ISOTOPY at a glance

                                         ISOTOPY /ISOTOPES

Isotopy is the existence of atoms of the same element having the same atomic number but different mass number or atomic mass. 

The different atoms are called isotopes. Hence

Isotopes are atoms of a particular element with the same atomic number but different mass number.

Examples of isotopes are

i.      ³⁵₁₇Cl and ³⁷₁₇Cl

ii.      ¹⁵₈O, ¹⁶₈O, ¹⁷₈O and ¹⁸₈O

iii.    ¹₁H, ²₁H and ³₁H

Relative Abundance: - when atoms exist naturally in isotopic mixture, they do so in a particular amount, when this amount is expressed in percentage, it is known as relative abundance. 

For example, chlorine is found always in two isotopic mixtures out of 100, 75 is Cl-35 and 25 is Cl-37. so, 75 and 25 when expresses in percentage are the relative abundance of the two isotopes. 

The relative abundance of an element is the amount (expressed in %) in which a particular isotope occurs in nature

Examples of isotopes are

i.   ³⁵₁₇Cl with relative abundance of 75% and ³⁷₁₇Cl with relative abundance of 25%.

ii.    ¹⁵₈O with relative abundance of 0.17, ¹⁶₈O with relative abundance of 99.76%, ¹⁷₈O with abundance of 0.05% and ¹⁸₈O with abundance of 0.02%

 RELATIVE ATOMIC MASS: - The relative atomic mass of an element is the number of times the average mass of one atom of the element is as heavy as 8one-twelfth the mass of carbon-12.

DETERMINATION OF RELATIVE ATOMIC MASS OF AN ELEMENT GIVEN THE RELATIVE ABUNDANCE

The relative abundance of an element can be calculated from their various isotopic masses, and their relative abundance as follows: -

1.   Naturally occurring exist in two isotopic mixtures ³⁵₁₇Cl with relative abundance of 75% and ³⁷₁₇Cl with relative abundance of 25% respectively. Calculate the relative atomic mass of Cl

            SOLUTION

    75 x 35 + 25 x 37 =
   100           100   

  
     26.25 + 9.25
          = 35.5

2.   An element Q has two isotopes ⁶³₂₉Q and ⁶⁵₂₉Q with relative abundance of 70% and 30% respectively. Calculate the relative atomic mass of Q

SOLUTION

70 x 63 + 30 x 65 =
100         100
  44.2 +   19.5
      = 63.70

3. X is an element which exists as an isotopic mixture containing 90% of ³⁹₁₉Xand 10% of ⁴¹₁₉X

a. How many neutrons are present in ⁴¹₁₉X isotope

b. Calculate the mean relative atomic mass of X

Solution

a. Neutrons in ⁴¹₁₉X

                    = 41-19 = 22

b. R.A.M =

               90 x 39 + 10 x 41 =
              100           100
                

                35.10 + 4.10 

                   = 39.2

 

CALCULATIONS

1. The following are more examples of calculations of relative atomic masses of elements.

2. An element Y exist in two isotopic forms ³⁹₁₈R and ⁴⁰₁₈R in the ratio 3:2 respectively. What is the relative atomic mass of the element?

SOLUTION

First you add the ratio together, that is, 3+2 =5

R.A.M of Y
    = 3 x 39 + 2 x40 = 
       5             5

    0.6 x 39 + 0.4 x 40 =
      23.4 + 16 =
        = 39.4   

3. An element with relative atomic mass 16.2 contains two isotopes ¹⁶₈R with relative abundance 90% and ^m₈R with relative abundance 10%. What is the value of m?

SOLUTION 

   16.2 = 90 x 16 + 10 x m
             100           100
   16.2 = 0.9 x 16 + 0.1m
   16.2 = 14.4 + 0.1m
   16.2 – 14.4 = 0.1m
   1.8 = 0.1 m
   m = 1.8 = 18
          0.1
  The value of m is 18

OBJECTIVE QUESTIONS

1.    The atomic number of an element is precisely

  (a) the number of protons in the atom

 (b)  the number of electrons in the atom 

 (c)  the number of neutrons in the atom

 (d)  the number of protons and neutrons in an atom

2.    An atom can be defined more accurately as 

(a)  the smallest indivisible particle of an element that can take part in a chemical reaction 

(b)  the smallest part of an element that can take part in a chemical reaction

 (c)  a combination of protons and neutrons

(d)   is the simplest unit of an element

3. The mass number is 

(a)  proton number +   neutron number

(b)   electron number + proton number 

(c)   neutron number + electron number

(d)   electron number + atomic number 

4. Calculate the relative atomic mass of an element having two isotopes ¹⁰⁷Ag and ¹⁰⁹Ag in the ratio 1:1

 (a) 106 

(b) 107 

(c) 108

(d)  109

5. An element X has two isotopes ^18.8X and ^15.8X in the proportion of 1:9 respectively. Find the relative atomic mass of X 

(a) 16.1 

(b) 13.6 

(c) 16.8

(d) 17.0

THEORY

1. (a) Define the term isotopy.

(b) Determine the number of electrons, protons and neutrons in each of the following:

  i.³⁹₁₉K    ii. ⁶³₂₆Cu    iii. ²³₁₁Na

2. If an element R has isotopes 60% of ¹²₆Qand 40% ^x₆Q and the relative atomic mass is 12.4, find x.

3. Consider the atoms represented below: ^q_rX and ^s_rX

a. State the relationship between the two atoms.

b. What is the difference between them?

c. Give two examples of other elements which exhibit the phenomenon illustrated.

4. State the number of electrons, protons and neutrons present in the following atoms/ions

(a)  ₂₀Ca 

(b)  ₁₆S^2- 

(c)  ₁₃Al³⁺ 

(d)  ₁₅P

THEORY QUESTIONS

1 a.(i)   Define Isotopy

     

      (ii). Element 3315Z and 3113Z occur in the ratio 1:3 

1. Calculate the relative atomic mass of Z

2. Give a reason why the relative stomach mass of Z is not a whole number

IUPAC NOMENCLATURE at a glance

          IUPAC NOMENCLATURE

This is a systematic method of naming organic compounds using IUPAC system. That is, the International Union of Pure and Applied Chemistry.  

The IUPAC system of naming organic compounds, the following rules are to be followed.

Every organic compound has 2 or three parts to its name, 

**the root and the suffix or 

**the prefix, the root and thwe suffix  

Organic Compounds without branches or that does not have any subtituents apart from the main the main functional group will have only two parts to their name.

While those with branches will have three parts to their names.

***The ROOT: - This is got from the longest continuous carbon chain in the molecule. We use the Greek system of numbering to indicate the root as follows

Number of C-atom	Name
one C-atom	Meth-
two C-atoms	Eth-
Three C-atoms	Prop
Four C-atoms	But-
Five C-atom	Pent -
Six    C-atoms	Hex-
Seven C-atoms	Hept-
Eight C-atoms	Oct-
Nine   C-atoms	Non-
Ten     C-atoms	Dec-

*** The SUFIX: - This is usually added after the root hydrocarbon, it indicates the family or homologous series that the organic compound belongs to. 

***PREFIX / PREFIXES: - These are usually hanging atoms or groups that are not the main functional group of the compound. They are usually named before the root hydrocarbon. Their positions are usually mentioned (using arithmetic integers like 1,2,3,...)  before their names, and where you have more than one of the same types, we use mono -, di-, tri-, tetra- to indicate the number.  

when we have more than one functional group in a compound, then one is named as the prefix and the other the sufix. In this case we will not use the name as a suffix but as a prefix. (see functional groups)

RULE 1: Select the longest continuous carbon chain in the molecule and use it as the root or the parent hydrocarbon name of this chain.

RULE 2: Every branch off the main chain should be considered as a substituent derived from a corresponding hydrocarbon or any other hanging group that is not a hydrogen 

 CH₄ -methane      CH₃ –   methyl, 

C₂H₆-ethane         CH₃CH₂- ethyl, 

C₃H₈ - propane      C₃H₇ - propyl  

All of these functions as prefix

RULE 3: Number the carbon atoms of the continuous chain from a direction that gives the Carbon atom carrying the substituent ( i.e the Carbon atom carrying anything other than a H-atom will have a lower number) a small number

RULE 4: Give each substituent a name and number

RULE 5: For identical substituents, use di, tri, tetra, penta, hexa, and so on to indicate the number of identical substituents.

RULE 6: Where you have more than one substituent that are different, name them alphabetically

RULE 7: Give the lowest possible number to the functional group.

NOTE: Halogen when they occur in organic compounds are named thus, chlorine = chloro, fluorine = fluoro, bromine = bromo, iodine = iodo.

Always show your bonds when you write or draw structures of organic compounds 

 E.g .1 write the structure of 2-methyl butane. This should be written as shown below:

                       

H  H  H  H                 ׀   ׀   ׀    ׀            H-C-C-C-C-H                 ׀   ׀    ׀    ׀                H   ׀   H  H                 H-C-H                      ׀                     H

                         

Following strictly the above rules, one can then name perfectly any organic compound with ease. For example, in naming the compound below, following the rules

 

                                          CH₃ 
                                                    |                                                                         
                                 C¹H₃C²H₂C³H₂C⁴H₂C⁵H₂C⁶H₃
                                                    |
                                                    CH₃ 

In the above structure, there are six carbon atoms in the longest continuous chain. Therefore, the parent or the ROOT hydrocarbon has a name Hex (RULE 1). There are 2 methyl substituents derived from another hydrocarbons (RULE 2)

Counting the position of the substituents on the continuous root chain, they both stand on the 4^th carbon atom when counting from left to right, but when counting from right to left, they appear on the 3rd carbon atom, hence the lowest position of 3rd carbon atom is considered (RULE 3)

Since the two substituents are similar, they are named dimethyl (RULE 4), hence the two substituents have a number 3 and name 3 and so can be named as 3,3-dimethyl (RULE 5). When this is combined with the parent hydrocarbon (RULE 1), the name of the compound becomes 3,3-dimethylhexane.

Other examples with their names are shown below following the stated rules.

                       CH₃                                                             CH₃
(i)                    |                                                                   |
              C1H₃-C2-C3H₂-C4H-C5H₃              (ii)        C1H₃C2H₂C3H-C4HC5H2C6H₃      
                       |            |                                                             |
                      CH₃     CH₃                                                        CH₃                                       
            2,2,4-trimethylpentane                                 3,4-dimethylhexane

                               CH₂CH₃                                                 CH₃
                                      |                                                               |
(iii)       C1H₃C2H₂-C3-C4H₂C5H₃                   (iv)       C1H₃-C2-C3H₃
                                      |                                                               |
                               CH₂CH₃                                                 CH₃
             3,3-diethylpentane                              2,2-dimethylpropane
                                                                                   

                                                                                        CH₃      CH₃
                                                                                                            |            |
(v)        CH₃CH₂CHCH₃                         (vi)       CH₃—C−CH2-CHCH₃
                     |                                                                    |
                    CH₃                                                              CH₃    
            2-methylbutane                                  2,2,4-trimethylpentane

(vii)      CH₃CH₂CH—CHCH₃                      (viii)     CH₃CH₂CHCH₃
                              ׀                                                          ׀       ׀               
                            CH₃  CH₃                                                    CH₃

             2,3-dimethylpentane                          2-methylbutane

                   

                   CH3                                                                                 CH3 
                       |                                                                                       |
(vi)       CH3 − C −CH− CH2−CH2CH2CH3                     (x)        CH3CH2CHCH2CHCH2CH2CH3 
                       |     |                                                                                             |
                     CH3  CH2CH3                                                                               CH2CH3  

            2,2-dimethyl, 3-ethylheptane                                        5-ethyl, 3-methyloctane

•                                                                                  Cl
•                                                                                  |
• (vii)       CH3CH2CHCH3                         (xii)      CH3-C-CH2CH3 
•                           |                                                       |
•                           Cl                                                    Cl
•             2-chlorobutane                                   2,2-dichlorobutane

(viii)     CH3CH2CHCH2CHCH3              (xiv)     CH3CHCH2CHCH3  

                          |            |                                          |          |

                         Cl         Br                                         Cl        Cl

            2-bromo, 4-chlorohexane                   2,4-dichloropentane

                           Cl        Br                                            Cl

                           |           |                                              |

(ix)      CH3CH2-C-CH2-C-CH3              (xvi)     CH3CHCH2-C-CH3  

                           |          |                                          |            | 

                          Cl         Br                                       Cl         Cl

            2,2-dibromo, 4,4-dichlorohexane       2,2,4-trichloropentane

                        CH3 H     H    CH3                                                               CH3  H      H    CH3 

                        

(x)    CH3 – C –  C –   C – C – CH3                  (xviii)   CH3 – C –   C –   C – C – CH3    

                       CH2 CH2 CH2 CH2                                                  CH2  CH2 CH2 CH2

                       CH3  CH3 CH3 CH3                                                       CH2  CH3 CH3 CH2

            4,5-diethyl,3,3,6,6-tetramethyloctane                      CH3                  CH3

                                                                                    5,6-diethyl,4,4,7,7-tetramethyldecane

(xi)     CH3 – CH2 – CH – CH3

                                    CH2Cl

            1-chloro – 2-methylbutane

          

ALKANES

ALKANES 

The alkanes are a homologous series of saturated aliphatic hydrocarbons with a general molecular formula of CnH2n+2.  The are the only saturated hydrocarbons because they contain only single bonds. The are found mainly in petroleum. The carbon atoms in the alkanes exhibit sp³ hybridization. There is a gradual change in the physical properties of the members from the gaseous state (for the lighter members) to the liquid state and then the solid state for the heavier compounds.

The first ten members of the series are.

1.       methane          CH₄

2.       Ethane             C₂H₆

3.       Propane           C₃H₈

4.       Butane             C₄H₁₀

_5.         Pentane           C₅H₁₂

6.        Hexane           C₆H₁₄

7.        Heptane          C₇H₁₆

8.        Octane            C₈H₁₈

9.        Nonane           C₉H₂₀

10.     Decane            C₁₀H₂₂

Chemical properties 

The alkanes undergo only two reactions, combustion and substitution reactions

1. Combustion Reaction: - this is a general property for all hydrocarbons. The alkanes burn in air or oxygen to yield Carbon (iv) oxide and water.

    CH_4(g) + O_2(g) → CO_2(g) + H₂O_(g)

2. Substitution Reaction: - Substitution reaction is the only other reactions that the alkanes can undergo because they are saturated.

A Substitution reaction is a reaction where one element (atom) replaces one or more of the hydrogen atoms in an alkane molecule.

METHANE: - This is the first and the simplest member of the alkane family. It has a molecular formula of CH₄ and a structural formula of

 

              H
               ׀
       H − C − H
               ׀
              H

It is found naturally in swamps or swampy areas when vegetations and dead organic matter decompose. It is also one of the major components in natural gas

Laboratory Preparation 

Methane is prepared in the laboratory by the action of sodium ethanoate on soda- lime.  (soda-lime is sodium hydroxide that has been slaked with lime CaO).

Soda- lime is preferred to caustic soda because 

1. It is not Deliquescent and 

2. It does not attack glass.

                                       Laboratory Preparation of methane 

CH₃COONa(S) + NaOH(s) → Na₂CO_3(s) + CH_4(g)  

Physical properties 

i.  It is a colourless gas. 

ii. It is less dense than air.

iii. It is insoluble in water.

Chemical properties 

I. Combustion: - methane burns with a pale blue flame in air or oxygen

CH_4(g) + O_2(g) → CO_2(g) + H₂O_(l) 

2. Substitution reaction: - Methane combines with Chlorine to produce various products.

The reaction is faster in the presence of light (photochemical reaction) and it occurs in stages.

I. CH_4(g) + Cl_2(g) → CH₃Cl + HCl

                             Mono chloromethane

II. CH₃Cl + Cl_2(g) → CH₂Cl₂ + HCl

                             dichloromethane

III. CH₂Cl₂ + Cl_2(g) → CHCl₃ + HCl

                            trichloromethane

IV.  CHCl₃ + Cl_2(g) → CCl₄ + HCl

                             tetrachloromethane

Uses of Methane 

I. It is used mainly as fuel sometimes mixed with other fuels

2. It is used to make hydrogen

3. It is used to make carbon black

4. When refined it is can be used as a rocket fuel

OBJECTIVE QUESTIONS 

1. What is the IUPAC name of the compound with this structure 

         H     H    H     H
         |        |      |        |   
H— C — C — C — C — H
         |        |       |         | 
         H      |      H       H
                  | 
          H —C—H
                  | 
                 H 
a. 3-methylbutane

b. 3-methylpentane

c.2-methylbutane 

d.2-methylpropane 

2.  Which of the following compounds is the structural isomer of the compound above?

a. 2,2-dimethylpropane 

b. 2-ethylpropane

c. 1,2-methylbutane 

d. 2-methylpentane

3. the energy value of petrol can be determined by a 

a. bomb calorimeter

b. catalytic cracker

c. fractionating column

d. thermometer

4. C8H18 will undergo the following reactions except 

a.  Cracking 

b. Combustion 

c. Substitution

d. Addition

5. One of the products of pentane in excess air 

a. pentanol 

b. pentene

c. nitrogen (II) oxide 

d. carbon (IV) oxide 

6. The gas produced when a mixture of sodium propanoate and soda lime is heated is 

a. Methane.

b. Pentane

c.. Ethane

d. Butene

 

THEORY QUESTIONS 

1. Methane reacts with chlorine under certain conditions to produce tetrachloromethane 

i.). State the condition for the reaction 

ii). Name the type of reaction

iii). Give two uses of methane 

iv). Name one natural source of methane.

                                                                                           ANSWERS

                                                                                           1. C 

                                                                                           2.  A