy electron beam that reasons the sample’s atoms (or molecules) to come to be electrically charged, generally by losing one or an ext electrons. This cations then pass v a (variable) electric or magnetic field that deflects every cation’s course to an degree that counts on both its mass and also charge (similar to just how the course of a huge steel sphere bearing rolling previous a magnet is deflected come a lesser extent that that of a small steel BB). The ions are detected, and a plot of the relative number of ions generated versus their 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 portion of cations with the stated mass-to-charge ratio. Due to the fact that its initial use during the breakthrough of modern atomic theory, ms has evolved to become a powerful tool for chemical analysis in a wide selection of applications.

Figure 5. analysis of zirconium in a mass spectrometer produces a mass spectrum with peaks mirroring the various isotopes that Zr.

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

Key Concepts and also Summary

An atom consists of a small, positively fee nucleus surrounding by electrons. The nucleus consists of protons and neutrons; that diameter is about 100,000 times smaller sized than the of the atom. The fixed of one atom is normally expressed in atom mass devices (amu), i m sorry is referred to as the atomic mass. An amu is identified as specifically frac112 that the fixed of a carbon-12 atom and also is same to 1.6605 × 10−24 g.

Protons are reasonably heavy particles through a fee of 1+ and also a fixed of 1.0073 amu. Neutrons are fairly heavy particles with no charge and also a mass of 1.0087 amu. Electrons room light particles v a charge of 1− and also a mass of 0.00055 amu. The number of protons in the cell core is referred to as the atomic number (Z) and also is the building that defines an atom’s elemental identity. The sum of the numbers of protons and also neutrons in the cell nucleus is called the massive number and, to express in amu, is approximately equal to the mass of the atom. One atom is neutral when it consists of equal numbers of electrons and also protons.

Isotopes that an element are atoms with the exact same atomic number yet different mass numbers; isotope of one element, therefore, differ from every other just in the number of neutrons within the nucleus. When a naturally arising element is created of numerous isotopes, the atomic mass that the facet represents the typical of the masses the the isotope involved. A chemical symbol identifies the atoms in a substance making use of symbols, which are one-, two-, or three-letter abbreviations for the atoms.

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

Chemistry end of thing Exercises

In what means are isotopes of a provided element constantly different? In what way(s) room they constantly the same?Write the symbol for each the the adhering to ions:

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

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

(c) the ion with atomic number 15, fixed number 31, and a 3− charge

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

Write the symbol for each of the following ions:

(a) the ion through a 3+ charge, 28 electrons, and a mass variety of 71

(b) the ion with 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 mass number 87

Determine the number of protons, neutrons, and electrons in the following isotopes the are provided in clinical diagnoses:

(a) atom number 9, fixed number 18, charge of 1−

(b) atom number 43, mass number 99, fee of 7+

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

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

(e) surname the elements in parts (a), (b), (c), and (d).

The complying with are nature of isotope of two elements that are crucial in our diet. Determine the number of protons, neutrons and electrons in each and name them.

(a) atomic number 26, fixed number 58, fee of 2+

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

Give the variety of protons, electrons, and also neutrons in neutral atom of every of the adhering to 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 every of the following isotopes:

(a) _3^7 extLi

(b) _52^125 extTe

(c) _47^109 extAg

(d) _7^15 extN

(e) _15^31 extP

An aspect has the complying with natural abundances and also isotopic masses: 90.92% abundance v 19.99 amu, 0.26% abundance with 20.99 amu, and also 8.82% abundance with 21.99 amu. Calculation the average atomic mass of this element.Average atom masses noted by IUPAC are based upon a study of speculative results. Bromine has two isotope 79Br and 81Br, who masses (78.9183 and 80.9163 amu) and also abundances (50.69% and 49.31%) were determined in previously experiments. Calculation the typical atomic fixed of bromine based upon these experiments.Variations in average atomic mass might be it was observed for aspects obtained from various sources. Lithium provides an example of this. The isotopic composition of lithium native naturally developing minerals is 7.5% 6Li and 92.5% 7Li, which have actually masses of 6.01512 amu and 7.01600 amu, respectively. A commercial resource of lithium, recycled from a armed forces source, to be 3.75% 6Li (and the rest 7Li). Calculation the mean atomic mass worths for each of these two sources.The typical atomic masses that some facets may vary, relying on the sources of their ores. Naturally emerging boron is composed of 2 isotopes with accurately recognized masses (10B, 10.0129 amu and also 11B, 11.0931 amu). The actual atomic mass of boron can vary native 10.807 to 10.819, depending upon whether the mineral source is from Turkey or the joined States. Calculation the percent abundances resulting in the 2 values of the average atomic masses the boron native these two countries.The 18O:16O abundance proportion in some meteorites is better than that provided to calculate the mean atomic massive of oxygen on earth. Is the typical mass of one oxygen atom in this meteorites higher than, much less than, or same to the of a terrestrial oxygen atom?


anionnegatively fee atom or molecule (contains an ext electrons than protons)atomic massaverage massive of atom of one element, express in amuatomic massive unit (amu)(also, linked atomic mass unit, u, or Dalton, Da) unit of mass same to frac112 that the massive of a 12C atomatomic number (Z)number of proton in the nucleus of an atomcationpositively fee atom or molecule (contains under electrons 보다 protons)chemical symbolone-, two-, or three-letter abbreviation used to represent an aspect or the atomsDalton (Da)alternative unit indistinguishable to the atom mass unitfundamental unit that charge(also dubbed the elementary school charge) equates to the size of the charge of one electron (e) with e = 1.602 × 10−19 Cionelectrically charged atom or molecule (contains unequal numbers of protons and electrons)mass number (A)sum the the numbers of neutrons and also protons in the cell core of an atomunified atom mass unit (u)alternative unit identical to the atomic mass unit


Answers to Chemistry finish of chapter Exercises

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

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

See more: 13 Popular Names That Are The Same Backwards And Forwards!, Backwards Names Have Hidden Meaning

6. (a) Lithium-6 includes three protons, 3 neutrons, and also three electrons. The isotope price 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. Because there room three isotopes, there is no method to be sure to accurately suspect the abundances to make the complete of 20.18 amu average atomic mass. Let us guess that the abundances are 9% Ne-22, 91% Ne-20, and only a map of Ne-21. The mean mass would be 20.18 amu. Checking the nature’s mix the isotopes reflects that the abundances space 90.48% Ne-20, 9.25% Ne-22, and 0.27% Ne-21, so our guessed quantities have come be slightly adjusted.

14. 79.904 amu

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