The changing atom

 

History of the atom

 

        The model of the atom has changed as our observations of its behaviour and properties have increased.

        A model is used to explain observations.  The model changes to explain any new observations.

        A map gives you an overview of a town/city.  As technology has improved our maps have become more accurate and detailed.  This is the same for scientific models.

        Research one of the stages in the development of the atom and present your findings to the group:-

 

Democritus (5th century)                                  The Greek 'atom' - indivisible

 

John Dalton (1800's)                                          Atoms of the same element are the same

 

George Johnstone Stoney (1891)                   Electrolysis.  The charge of an electron.

 

Joseph J Thompson (1897)                             The cathode ray tube and e/m deflection.  The mass / charge of an electron.

 

Joseph J Thompson (1906)                             Plum Pudding model of an atom.

 

Robert Milikan (1909)                                         Oil drop experiment.  The mass / charge of an electron.

 

Rutherford (and Marsden) (1909)                    Alpha particle deflection.  The nuclear model.

 

Henry Moseley (1913)                                        Atomic number

 

Neils Bohr (1913)                                                 Planetary model of the atom

 

Louis De Broglie (1923)                                     Wave particle duality

 

Erwin Shrodinger (1926)                                   Atomic orbitals

 

James Chadwick (1932)                                    Discovery of the neutron

 

Atomic structure

 

Protons, electrons and neutrons

Text Box: Sub-Atomic
Atomic 
Atomic
working
Particle
Mass
Charge
it out
Proton
1
+1
bottom
Electron
1/2000
-1
bottom
Neutron
1
0
top - bottom
 
Text Box:         From GCSE this table shows that only protons and neutrons have a mass.
        Since different elements have different atoms.
        These atoms have different numbers of protons and neutrons (and electrons).
        This means that different elements atoms must have different masses.

 

 

 

 

        So an atom of one element must have a different mass from another element, we call this the Mass Number.

        The number of protons determines which element an atom is and the bottom number tell us this, we call this the Atomic number.

 

Examples:-

1) Lithium

7 Li
3

                            Protons = 3

                            Electrons = 3

                            Neutrons = 4  (7-3)

 

2)  Nitrogen 

14 N
7

                            Protons = 7

                            Electrons = 7

                            Neutrons = 7  (14-7)

Isotopes 

 

An atom of the same element that has the same number of protons and electrons but a different number of neutrons.

 

 

                       

 

Atomic structure of ions

Sodium ion

Mass number, A = 23 Mass number, A = 23
Atomic number, Z, P = 11  (+) Atomic number, Z, P = 11  (+)
Electrons, e = 11  (-) Electrons, e = 10  (-)
Neutrons, N = 12 Neutrons, N = 12
charge is neutral, 11P (+) = 11e (-) charge is 1+, 11P (+) = 10e (-)

 

Chloride ion:

Mass number, A = 35 Mass number, A = 35
Atomic number, Z, P = 17  (+) Atomic number, Z, P = 17  (+)
Electrons, e = 17  (-) Electrons, e = 18  (-)
Neutrons, N = 18 Neutrons, N = 18
charge is neutral, 17P (+) = 17e (-) charge is 1-, 17P (+) = 18e (-)

 

Questions 1, 2 p7 / 1 p35 / 1p36

Chemical equations:

Writing formulae and balancing equations

        The first part of this is to be able to write formulae.

        Some names are straightforward whereas others are unexpected.

 

Writing formula

        A formulae is a shorthand way of describing a chemical substance.

        The composition and elements present are represented in the chemical formulae.

        Following a set of rules the chemical formula for a substance can deduced:-

 

Periodic Table Group

Charge on ion

Other ions

1

1+

H+  NH4+  Ag+

2

2+

Co2+  Cu2+  Fe2+

3

3+

Fe3+

6

2-

SO42-  CO32-

7

1-

OH-  NO3-

 

         The ions in a chemical formula must add up to zero.

         Use subscripts after an ion in a formula to double/triple that ion so the sum=0.  eg. CuCl2

         If you are double/tripling ions that consist of more than one element brackets must be used. eg.  Ca(OH)2

         If Roman numeral numbers follow a metal ion in brackets, that tells you the positive charge of that metal ion.  They are usually Transition Metals as they can have more than one oxidation state.  eg.  Copper (II) Chloride.  Copper can also exist as copper (I).

         Water of crystallization can be added to compounds after a dot.  eg.  CuSO4.7H2O

 

How to work out formulae

                Sodium Chloride -            Na+               Cl-                 Write the ions with charges

                                                           1          :        1                    Write the ratio of the charges

                                                                                                       Scale up if necessary to =0

                                                                     NaCl                          Bring together omitting the charges

 

                Copper (II) Chloride          Cu2+              Cl-                 Write the ions with charges

                                                           2          :        1                    Write the ratio of the charges

                                                           Cu2+     :        Cl-2                     Scale up if necessary to =0

                                                                     CuCl2                                Bring together omitting the charges

 

                Calcium Hydroxide           Ca2+              OH-               Write the ions with charges

                                                           2          :        1                    Write the ratio of the charges

                                                           Ca2+    :        (OH-)2           Scale up if necessary to =0

                                                                     Ca(OH)2                   Bring together omitting the charges

 

 

FormulaWriting simple formula from the periodic table

 

FormulaWriting complex formula

 

 

Balancing equations

 

Step 1          Write out the word equation.

 

             Iron     +       Hydrochloric acid         Iron (II) chloride        +       Hydrogen

 

Step 2          Write the correct formula underneath.

 

             Iron      +       Hydrochloric acid             Iron (II) chloride            +       Hydrogen

             Fe        +       HCl                                    FeCl2                           +       H2

 

Step 3          Balance using large numbers to scale up the number of molecules.  A rule of thumb to help you balance is to balance the elements in this order - MACHO:

                                    Metal

                                    Any other element

                                    Carbon

                                    Hydrogen

                                    Oxygen

                      A table helps.

 

             Iron      +       Hydrochloric acid             Iron (II) chloride            +       Hydrogen

             Fe        +       2HCl                                  FeCl2                            +       H2

                      Text Box: Atoms
No
No
Balanced
Fe
1
1
Yes
Cl
1x2=2
2
Yes
H
2
2
Yes
 

 

                                                                                                                                                                                                                                                                                                   

Put the 2 before the molecule with the element you are scaling up.  You now have the same number of atoms on each side.

 

Step 4          All that remains is to add the state symbols.

 

             Iron      +       Hydrochloric acid             Iron (II) chloride            +       Hydrogen

             Fe        +       2HCl                                  FeCl2                            +       H2

                      Text Box: Atoms
No
No
Balanced
Fe
1
1
Yes
Cl
1x2=2
2
Yes
H
2
2
Yes
 

 

                                                                                                                                                                                                                                                                                                                                                     

             Fe(s)     +       2HCl(aq)                             FeCl2(aq)                       +       H2(g)

 

You now have a balanced chemical equation.  This can be used to record an experiment or calculate amounts to mix.

Questions 1-2 p19 / 11 p35

Acids and Bases

Strong acids - Hydrochloric acid:

 

                                        Strong acids dissociate fully in water releases H+ ions into the solution:

                      HCl(g)                            H+(aq)             +                          Cl-(aq)

 

                                        Acids can be defined as proton donors

Weak acids - Ethanoic acid:

 

                                        Weak acids partially dissociate in water

                      CH3COOH(l)        +        aq                D            H+(aq)             +                          CH3COO-(aq)

Bases

                                        Bases can be defined as proton acceptors

 

Alkalis

                      NaOH(s)        +        aq                         Na+(aq)             +                          OH-(aq)

 

                                        Alkalis dissociate to give hydroxide ions in solution

                      NH3(g)        +        H2O(l)               D               NH4+(aq)        +        OH-(aq)

 

Biological acids and bases

 

Acids:  fatty acids, amino acids, nucleic acids

 

Amphoteric:  This means they have acidic and basic properties - amino acids - NH2 and CO2H

 

Questions 1-2 p23 / 12 p35

Salts

Practical (investigative)

 

Practical (Acid / Base - done earlier with moles)

 

(NOTE:  Alternatively do the practical as a circus cf GCSE)

 

All salts contain the following:

Acid Cation...Salt
Sulphuric ...sulphate
H2SO4 ...SO42-
Nitric ...nitrate
HNO3 ...NO3-
Hydrochloric ...chloride
HCl ...Cl-

1    Metal                                  +    Acid           Salt         +    Hydrogen

 

2    Metal Oxide                       +    Acid           Salt         +    Water

 

3    Metal Hydroxide (alkali)   +    Acid           Salt         +    Water

 

4    Metal Carbonate               +    Acid           Salt         +    Water        +       Carbon dioxide

Constructing balanced chemical equations:

Example:  Sodium hydroxide reacts with sulphuric acid, write a balanced chemical equation for this reaction:

 

Step 1          Use the 4 reactions above to identify your reaction - Sodium hydroxide tells us it is reaction (3)

 

             Sodium hydroxide     +       Sulphuric acid         Salt        +       Water

 

Step 2          Work out the name of your salt:

 

             Sodium hydroxide     +       Sulphuric acid         Sodium sulphate        +       Water

 

Step 3          Write the correct formula underneath.

 

             Sodium hydroxide     +       Sulphuric acid         Sodium sulphate        +       Water

                    NaOH                  +            H2SO4                       Na2SO4                +          H2O

 

Step 4          Balance the equation, in this order - works 90% of the time

 

                                    Metal

                                    Any other element

                                    Carbon

                                    Hydrogen

                                    Oxygen

 

             Sodium hydroxide     +       Sulphuric acid         Sodium sulphate        +       Water

                  2NaOH                   +            H2SO4                       Na2SO4                +          H2O

Element     Balanced
Na 1 2 NO
S      
H      
O      

 

             Sodium hydroxide     +       Sulphuric acid         Sodium sulphate        +       Water

                  2NaOH                   +            H2SO4                       Na2SO4                +        2H2O

Element     Balanced
Na 1 x 2 = 2 2 Yes
S 1 1 Yes
H 4 2 NO
O      

 

             Sodium hydroxide     +       Sulphuric acid         Sodium sulphate        +       Water

                  2NaOH                   +            H2SO4                       Na2SO4                +        2H2O

Element     Balanced
Na 1 x 2 = 2 2 Yes
S 1 1 Yes
H 4 2 No
O      

 

             Sodium hydroxide     +       Sulphuric acid         Sodium sulphate        +       Water

                  2NaOH                   +            H2SO4                       Na2SO4                +        2H2O

Element     Balanced
Na 1 x 2 = 2 2 Yes
S 1 1 Yes
H 4 2 x 2 = 4 Yes
O 6 6 Yes

 

 

Step 4          Add the state symbols:

 

                  2NaOH(aq)              +         H2SO4(aq)                     Na2SO4(aq)            +        2H2O(l)

 

You now have a balanced chemical equation.

 

Balance Eq

 

Acid salts:

                  H2SO4                     NaHSO4        Sodium hydrogen sulphate

                  NaHSO4                  Na2SO4        Sodium sulphate

 

Ammonium salts and fertilisers

                  NH3(aq)              +         HNO3(aq)                     NH4NO3(aq)

 

Calculating the % of N in a fertiliser:

                            % Element    =    No of atoms of that element in the formula    x    Ar of the element   x   100

                                                                                                    Mr of the compound

 

 

Worked example:

1)    Write out the formula:

 

                            % Element    =    No of atoms of that element in the formula    x    Ar of the element    x    100

                                                                                                           Mr of the compound

 

2)    Calculate the Mr of the compound:   

Element Ar No that element Sub total
N 14 2 28
H 1 4 4
O 16 3 48

Mr =

80

                                                                           

3)    Fill in the rest of the formula and calculate the answer

 

                            % Element    =           2 (N)    x    14 (Ar)    x    100

                                                                                   80

 

                            % Element    =       35%

 

Practical

Questions 1-2 p25 / 13 p35

Oxidation numbers

Rules for assigning Oxidation Numbers

 

    1)    Ox. No. of an element = 0

    2)    Ox. No. of each atom in a compound counts separately.  Sum = 0

    3)    Ox. No. of an ionic element = charge on ion.

    4)    In a polyatomic ion (SO42-), The sum of the Ox. No.s of the atoms = charge on ion.

    5)    H = +1  except with metals (metal hydrides = -1).

    6)    Gp 7 (Halogens) = -1  (except with oxygen)

    7)    O = -2  except in peroxides  (H2O2,  O = -1)

 

Examples:

 

Compounds -

      CO2           O = - 2 each, total = - 4

                         The molecule is neutral so   C + - 4 = 0

                         C = + 4

 

Formula -                        

      MgCl2        Mg = + 2

                         Cl = - 1 each, total = - 2

 

Molecular ions -                       

      NO3-           O = - 2 each, total = - 6

                         The molecule = - 1 so             N + - 6 = -1

                         N = + 5

 

Oxidation numbers in chemical names:

Transition metals:

Compound Name Element with different Ox. No. Ox. No. of that element
FeCl2 Iron (II) chloride Fe +2
FeCl3 Iron (III) chloride Fe +3

Oxyanions:

Oxyanion Name Element with different Ox. No. Ox. No. of that element
NO2- Nitrate (III) N +3
NO3- Nitrate (V) N +5

Ox No.s

Questions 1-4 p31 / 14 p35

 

Redox reactions

        2Mg(s)               +               O2(g)                     2MgO(s)

        Mg(s)               -               2e-                           Mg2+(s)

        O2(g)               +               4e-                           2O2-(s)

Oxidation                                  - is addition of oxygen / loss of hydrogen /

Is

Loss of electrons

 

Reduction                                 - loss of oxygen / addition of hydrogen /

Is

Gain of electrons

REDuction      and               OXidation

2Fe(s)               +            3Cl2(g)                    2FeCl3(s)

 

2Fe(s)               +            3Cl2(g)                    2FeCl3(s)

 

Fe(s)                                                             Fe3+(s)                  +              3e-

0                                                                        +3

 

1/2Cl2(g)                +            e-                         Cl -(s)

    0                                                                        -1

Oxidising and reducing agents:

 

2Fe(s)               +            3Cl2(g)                    2FeCl3(s)

 

Fe(s)                                                             Fe3+(s)                  +              3e-          Oxidation

0                                                                        +3

 

1/2Cl2(g)                +            2e-                         Cl -(s)                                               Reduction

0                                                                          -1

Oxidation                                  Reducing agent

Is

Loss of electrons

 

Reduction                                 Oxidising agent

Is

Gain of electrons

 

Examples of oxidising and reducing agents

            Fe2+            Fe3+        +        e-                Oxidation (requires an oxidising agent)

          Green              Brown

            Fe3+         +           e-               Fe2+         Reduction (requires a reducing agent)

          Brown                                          Green

 

Practical 

 

Using oxidation numbers with equations:

Mg(s) + 2HCl(aq) MgCl2(aq) + H2(g)

 

  Mg(s) + 2HCl(aq) MgCl2(aq) + H2(g)
Ox No's 0       +1 -1         +2 -1   0
                                               
      0 +2   +1 0   (x2)
      Up 2   Down 2    

Things to note:

Mg(s) + 2H+(aq) Mg2+ (aq) + H2(g)

REDOX

Qu 1-2 p33 / Qu 1, 11, 12, 13, 14, 18 P35 / Qu 1, 2a, 5a, 6a, 6b P36, 37