Mass Of Chlorine




Chlorineis a chemical element with the symbol Cland it has atomic number 17. Chlorine is the second member of halogen group ithas similar properties like fluorine, bromine and iodine. Chlorine haselectronic configuration [Ne] 3s23p5 withthe seven electrons in the third outermost shell acting as its valenceelectrons.

Chlorine Gas Cl2 Molar Mass, Molecular Weight. PCl3 + Cl2 = PCl5. Chlorine (17 Cl) has 25 isotopes with mass numbers ranging from 28 Cl to 52 Cl and 2 isomers (34m Cl and 38m Cl). There are two stable isotopes, 35 Cl (75.77%) and 37 Cl (24.23%), giving chlorine a standard atomic weight of 35.45. The longest-lived radioactive isotope is 36 Cl, which has a half-life of 301,000 years. The average mass of chlorine is 35.45 amu. Chlorine has two stable isotopes: chlorine – 35 and chorine – 37. The atomic weight of chlorine given on the periodic table is 35.47 u. Different isotopes have different relative abundances,chlorine – 35 has a relative abundance of 75.76%,whereas chlorine – 37 has a relative abundance of 24.24%. Chlorine has two stable isotopes chlorine-35 and chlorine-37with Chlorine-35 accounting for roughly 3 out of every 4 naturally occurring chlorine atoms. Chlorine-36 is also known naturally and is a radioactive isotope with a half life of about 30,000 years.

Chlorine has two stable isotopes: chlorine – 35and chorine – 37. The atomic weight of chlorine given on the periodic table is35.47 u. Different isotopes have different relative abundances ,chlorine – 35has a relative abundance of 75.76% ,whereas chlorine – 37 has a relativeabundance of 24.24%. Chlorine -35 is about 3 times more abundant than chlorine– 37, the weighted average is closer to 35 than 37.

The mass spectrum of Chlorine

Chlorine is such an element which contain morethan one atom per molecule. It has two isotopes Cl-35 and Cl-37, so it contain3 atoms of Cl-35 and 1 atom of Cl-37. It consist of molecules so when it passedinto the ionization chamber, the electrons are knocked off, and give molecularion, Cl2+. The ions are not stable so some will formchlorine atom and a Cl+ ion. This process is called fragmentation.

Cl2+→ Cl + Cl+

The Cl atom is neither accelerated nor deflected in the machine it is not ionized in the ionization chamber and simply lost. But Cl+ ions will pass through the machine and give lines at 35 and 37 it is depend upon isotopes.

Mass Spectrometry

Mass spectrometry is an important method which is used to identify elements and compounds by their mass spectrum. Mass spectrometry is a technique used to determine relative isotopic masses of different elements and relative abundance of the isotopes.

Inall types of mass spectroscopy they include vaporing atoms or molecules in highvacuum and create electron bombardment to generate a beam of positive ionscalled ionization. Mass spectrometer separates and counts the numbers ofdifferent positive ions particles are released, the resulting product from thedetector is known as mass spectrum (plural mass spectra).

Mass Of Chlorine

Theseare different types of mass spectrometer:

Method No. 1. Deflection Mass Spectrometer:

It is also called as TOF type. Deflection mass spectrometer consists of ionization, acceleration the positive ions, which in turn deflection of ions and ion detection followed by deflection, separation and detection. The substance which is to be analyzed is injected in the high vacuum tube system which has extremely low pressure particles are ionized through colliding with beam of high speed electron.

In the above diagram the symbol K as sampleand Q as high voltage supply label. High voltage electron gun falls a beam ofhigh energy electrons from a heated metal element into the vaporized sampleunder analysis and causes ionization of the atoms or molecule form positiveions. The collision of high energy electrons with atoms or molecules causesanother electron to remove the particle as positively charged particle. The sample must in the gaseous phase, In caseof chlorine:

Cl (g) + e = Cl +(g) + e

Mass of chlorine gas

The low pressure vacuum is needed to stop theions to collide with air particles which affect the motion of particles toreach the ion detector system. Fluorine atomic number of neutrons.

Negativeplate as P in above diagram. Negative plates accelerate the positive ions tomove through the tube. The moving charge particles create a magnetic fieldaround itself which interact with magnetic field of the system at point R [2].

Deflectionof ions due to magnetic field is label as R. Magnetic field deflect the mono-positiveions according to their increasing mass towards the ion detection system. Alllike mass particles move down the tube. Ion detection system is labelled as N. ionsstrike the ion detection system where they generate a small electrical current.This small current convert into electronic signals appear as ion peaks whichsent to computer for analysis and display as mass spectrum. The data isdisplayed as m/z versus peak height.m/z means relative mass over chargewhich help to know relative atomic mass of ionized particle.

The mass spectrum of chlorine is good example of molecular element. Chlorine has two stable isotopes; chlorine-37 (25%) and chlorine -35 (75%).There are five main peaks of isotopes of chlorine of various isotopic monatomic ions.

  1. [37Cl2]+ mz =74 ( molecular ion)
  2. [37Cl 35 Cl]+ mz = 72
  3. [35Cl2]+ m/z=70 (molecular ion)
  4. [37Cl]+ m/z = 37 (monatomic ion)
  5. [35Cl]+ m/z =35 (monoatomic ion)

Thepresence of five peaks for chlorine shows the ratio of heights for peaks 1 and2 is 3: 1. For bimolecular ions, ¾ of the chlorine isotopes are Cl 35 and ¼ ofthe isotope of chlorine is cl37.

Themass spectrum of an organic compound having chlorine atoms also show differentmass spectrum. Organic compound having one chlorine atom show molecular peaksM+ and M+2.the molecular ion containing 35Cl isotope has relativeatomic mass 78 whereas molecular ion containing 37Cl has relativeatomic mass 80.

Mass spectrum of 2-chloropropane is given below. In this mass spectrum the peak heights of chlorine are in the ratio of 3:1 which show that lighter isotope of chlorine is attached with more number of molecules as compared to the heavier isotope. The fragmentation of 2-chloropropane formed are

Same pattern of peaks are observed at m/z = 63 and m/z = 65 due to chlorine atoms which is attached to CH3CH forming positive ion.

The organic compound having two chlorine atoms show three peaks due different combinations of isotopes of chlorine are attached with carbon and hydrogen. The ratio observed in this case is 9:6:1.the compound containing 2 chlorine atom have difference in ratio due to isotopes attached with fragments.

Method No. 2. Time of Flight (TOF) Mass Spectrometer

The principle of this method is also include ionization, acceleration to donate constant kinetic energy to all ions, ions drift, ions detection and also data analysis, all things are controlled and carried out with the help of computers now a days.

Theions are formed similarly through electron bombardment and the ions which areproduced as a result of bombardment are accelerated between electricallycharged plates. The sample must be in gaseous form, laser beam is used toaccelerate electrons and produced positive ions. In this process magnetic fieldis not used to separate the positive ions.

Inthis process the ionized particle which has smaller mass has smaller time offlight in the drift region so in this case ions are separate on the base oftheir time of flight.

t=Kinst √(m/q)

t=timeof flight

m=massof ion

q=chargeon ion

√=squareroot of

Kinst= a proportionality constant based on the instrument settings andcharacteristics e.g. the electric field strength, length of analyzing tube etc.


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Learning Objectives

  • Define atomic mass.
  • Calculate atomic mass given relevant information about the isotopes.

Have you ever tried to move a boulder?

You have a pile of rocks to move and need to decide what equipment you want to rent to move them. If the rocks are fairly small, you can get a shovel to pick them up. Larger rocks could be moved by hand, but big boulders will need some sort of mechanical scoop. The amount of each kind of rock will also determine how much time you will need to get the job done. Knowing the relative amounts of large, medium, and small rocks can be very useful in deciding how to approach the job.

Most elements occur naturally as a mixture of two or more isotopes. Table below shows the natural isotopes of several elements, along with the percent natural abundance of each.

Atomic Masses and Percent Abundances of Some Natural Isotopes
Element Isotope (symbol)Percent natural abundanceAtomic mass (amu)Average atomic mass (amu)



[latex]^3_1text{H}[/latex] negligible 3.0160



[latex]^{14}_6text{C}[/latex]trace 14.003




[latex]^{18}_8text{O}[/latex]0.204 17.999




[latex]^{37}_{17}text{Cl}[/latex]24.23 36.966




[latex]^{65}_{29}text{Cu}[/latex] 30.83 64.928

For some elements, one particular isotope predominates greatly over the other isotopes. Naturally occurring hydrogen is nearly all hydrogen-1 and naturally occurring oxygen is nearly all oxygen-16. For many other elements, however, more than one isotope may exist in more substantial quantities. Chlorine (atomic number 17) is a yellowish-green toxic gas. About three quarters of all chlorine atoms have 18 neutrons, giving those atoms a mass number of 35. About one quarter of all chlorine atoms have 20 neutrons, giving those atoms a mass number of 37. Were you to simply calculate the arithmetic average of the precise atomic masses, you would get 36.

[latex]displaystylefrac{left(34.969+36.966right)}{2}=35.968text{ amu}[/latex]

Clearly the actual average atomic mass from the last column of the table is significantly lower. Why? We need to take into account the percent natural abundances of each isotope in order to calculate what is called the weighted average. The atomic mass of an element is the weighted average of the atomic masses of the naturally occurring isotopes of that element. The sample problem below demonstrates how to calculate the atomic mass of chlorine.

Sample Problem: Calculating Atomic Mass

Use the atomic masses of each of the two isotopes of chlorine along with their percent abundances to calculate the average atomic mass of chlorine.

Step 1: List the known and unknown quantities and plan the problem.


  • chlorine-35: atomic mass = 34.969 amu and % abundance = 75.77%
  • chlorine-37: atomic mass = 36.966 amu and % abundance = 24.23%


  • Average atomic mass of chlorine

Change each percent abundance into decimal form by dividing by 100. Multiply this value by the atomic mass of that isotope. Add together for each isotope to get the average atomic mass.

Step 2: Calculate

Mass Of Chlorine Atom

chlorine-350.7577 × 34.969 = 26.50 amu
chlorine-370.2423 × 26.966 = 8.957 amu
average atomic mass26.50 + 8.957 = 35.45 amu

Note: Applying significant figure rules results in the 35.45 amu result without excessive rounding error. In one step:

(0.7577 × 34.969) + (0.2423 × 36.966) = 35.45 amu

Step 3: Think about your result.

The calculated average atomic mass is closer to 35 than to 37 because a greater percentage of naturally occurring chlorine atoms have the mass number of 35. It agrees with the value from the Table above .

Watch these videos to learn more about these calculations:

Mass of chlorine 35


  • The atomic mass of an element is the weighted average of the atomic masses of the naturally occurring isotopes of that element.
  • Calculations of atomic mass use the percent abundance of each isotope.


Click on the link below to get some experience in atomic mass determinations:


  1. Define atomic mass.
  2. What information do you need to calculate atomic mass for an element?
  3. Calculate the atomic mass for carbon using the data provided in Table below.

mass number

exact weight

percent abundance








Chlorine Mass Spec

  • atomic mass: The weighted average of the atomic masses of the naturally occurring isotopes of that element.
  • percent abundance: To calculate the weighted average, take into account the percent natural abundances of each isotope. The atomic mass of an element is the weighted average of the atomic masses of the naturally occurring isotopes of that element.
Show References

Atomic Mass Of Chlorine


Mass Of Cl

  1. Devon Fyson, based on image created by the U.S. Geological Survey.
  2. George M. Groutas.