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SAMPLE EXERCISE 2.7

Give the chemical symbols, including mass numbers, for the following ions: (a) The ion with 22 protons, 26 neutrons, and 19 electrons; (b) the ion of sulfur that has 16 neutrons and 18 electrons.

Solution

In this video learn the difference between atomic mass and mass number. Learn how to calculate the atomic mass of Chlorine.Why chlorine have atomic mass of 3. Element Chlorine (Cl), Group 17, Atomic Number 17, p-block, Mass 35.45. Sources, facts, uses, scarcity (SRI), podcasts, alchemical symbols, videos and images.

(a) The number of protons (22) is the atomic number of the element, so the element is Ti (titanium). The mass number of this isotope is (the sum of the protons and neutrons). Because the ion has three more protons than electrons, it has a net charge of Thus, the symbol for the ion is

(b) By referring to a periodic table or table of elements, we see that sulfur (symbol S) has an atomic number of 16. Thus, each atom or ion of sulfur has 16 protons. We are told that the ion also has 16 neutrons, so the mass number of the ion is Because the ion has 16 protons and 18 electrons, its net charge is Thus, the complete symbol for the ion is

In general, we will focus on the net charges of ions and ignore their mass numbers unless the circumstances dictate that we specify a certain isotope.

PRACTICE EXERCISE

How many protons and electrons does the ion possess?

Answer: 34 protons and 36 electrons

Isotope patterns for -Cl and -Br

• Mass spectrometers are capable of separating and detecting individual ions even those that differ only by a single atomic mass unit (note in rality they are far more sensitive than that!)
  
• As a result, molecules containing different isotopes can be distinguished.
• This is most apparent (at this level) when atoms such as bromine or chlorine are present in a molecule because those elements naturally exist with a significant % of the heavier isotope.
• For example, while C has 2 common isotopes, ¹²C and ¹³C, ¹³C represents only about 1% of natural carbon. In contrast, Cl has 2 common isotopes, ³⁵Cl and ³⁷Cl, with about 25% being ³⁷Cl.
• Typically, one looks at the molecular ion peak, 'M' (since this is being identified and used to determine the MW).
• When working with MW from the molecular ion in MS, the best approach is to always use the lighter ion (i.e. M) and the mass of the lighter isotope (i.e. for Cl use 35 not 35.5, or, for Br use 79 and not 80)

• ³⁵Cl : ³⁷Cl exists naturally in an almost 3:1 ratio, so we observe peaks at 'M' (molecules with an atom of ³⁵Cl) and 'M+2' (molecules an atom of ³⁷Cl) are obtained with relative intensity 3:1
  

• ⁷⁹Br : ⁸¹Br exists naturally in an almost 1:1 ratio, so we observe peaks at 'M' (molecules with an atom of ⁷⁹Br) and 'M+2' (molecules an atom of ⁸¹Br) are obtained with relative intensity 1:1
  

Chlorine 35 Atomic Number

• Note that since the relative natural abundances of the isotopes are different, you can tell the difference between the presence of Cl and Br. The patterns are different, they look different.
• 'M+1' peaks are usually seen due to the presence of ¹³C in the sample but because 13C is only about 1% of natural carbon, the peaks tend to be small (unless there is a large number of C atoms present). Note that you can see the small peaks due to the presence of ¹³C in the figures shown for Cl and Br, they look like little shadows on the right of the other peaks.

The following two examples of mono-haloalkanes mass spectra show the characteristic isotope patterns of monohalogenated molecules. The patterns are highlighted in the green boxes:

Example 1 :

This MS is of 2-chloropropane, C₃H₇Cl.

37 Cl Mass Number

Note the characteristic isotope pattern at 78 (M) and 80 (M+2) in a 3:1 ratio.
Loss of ³⁵Cl from 78 or³⁷Cl from 80 gives the base peak a m/z = 43 (M - 35 = M+2 - 37 = 43) corresponding to the secondary propyl cation.
Note that the peaks at m/z = 63 and 65 represent fragment ions that still contain Cl and therefore also show the 3:1 isotope pattern.
The very small peak at 79 represents M+1, the small number of molecules that contain ³⁵Cl and an atom of ¹³C rather than ¹²C.
The even smaller peak at 81 presents M+2+1 = M+3, a very small number of molecules that contain ³⁷Cl and an atom of ¹³C rather than ¹²C.

Example 2 :

This MS is of 1-bromopropane, C₃H₇Br.

Note the isotope pattern at 122 and 124 represents the M and M+2 in a 1:1 ratio.
Loss of ⁷⁹Br from 122 or ⁸¹Br from 124 gives the base peak a m/z = 43, corresponding to the propyl cation.
Note that other peaks, such as those at m/z = 107 and 109 (yes, they are small) still contain Br and therefore still show the 1:1 isotope pattern.

Note: the isotope patterns for polyhalogenated molecules (such as having both -Cl and -Br or with multiple -Cl or -Br) give different (but still characteristic isotope patterns).