Revise Periodicity of physical properties of the elements in Period 3 for Cambridge 9701 (9.1)

As we move from sodium to argon, a proton is added to the nucleus and an electron is added to the outer shell for each successive element. Crucially, all the outer electrons are in the third principal energy level ($n=3$). The increasing number of protons leads to a greater nuclear charge. While there are more electrons, the shielding effect from the inner shells (n=1 and n=2) remains constant. Consequently, the electrostatic attraction between the nucleus and the outermost electrons strengthens, pulling the electron cloud in more tightly. This results in a steady decrease in atomic radius across the period.

First ionisation energy is the energy required to remove one mole of electrons from one mole of gaseous atoms to form one mole of gaseous 1+ ions. The general trend across Period 3 is an increase. This is a direct consequence of the decreasing atomic radius and increasing nuclear charge. The outermost electron is held more tightly, so more energy is needed to remove it. However, there are two important exceptions to this smooth increase.

X(g) → X⁺(g) + e⁻

The first anomaly occurs between Group 2 (Magnesium) and Group 13 (Aluminium). Mg has the electron configuration [Ne]3s², while Al is [Ne]3s²3p¹. The electron removed from Al is from the 3p orbital, which is at a slightly higher energy level and is partially shielded by the 3s electrons. This makes it energetically easier to remove than one of Mg's 3s electrons.

The second anomaly is between Group 15 (Phosphorus) and Group 16 (Sulfur). P has the configuration [Ne]3s²3p³, with three unpaired electrons in the 3p subshell (a stable half-filled subshell). S is [Ne]3s²3p⁴, where one 3p orbital contains a pair of electrons. The repulsion between these two electrons in the same orbital makes it easier to remove one of them compared to removing an electron from a singly occupied orbital in phosphorus.

The melting point trend across Period 3 is not a simple increase or decrease. It is directly related to the change in bonding and structure from metallic to giant covalent to simple molecular.

• Na, Mg, Al: These are metals. Melting point increases as the strength of the metallic bond increases. This is due to the increasing number of delocalised electrons per atom (1 for Na, 2 for Mg, 3 for Al) and the increasing charge of the metal cation, leading to stronger electrostatic forces of attraction.
• Si: Silicon has a giant covalent structure, similar to diamond. To melt it, many strong covalent bonds must be broken, which requires a very large amount of energy. This gives Si the highest melting point in the period.
• P, S, Cl: These are non-metals that exist as simple discrete molecules (P₄, S₈, Cl₂). Their melting points are low because only weak intermolecular van der Waals' forces need to be overcome. The strength of these forces depends on the number of electrons in the molecule, so S₈ (more electrons) has a higher melting point than P₄, which is higher than Cl₂.
• Ar: Argon is a noble gas and exists as individual atoms (monatomic). The forces between atoms are extremely weak van der Waals' forces, resulting in a very low melting point.

Electrical conductivity depends on the presence of mobile charge carriers. The trend across Period 3 reflects the change in bonding.

• Na, Mg, Al: As metals, they possess a lattice of positive ions surrounded by a 'sea' of delocalised electrons. These electrons are mobile and can move through the structure to carry an electrical current. Conductivity increases from Na to Al due to the increasing number of delocalised electrons.
• Si: A metalloid, silicon is a semiconductor. Its electrons are held in covalent bonds, but a small number can gain enough thermal energy to break free and move, allowing for a small current to flow. Its conductivity is many orders of magnitude lower than metals.
• P, S, Cl, Ar: These are electrical insulators. Their electrons are either held tightly in covalent bonds (P, S, Cl) or within stable atoms (Ar). There are no mobile charge carriers, so they cannot conduct electricity.