Periodicity (hl)

13.1 - Trends across period 3

Characteristic	Trend (left to right)	Reason
Atomic radius	decreases in size from left to right	increased attractive force (acting on the same energy shell) from the nucleus as the number of protons (and hence the nuclear charge) increases
Ionic radius	decreases across the period until formation of the negative ions then there is a sudden increase followed by a steady decrease to the end	In general as above. The sudden increase on formation of negative ions is due to the new (larger) outer shell
Electronegativity	Increases	More electron attracting power of the larger nuclear charge as we move to the right
Metallic character	Decreases - Na, Mg, Al metals; Si metalloid; P, S, Cl, Ar non-metals	Metallic character is a measure of the ease of loss of electrons from the outer shell. This decreases with increasing nuclear charge.
Oxides	Na, Mg - alkaline Al - amphoteric Si, P, S, Cl -acidic
Chloride character	NaCl - ionic MgCl2 - some covalent character AlCl3 - covalent The remainder covalent	Increasing charge density of the positive ion polarises the chloride ion as we move to the right hand side
Melting point	NaAl steady increase	Increasing availability of electrons in the metallic bonding associated with greater charge density of the metal ion
	Si massive increase	Si giant macromolecular structure
	P large decrease	P4 molecules
	S small increase	S8 molecules
	Cl Ar decrease	Cl2 molecules and Ar atoms

Chemical periodicity of period 3 oxides

Chemical periodicity of period 3 chlorides

13.2 - First-row d-block elements

Characterised by the following properties:

• Variable oxidation states

• Coordinated ligands

• Complex ions

• Coloured compounds

• Magnetic properties

• Catalytic properties

Variable oxidation state

The multiple oxidation states of the d-block (transition metal) elements are due to the proximity of the 4s and 3d sub shells (in terms of energy). All transition metals exhibit a 2+ oxidation state (both electrons being lost from the 4s and all have other oxidation states (common).

Coordinated ligands

Ligands are the molecules (or ions) which donate an electron pair to form a dative covalent bond with the central transition metal atom (forming a complex molecule or ion).

Coordination

Complexes

These are species which are formed around a central atom, with other atoms, ions or molecules donating an electron pair to form a covalent bond to this central atom. The result is a "complex" usually an ion but may also be a molecule.

Coloured compounds

The color in the transition metals (d-block) is predominantly due to the splitting of the d shell orbitals into slightly different energy levels. As a result, certain wavelengths of energy can be absorbed by the d-block elements (with electrons jumping between these slightly different energy levels), resulting in the complement color being visible.

Colour is affected by both the oxidation state of the transition metal and the type of ligand

Crystal field theory

Magnetism

Transition metals and their ions often have unpaired 'd' electrons which produce an asymmetric magnetic field that can be detected. This is called paramagnetism

examples

Catalytic activity

'd' block elements make good catalysts due to their multiple oxidation states (hence their ability to react with different species and produce a path of lower activation energy, and so allow the reaction to proceed at a faster rate). Another possible reason for their catalytic activity is their available 'd' orbitals which allow reacting molecules to co-ordinate to the surface of the transition metal which in turn weakens the bonding within the molecule allowing it to react.

Examples...

• MnO₂ in decomposition of hydrogen peroxide
• V₂O₅ in the contact process
• Fe in Haber process
• Ni in conversion of alkenes to alkanes