Unit 1 Section 3 Periodicity

Atomic radius decreases across a period.

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.

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

The melting and boiling points generally increase from sodium to silicon, but then decrease from silicon to argon.

The melting and boiling points depend on the structure and bonding of the elements.

Sodium, magnesium and aluminium are metals. Their melting and boiling points increase across the period because the metal-metal bonds get stronger.

The bonds get stronger because the metal ions have an increasing number of delocalised electrons and a decreasing nucleus.

This leads to a higher charge density, which attracts the ions together more strongly. 

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 has high melting and boiling points.

Silicon is similar to carbon in diamond form. They are both in group 4, so have the same number of electrons in their outer shell.

Phosphorus (P4), sulfur (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 forces are weak and easily overcome so these elements have low melting and boiling points.

More atoms in a molecule mean stronger van der waals forces.

Sulfur is the biggest molecule (S8), so it's got higher melting and boiling points than phosphorus or chlorine.

Argon has very low melting and boiling points because it exists as individual atoms (monatomic) resulting in very weak van der waals forces.

The first ionisation energy is the energy needed to remove 1 electron from each atom in 1 mole of gaseous atoms to form 1 mole of gaseous 1+ ions.

There's a general increase in the first ionisation energy as you go across a period (period 3).

This is because of the increasing attraction between the outer shell electrons and the nucleus, due to the number of protons increasing.  

Macromolecular: A structure consisting of a huge network of covalently bonded atoms. They're also called giant covalent structures.

First ionisation energy: The energy needed to remove 1 electron from each atom in 1 mole of gaseous atoms to form 1 mole of gaseous 1+ ions.