Periodicity

Periodicity (hl)

13.1	- Periodic trends

13.2	- d block elements

13.1 - Periodic trends NaAr (the third period)

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

	Na₂O	MgO	Al₂O₃	SiO₂	P₄O₁₀ (or P₄O₆)	SO₃ (or SO₂)	Cl₂O₇ Cl₂O
Add H2O	Na₂O + H₂O -> 2NaOH	MgO + H₂O -> Mg(OH)₂	Insoluble	Insoluble	P₄O₁₀ + 6H₂O -> 4H₃PO₄ P₄O₆+ 6H₂O -> 4H₃PO₃	SO₃ + H₂O -> H₂SO₄ SO₂ + H₂O -> H₂SO₃	Cl₂O₇ + H₂O -> 2HClO₄ Cl₂O + H₂O -> 2HOCl
Add HCl	Na₂O + H⁺ -> 2Na⁺ + H₂O	MgO + 2H⁺ -> Mg²⁺ + H₂O	Al₂O₃ + 6H⁺ -> 2Al³⁺ + 3H₂O	No reaction	No reaction	No reaction	No reaction
Add NaOH	No reaction	No reaction	Al₂O₃ + 2OH^- + 3H₂O -> 2Al(OH)₄	SiO₂ + 2OH^- -> SiO₃^2- + H₂O	H₃PO₄ + OH^- -> H₂PO₄^- + H₂O H₃PO₃ + OH^- -> H₂PO₃^- + H₂O	SO₂ + OH^- -> HSO₄^- SO₂ + OH^- -> HSO₃^-	HCl₂O₇ + OH^- -> Cl₂O₇^2- + H₂O HOCl + OH^- -> OCl^- + H₂O
Nature	Basic Oxide	Basic Oxide	Amphoteric Oxide	Acidic Oxide	Acidic Oxide	Acidic Oxide	Acidic Oxide
Conductivity	Good	Good	Good	None	None	None	None
Melting Point	1275	2852	2027	1610	24	17	-92

Chemical periodicity of period 3 chlorides

	NaCl	MgCl₂	Al₂Cl₆	SiCl₄	PCl₃	PCl₅	Cl₂
Add H₂O	Dissolves to give free ions	Dissolves to give free ions	Hydrolysis to give [Al(H₂O)₆]3+ and Cl- ions	Reacts to produce HCl and Si(OH)₄	Reacts to produce H₃PO₃ and HCl	Reacts to produce H₃PO₄ and HCl	Dissociates to give HOCl and HCl
Nature	ionic	ionic	covalent	covalent	covalent	covalent	covalent
Conductivity	Good	Good	None	None	None	None	None
Melting Point	801	714	178	-70	-112		-101

13.2 - d-block elements (first row: ScZn)

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).

Sc	Ti	V	Cr	Mn	Fe	Co	Ni	Cu	Zn
								+1
	+2	+2	+2	+2	+2	+2	+2	+2	+2
+3	+3	+3	+3	+3	+3	+3	+3
	+4	+4		+4
		+5
			+6	+6	+6
				+7

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).

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.

Complex	shape	ligands	coordination number	name
[Fe(H₂O)₆]³⁺	octahedral	water	6	hexa-aqua iron III ion
[Fe(CN)₆]^3-	octahedral	cyanide CN-	6	hexacyano ferrate III ion
[CuCl₄]^3-	tetrahedral	chloride Cl-	4	tetrachloro cuprate I ion
[Cu(NH₃)₄]²⁺	square planar	ammonia	4	tetra-ammine copper II ion
[Ag(NH₃)₂]⁺	linear	ammonia	2	diammine silver I ion
Ni(CO)4	tetrahedral	carbon monoxide	4	tetracarbonyl Nickel 0 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

Complex ion	Oxidation state of metal	colour	ligand
[Fe(H₂O)₆]³⁺	III	pale green	water
[Fe(H₂O)₆]²⁺	II	yellow	water
[Cu(H₂O)₆]²⁺	II	blue	water
[Cu(NH₃)₄]²⁺	II	deep blue	ammonia
[CuCl₄]^2-	II	green	chloride ion

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

Complex ion	electronic configuration	no of unpaired electrons	magnetism
[Fe(H₂O)₆]³⁺	[Ar]4s0 3d5	5	paramagnetic
[Cr(H₂O)₆]³⁺	[Ar]4s0 3d3	3	paramagnetic
[Cu(H₂O)₆]²⁺	[Ar]4s0 3d9	1	paramagnetic
[Ni(NH₃)6]²⁺	[Ar]4s0 3d8	2	paramagnetic
[CoCl₄]^2-	[Ar]4s0 3d7	3	paramagnetic

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
V₂O₅in the contact process
Fe in Haber process
Ni in conversion of alkenes to alkanes

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Copyright: 2003 Isis Publication