The periodic table and noble gases

The idea

Elements are arranged in order of atomic number; elements in the same group have the same number of outer electrons and similar reactions. Mendeleev left gaps for undiscovered elements and changed the order of some to fit patterns; the discovery of isotopes later explained this. Metals, on the left, form positive ions; non-metals do not. Group 0 noble gases are unreactive because they have full outer shells; their boiling points increase down the group.

1. Use the group to find outer electrons.
2. Use the period to find the number of shells.
3. Explain trends in terms of electrons.

Watch out Mendeleev's table was ordered by atomic weight, with changes; today's is ordered by atomic number.