Chemistry Unit 3

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The Periodic Table

The stucture of the Periodic Table

History:

- 1808: John Dalton -> arranged all known elements in order of their masses.

- 1863: John Newlands -> law of octaves.

- 1869: Dimitri Mendeleev -> arranged all known elements in order of mass and properties, and he left gaps for the undiscovered alements.

Protons and Neutrons dicovered!

- 1900+: Modern Periodic Table -> all elements arranged according to atomic number (number of protons)


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The Modern Periodic Table

Elements are arranged in order of atomic number (number of protons) and NOT according to mass.

e.g 40                                 39

            Ar is placed before          K in the periodic table, despite it being heavier than K.

       18                                 19

e.g 128                       127

              Te is before            I

        52                        53

                                                   Te has one proton less than I, but must have 2 more                                                    neutrons than I, and this is why it's heavier.

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The Structure of the Periodic Table

Groups = columns

        = group number = number of electrons in the outer shell

        = elements in the same group have similar properties (chemical and physical)

        = some groups have names

Group 1 = alkali metals                    Periods = rows

       2 = alkaline earth metals                    = period number = number of shells

       7 = halogens                                     altogether

       0/8 = noble gases                             = elements change properties from                                                            stronglty metallic -> metalloids                                                                -> non-metals (periodicity)

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The Structure of the Periodic Table (continued)

                           no. of protons + neutrons

                mass number             23

                                                              Na

          atomic number       11

                                                  number of protons

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Group 1: Alkali metals

Physical properties:                        Chemical properties:

- soft (can be cut with a knife)            Metal                Obvservations              Reactivity (1-most

- low density (floats on water)                                                                                       2-least)

- relatively low melting point               Lithium (Li)        - fizzing.                                   3

- shiny when freshly cut                                             - moving about the

                                                                               surface of the water.

                                                                                - Quick reaction.

                                                       Sodium (Na)       - more fizzing                           2

                                                                                - moves about faster

                                                                                - quicker reaction

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Group 1: Alkali metals (continued)

Metal                            Observations             Reactivity (1-most 3-least)

Potassium (K)              - violent fizzing                    1

                                   - catches fire

                                   - very quick reaction

                                     (quickest)

another observation: solution left after reaction turns universal indicator blue/purple

Conclusions

This reaction forms two products:

1) flammable gas -> hydrogen, H2

 2) alkaline solution -> metal hydroxide e.g LiOH, NaOH, KOH


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Group 1: Alkali metals (continued 2)

Equation:

                   metal + water -> metal hydroxide + hydrogen

                   2Li(s) + 2H2(l) -> 2LiOH(aq) + H2(g)

                    2Na(s) + 2H2O(l) -> 2NaOH(aq) + H2(g)

                    2K(s) + 2H2O(l) -> 2KOH(aq) + H2(g)

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