Average Atomic Mass Of Chlorine

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Average Atomic Mass Of Chlorine

Naturally occurring chlorine is 75.78% 35 Cl, which has an atomic mass of 34.969 amu, and 24.22% 37 Cl, which has an atomic mass of 36.966 amu. Calculate the average atomic mass (that is, the atomic weight) of chlorine. The average atomic mass is found by multiplying the abundance of each isotope by its atomic mass and summing these. The element chlorine has two naturally occurring isotopes. About 75% of chlorine isotopes are Cl-35 and about 25% are Cl-37. What is the average atomic mass of chlorine atoms?

Chlorine

Naturally occurring chlorine is 75.53 percent 35 Cl, which has an atomic mass of 34.969 amu, and 24.47 percent 37 Cl, which has an atomic mass of 36.966 amu. Calculate the average atomic mass (that is, the atomic weight) of chlorine. SOLUTION As in the preceding example, the average atomic mass is found by multiplying the abundance of each. A brief explanation of the process used to gather data on isotopes (mass spectrometry) and calculation of an element's average atomic mass. For an element relative atomic mass is the average mass of the naturally occurring isotopes of that element relative to the mass of an atom of 12C. Relative atomic mass = (75/100 x 35) + (25/100 x 37) Relative atomic mass = 35.5 u. Thus the atomic mass of chlorine is taken as 35.5u and not a whole number like 36u or 35u.

Explanation

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 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).

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:

Calculate The Average Atomic Mass Of Chlorine

Cl (g) + e– = Cl +(g) + e–

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.

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

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.

Average Atomic Mass Of Chlorine Isotopes

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.

References:

Average Atomic Mass Of Chlorine Percentage

  1. http://www.docbrown.info/page04/4_71atomMSintro.htm

Average Atomic Mass Of Cl 35

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