The Periodic Table
The Periodic Table tests the arrangement of elements, group trends and the properties of Groups 1, 2, 7 and 0 and transition metals.
Structure of the table
The Periodic Table arranges elements by increasing atomic number. A period is a horizontal row and corresponds to the number of occupied electron shells. A group is a vertical column and corresponds to the number of outer-shell electrons. Metals are on the left and centre, non-metals on the right, and the noble gases in Group 0. Group 1 contains the alkali metals, Group 2 the alkaline earth metals, and Group 7 the halogens. Transition metals form the central block.
Trends and properties
Reactivity in Group 1 increases down the group because the outer electron is further from the nucleus and more shielded, so it is lost more easily. Reactivity in Group 7 decreases down the group because the outer shell is further from the nucleus and gaining an electron is harder. Noble gases have full outer shells and are unreactive. Transition metals have variable oxidation states, form coloured compounds and often act as catalysts.
Worked example
Explain why potassium is more reactive than sodium. Both are Group 1 metals with one outer electron. Potassium has one more shell, so its outer electron is further from the nucleus and experiences more shielding by inner electrons. The attraction between the nucleus and the outer electron is weaker, so potassium loses the electron more easily and reacts more vigorously.
Traps
Group number is not the number of shells; period number is. Halogen displacement reactions show that a more reactive halogen displaces a less reactive one from its salt. Noble gases are unreactive because of their full outer shells, not because they have no electrons. Do not call all non-metals gases: carbon, sulfur and phosphorus are solids.