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

The advance of modern-day atomic theory revealed much around the inner structure of atoms. It was learned that an atom includes a very tiny nucleus composed of positively charged protons and uncharged neutrons, surrounding by a much larger volume of room containing negatively charged electrons. The nucleus includes the bulk of one atom’s mass since protons and also neutrons are lot heavier 보다 electrons, conversely, electrons occupy almost every one of an atom’s volume. The diameter of one atom is on the order of 10−10 m, vice versa, the diameter the the cell core is approximately 10−15 m—about 100,000 time smaller. Because that a perspective around their relative sizes, consider this: If the nucleus were the size of a blueberry, the atom would be around the dimension of a football stadion (Figure 1).

You are watching: The symbol na+ represents a sodium atom that has lost an electron.

Figure 1. If one atom could be increased to the dimension of a football stadium, the nucleus would be the dimension of a solitary blueberry. (credit middle: alteration of job-related by “babyknight”/Wikimedia Commons; credit transaction right: change of work by Paxson Woelber)

Atoms—and the protons, neutrons, and also electrons that compose them—are incredibly small. Because that example, a carbon atom weighs much less than 2 × 10−23 g, and also an electron has a charge of less than 2 × 10−19 C (coulomb). When describing the properties of small objects such together atoms, we use appropriately little units that measure, such as the atomic massive unit (amu) and the fundamental unit of fee (e). The amu was originally defined based on hydrogen, the lightest element, then later in regards to oxygen. Since 1961, it has been identified with regard come the most abundant isotope of carbon, atoms of which room assigned masses of precisely 12 amu. (This isotope is well-known as “carbon-12” as will be questioned later in this module.) Thus, one amu is precisely frac112 that the fixed of one carbon-12 atom: 1 amu = 1.6605 × 10−24 g. (The Dalton (Da) and also the unified atom mass unit (u) are alternate units that are identical to the amu.) The an essential unit of fee (also dubbed the elementary school charge) equates to the size of the charge of one electron (e) with e = 1.602 × 10−19 C.

A proton has a fixed of 1.0073 amu and a charge of 1+. A ghost is a slightly heavier bit with a massive 1.0087 amu and also a fee of zero; together its surname suggests, that is neutral. The electron has actually a charge of 1− and is a lot lighter particle with a massive of around 0.00055 amu (it would certainly take about 1800 electron to same the mass of one proton. The properties of these fundamental particles are summarized in Table 3. (An observant student might notice that the amount of an atom’s subatomic corpuscle does no equal the atom’s yes, really mass: The total mass of six protons, 6 neutrons, and also six electron is 12.0993 amu, slightly larger than 12.00 amu. This “missing” massive is well-known as the mass defect, and also you will learn about it in the thing on atom 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 variety of protons in the nucleus of one 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 type of atom that includes six protons is the facet carbon and also has the atom number 6, regardless of how numerous neutrons or electron it might have. A neutral atom have to contain the same variety of positive and an unfavorable charges, so the variety of protons equates to the number of electrons. Therefore, the atomic number also indicates the number of electrons in one atom. The total number of protons and neutrons in one atom is called its mass number (A). The variety of neutrons is therefore the difference between the mass number and also the atomic number: A – Z = number of neutrons.

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

Atoms space electrically neutral if castle contain the same variety of positively charged protons and negatively fee electrons. Once the numbers of these subatomic particles room not equal, the atom is electrically charged and also is called an ion. The charge of one atom is characterized as follows:

Atomic fee = variety of protons − number of electrons

As will be questioned in an ext detail later on in this chapter, atom (and molecules) frequently acquire charge by acquiring or shedding electrons. An atom the gains one or an ext electrons will exhibit a an adverse charge and also is referred to as an anion. Positively fee atoms referred to as cations are developed when one atom loses one or an ext electrons. For example, a neutral sodium atom (Z = 11) has actually 11 electrons. If this atom loses one electron, it will come to be a cation with a 1+ charge (11 − 10 = 1+). A neutral oxygen atom (Z = 8) has eight electrons, and if it gains 2 electrons the will end up being an anion v a 2− charge (8 − 10 = 2−).

Example 1

Composition of an AtomIodine is an important trace facet in ours diet; it is necessary to create thyroid hormone. Insufficient iodine in the diet deserve to lead come the development of a goiter, one enlargement the the thyroid gland (Figure 2).

Figure 2. (a) inadequate iodine in the diet can reason an enlargement the the thyroid gland dubbed a goiter. (b) The addition of small amounts the iodine come salt, which prevents the development of goiters, has helped remove this problem in the us where salt consumption is high. (credit a: change of job-related by “Almazi”/Wikimedia Commons; credit b: change of job-related by Mike Mozart)

The addition of little amounts of iodine to table salt (iodized salt) has actually essentially removed this health worry in the united States, however as much as 40% the the world’s population is still at threat 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 electrons in one of these iodine anions.


The atomic number of iodine (53) tells us that a neutral iodine atom has 53 proton in its nucleus and 53 electrons exterior its nucleus. Since the amount of the numbers of protons and also neutrons equates 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 number of electrons is 54 <53 – (1–) = 54>.

Check your Learning

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

Chemical Symbols

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

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

The symbols for several usual elements and also their atom are detailed in Table 4. Some symbols are acquired from the usual name of the element; others room abbreviations the the name in another language. Most symbols have one or two letters, however three-letter symbols have actually been provided to explain some elements that have actually atomic numbers greater than 112. To protect against confusion with various other notations, only the very first letter the a symbol is capitalized. Because that example, Co is the symbol for the facet cobalt, but CO is the notation for the link carbon monoxide, which consists of atoms that the facets carbon (C) and also oxygen (O). All known elements and their symbols are in the routine table in number 2 in chapter 2.5 The routine Table (also found in appendix 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 common Elements and also Their Symbols

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

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


The symbol because that a particular isotope of any type of element is written by put the mass number together a superscript to the left that the element symbol (Figure 4). The atomic number is periodically written as a subscript preceding the symbol, but because this number specifies the element’s identity, as does its symbol, it is often omitted. Because that example, magnesium exists together a mixture of three isotopes, each through an atomic variety 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 also 26Mg. These isotope signs are review as “element, massive number” and also can it is in symbolized constant with this reading. Because that instance, 24Mg is read as “magnesium 24,” and can be created as “magnesium-24” or “Mg-24.” 25Mg is check out as “magnesium 25,” and can be written as “magnesium-25” or “Mg-25.” all magnesium atoms have actually 12 proton in your nucleus. They different only due to the fact that a 24Mg atom has 12 neutron in the nucleus, a 25Mg atom has actually 13 neutrons, and also a 26Mg has actually 14 neutrons.

Figure 4. The symbol for an atom indicates the facet via its normal two-letter symbol, the mass number together a left superscript, the atom number together a left subscript (sometimes omitted), and also the charge as a appropriate superscript.

Information about the naturally emerging isotopes of facets with atom numbers 1 v 10 is given in Table 5. Note that in addition to traditional names and also symbols, the isotopes of hydrogen are often referred come using typical names and also accompanying symbols. Hydrogen-2, symbolized 2H, is additionally called deuterium and also sometimes symbolized D. Hydrogen-3, symbolized 3H, is additionally called tritium and also sometimes symbolized T.

ElementSymbolAtomic NumberNumber of ProtonsNumber the 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 atom of the very Light Elements

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

Atomic Mass

Because every proton and also each spirit contribute roughly one amu to the fixed 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 atoms of most aspects are not entirety numbers since most elements exist normally as mixture of 2 or much more isotopes.

The massive of an facet shown in a routine table or noted in a table of atomic masses is a weighted, average mass of all the isotopes current in a naturally arising sample of that element. This is same to the sum 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 space 10B through a massive of 10.0129 amu, and also the continuing to be 80.1% are 11B with a massive of 11.0093 amu. The typical atomic mass for boron is calculated to 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 vital to understand that no single boron atom weighs exactly 10.8 amu; 10.8 amu is the median mass of every boron atoms, and also individual boron atoms sweet either around 10 amu or 11 amu.

Example 2

Calculation of typical Atomic MassA meteorite discovered in main Indiana includes traces of the noble gas neon choose up indigenous the solar wind throughout the meteorite’s expedition through the solar system. Analysis of a sample the the gas verified that it had 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 the 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 typical mass the a neon atom in the solar wind is 20.15 amu. (The mean mass the a terrestrial neon atom is 20.1796 amu. This an outcome demonstrates that us may discover slight differences in the natural abundance that isotopes, depending upon their origin.)

Check her LearningA sample that magnesium is uncovered to contain 78.70% that 24Mg atom (mass 23.98 amu), 10.13% the 25Mg atoms (mass 24.99 amu), and 11.17% that 26Mg atoms (mass 25.98 amu). Calculation the typical mass of a Mg atom.

Example 3

Calculation the Percent AbundanceNaturally emerging chlorine consists of 35Cl (mass 34.96885 amu) and also 37Cl (mass 36.96590 amu), v an mean mass that 35.453 amu. What is the percent composition of Cl in regards to these 2 isotopes?

SolutionThe typical mass the chlorine is the fraction that is 35Cl time the fixed of 35Cl plus the fraction 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 represent the fraction that is 35Cl, climate the portion that is 37Cl is represented by 1.00 − x.

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

Substituting this right into the median mass equation, us 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 addressing yields: x = 0.7576, which means that 1.00 − 0.7576 = 0.2424. Therefore, chlorine is composed of 75.76% 35Cl and 24.24% 37Cl.

Check her LearningNaturally emerging copper is composed of 63Cu (mass 62.9296 amu) and also 65Cu (mass 64.9278 amu), through an typical mass of 63.546 amu. What is the percent composition of Cu in terms of these two isotopes?

Visit this website to do mixtures the the key isotopes the the an initial 18 elements, get experience with median atomic mass, and also check naturally occurring isotope ratios using the Isotopes and also Atomic fixed simulation.

The occurrence and natural abundances of isotopes have the right to be experimentally established using one instrument called a fixed spectrometer. Massive spectrometry (MS) is widely offered in chemistry, forensics, medicine, environmental science, and many other fields to analysis and assist identify the building material in a sample the material. In a usual mass spectrometer (Figure 5), the sample is vaporized and also exposed to a high-energy electron beam that reasons the sample’s atoms (or molecules) to become electrically charged, generally by losing one or much more electrons. This cations climate pass with a (variable) electric or magnetic ar that deflects each cation’s course to an level that depends on both its mass and also charge (similar to just how the course of a big steel round bearing rolling past a magnet is deflected to a lesser level that the of a little steel BB). The ions space detected, and a plot the the relative number of ions generated versus your mass-to-charge ratios (a mass spectrum) is made. The elevation of every vertical function or height in a mass spectrum is proportional to the fraction of cations v the mentioned mass-to-charge ratio. Due to the fact that its initial use throughout the advance of contemporary atomic theory, multiple sclerosis has progressed to end up being a an effective tool because that chemical evaluation in a wide selection of applications.

Figure 5. analysis of zirconium in a fixed spectrometer produces a massive spectrum through peaks showing the various isotopes the Zr.

See an animation that describes mass spectrometry. Clock this video clip from the Royal society for Chemistry because that a quick description of the rudiments of fixed spectrometry.

Key Concepts and Summary

An atom is composed of a small, positively charged nucleus surrounded by electrons. The nucleus consists of protons and also neutrons; that is diameter is around 100,000 times smaller sized than that of the atom. The mass of one atom is commonly expressed in atomic mass devices (amu), i beg your pardon is referred to as the atom mass. An amu is identified as precisely frac112 that the massive of a carbon-12 atom and is same to 1.6605 × 10−24 g.

Protons are relatively heavy particles through a fee of 1+ and a mass of 1.0073 amu. Neutrons are relatively heavy particles through no charge and a massive of 1.0087 amu. Electrons space light particles v a charge of 1− and also a fixed of 0.00055 amu. The variety of protons in the cell core is referred to as the atomic number (Z) and also is the residential property that specifies an atom’s elemental identity. The sum of the number of protons and neutrons in the cell nucleus is dubbed the mass number and, express in amu, is around equal to the massive of the atom. An atom is neutral as soon as it includes equal number of electrons and also protons.

Isotopes the an facet are atoms with the exact same atomic number however different fixed numbers; isotopes of one element, therefore, different from each other only in the number of neutrons within the nucleus. When a naturally developing element is composed of numerous isotopes, the atomic mass that the element represents the typical of the masses of the isotopes involved. A chemical symbol identify the atoms in a substance utilizing 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 constantly different? In what way(s) are they constantly the same?Write the symbol because that each of the following ions:

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

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

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

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

Write the symbol because that each of the complying with ions:

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

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

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

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

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

(a) atom number 9, massive number 18, fee of 1−

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

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

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

(e) surname the facets in components (a), (b), (c), and also (d).

The following are properties of isotopes of two aspects that are vital in our diet. Identify the variety of protons, neutrons and electrons in each and also name them.

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

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

Give the variety of protons, electrons, and also neutrons in neutral atoms 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 number of protons, electrons, and also neutrons in neutral atoms of each 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 with 19.99 amu, 0.26% abundance v 20.99 amu, and 8.82% abundance through 21.99 amu. Calculation the typical atomic mass of this element.Average atomic masses listed by IUPAC are based on a research of speculative results. Bromine has two isotope 79Br and 81Br, who masses (78.9183 and 80.9163 amu) and abundances (50.69% and also 49.31%) were identified in earlier experiments. Calculate the median atomic fixed of bromine based on these experiments.Variations in typical atomic mass may be observed for aspects obtained from various sources. Lithium provides an example of this. The isotopic composition of lithium indigenous naturally developing minerals is 7.5% 6Li and also 92.5% 7Li, which have masses that 6.01512 amu and also 7.01600 amu, respectively. A commercial source of lithium, recycled from a army source, to be 3.75% 6Li (and the remainder 7Li). Calculation the typical atomic mass values for every of these 2 sources.The typical atomic masses the some facets may vary, depending on the sources of your ores. Naturally emerging boron consists of two isotopes v accurately well-known masses (10B, 10.0129 amu and 11B, 11.0931 amu). The actual atomic mass that boron deserve to vary native 10.807 to 10.819, depending on whether the mineral resource is from Turkey or the united States. Calculate the percent abundances bring about the two values the the typical atomic masses the boron indigenous these 2 countries.The 18O:16O abundance proportion in some meteorites is higher than that offered to calculation the mean atomic mass of oxygen top top earth. Is the typical mass of one oxygen atom in these meteorites better than, much less than, or same to that of a terrestrial oxygen atom?


anionnegatively charged atom or molecule (contains more electrons than protons)atomic massaverage fixed of atoms of an element, expressed in amuatomic massive unit (amu)(also, merged atomic massive unit, u, or Dalton, Da) unit the mass equal to frac112 of the mass of a 12C atomatomic number (Z)number of protons in the cell core of one atomcationpositively fee atom or molecule (contains under electrons than protons)chemical symbolone-, two-, or three-letter abbreviation used to represent an element or its atomsDalton (Da)alternative unit equivalent 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 also electrons)mass number (A)sum of the number 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 thing Exercises

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

4. (a) Carbon-12, 12C; (b) This atom contains six protons and also six neutrons. Over there are six electrons in a neutral 12C atom. The net charge of together a neutral atom is zero, and also the massive number is 12. (c) The coming before answers space correct. (d) The atom will be stable due to the fact that C-12 is a secure isotope of carbon. (e) The coming before answer is correct. Other answers for this exercise are possible if a different facet of isotope is chosen.

See more: 2002 Nissan Frontier Service Engine Soon Light After Service

6. (a) Lithium-6 includes three protons, 3 neutrons, and also 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 also 32 neutrons; (b) iodine, 53 protons, 54 electrons, and 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 united state use neon together an example. Since there are three isotopes, there is no means to be certain to accurately guess the abundances to do the total of 20.18 amu typical atomic mass. Let united state guess the the abundances are 9% Ne-22, 91% Ne-20, and also only a trace of Ne-21. The mean mass would certainly be 20.18 amu. Checking the nature’s mix the isotopes shows that the abundances room 90.48% Ne-20, 9.25% Ne-22, and 0.27% Ne-21, so our guessed quantities have to 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)