Periodicity

1. The periodic table is arranged into periods (rows) and groups (columns), by atomic (proton) number.

2. All elements within a period have the same number of electron shells. E.g. the elements in period 2 have 2 electron shells.

3. All elements within a group have the same number of electrons in their outer shell - so they have similar properties.

4. The group number tells you the number of electrons in the outer shell, e.g. Group 1 elements have 1 electron in their outer shell, Group 4 elements have 4 electrons and so on...

The periodic table can be split into an s block, d block, and p block like below. Doing this shows you which sub-shells all the electroons go into.

1. The s-block elements have an outer electron configuration of s1 or s2

2. The p-block elements have an outer shell configuration of s2 p1 to s2 to p6

3. The d-block elements have electron configurations in which d-sub-shells are being filled.

1. As the number of protons increases, the positive charge of the nucleus increases. This means electrons are pulled closer to the nucleus, making the atomic radius smaller.

2. The extra electrons that the elements gain across a period are added to the outer energy level so they don't really provid any extra shielding effect (shielding works with inner shells mainly).

1. Sodium, magnesium and aluminium are metals. Their melting and boiling points increase across the period becasue the metal-metal bonds get stronger. The bonds get stronger because the metal ions have an increasing number of delocalised electrons and a decreasing radius. This leads to a higher charge density, which attracts the ions together more strongly.

2. Silicon is macromolecular, with a tetrahedral structure - strong covalent bonds link all its atoms together. A lot of energy is needed to break these bonds, so silicon hs high melting and boiling points.

3. Phosphorus (P4), sulphur (S8) and chlorine (Cl2) are all molecular substances. Their melting and boiling points depend upon the strength of the Van der Waals forces between their molecules. Van der Waals are weak and easily overcome so these elements have low melting and boiling points.

4. More atoms in a molecule mean stronger Van der Waals forces. Sulphur is the biggest molecule (S8), so it's got higher melting and boiling points than phosphorus or chlorine.

5. Argon has very low melting and boiling points because it exists as individual atoms (they're monatomic) resulting in very weak Van der Waals.

This is because of the increasing attraction between the outer shell electrons, and the nucleus, due to the number of protons increasing (there are a few blips in the the trend however, go back to the ionisation energies section for more details).