Chemistry Unit 3

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)

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.

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)

                           no. of protons + neutrons

                mass number             23

                                                              Na

          atomic number       11

                                                  number of protons

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

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

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)