INORGANIC Part 6 Period 4 survey & group trends page sub-index: 6.1 Survey of Period 4 elements * 6.2 Period 4 trends in physical properties * 6.3 Period 4 trends in bonding, formulae and oxidation state * 6.4 Important element trends down a Group

Advanced Level Inorganic Chemistry Periodic Table Index * Part 1 Periodic Table history * Part 2 Electron configurations, spectroscopy, hydrogen spectrum, ionisation energies * Part 3 Period 1 survey H to He * Part 4 Period 2 survey Li to Ne * Part 5 Period 3 survey Na to Ar * Part 6 Period 4 survey K to Kr and important trends down a group * Part 7 s-block Groups 1/2 Alkali Metals/Alkaline Earth Metals * Part 8  p-block Groups 3/13 to 0/18 * Part 9 Group 7/17 The Halogens * Part 10 3d block elements & Transition Metal Series * Part 11 Group & Series data & periodicity plots

6.4 A summary of important physical and chemical trends down a Group

GROUP DATA TABLES - data aligned down the group

z = at. no., cond'y = conductivity, config. = configuration, oxidation states are numerically equal to valencies

M = metal, SM = semi-metal (metalloid), NM = non-metallic element

Group 1 Alkali Metals

• One of the most consistent sets of data for any group of the periodic table, helped by the fact they are all metals.

• The melting/boiling points, electronegativity, 1st ionisation energy and electrical conductivity decrease down the group.

• The atomic radius and density increase down the group.

• The formulae are perfectly consistent with what you might expect from the principles of the Periodic Table e.g. the chlorides, the metal having the only possible oxidation state of +1 (valency 1).

• Down the group the peroxide (M₂O₂) and the 'superoxide' (MO₂) become more stable with respect to the expected oxide formula (M₂O).

• 7 Detailed advanced level chemistry notes s-block Group 1 Alkali Metals

• Explanation of general group trends down the groups of the periodic table (end of this page)

Group 2 Alkaline Earth Metals

• A reasonably consistent sets of data for a group of the periodic table, helped by the fact they are all metals..

• The melting/boiling points, electronegativity, 1st ionisation energy and electrical conductivity decrease down the group.

• The atomic radius and density increase down the group.

• There is total consistency in the formulae for the expected oxidation state of +2 (valency 2) for the chloride and 'normal oxide'.

• From magnesium onwards they all form a peroxide (MO₂) as well as the 'normal' expected oxide (MO).

• 7 Detailed advanced level chemistry notes s-block Group 2 Alkaline Earth Metals

• Explanation of general group trends down the groups of the periodic table (end of this page)

Group 3/13

• Apart from the atomic radius and density, the 'patterns' are not very consistent.

• The +1 oxidation state does get more stable down the group, but Group 3/13 chemistry is dominated by the +3 oxidation state compounds.

• Detailed advanced level chemistry notes p-block Group 3/13

• Explanation of general group trends down the groups of the periodic table (end of this page)

Group 4/14

• Apart from the atomic radius, 1st ionisation energy and density, the 'patterns' are not very consistent.

• The +2 oxidation state becomes more stable down the group.

• Detailed advanced level chemistry notes p-block Group 4/14

• Explanation of general group trends down the groups of the periodic table (end of this page)

Group 5/15

• The Pauling electronegativity, atomic radius and 1st ionisation energy all show reasonably consistent trends.

• Apart from nitrogen, the oxidation state pattern of +3 and +5 in compounds is quite consistent, though the +5 state generally becomes less stable with respect to the +3 state.

• Detailed advanced level chemistry notes p-block Group 5/15

• Explanation of general group trends down the groups of the periodic table (end of this page)

Group 6/16

• Melting points, boiling points, Pauling electronegativity, atomic radius, 1st ionisation energy and density all show reasonably consistent trends down the group.

• Apart from oxygen, the +6 oxidation state becomes less stable with respect to the +2 and +4 oxidation states.

• Detailed advanced level chemistry notes p-block Group 6/16

• Explanation of general group trends down the groups of the periodic table (end of this page)

Group 7/17 The Halogens

• All the physical properties show consistent trends down the group.

• Apart from fluorine, all the elements exhibit oxidation states of -1, +1, +3, +5 and +7 in their compounds.

• Detailed advanced level chemistry notes p-block Group 7/17 The halogens

• Explanation of general group trends down the groups of the periodic table (end of this page)

Group 0/18 The Noble Gases

• All the physical properties show a consistent trend down the group.

• Detailed advanced level chemistry notes p-block Group 0/18 The Noble Gases

• Explanation of general group trends down the groups of the periodic table (end of this page)

Summary and general arguments for group trends

• 1st Ionisation enthalpy of an element: ΔH for the process X_(g) ==> X⁺_(g) + e^- 

  • The 1st ionisation energy decreases down a group as the outer electrons are increasingly less strongly held.

  • As you go down the group from one element down to the next, the outer electron is further and further from the nucleus because of the extra full shell of electrons due to the change in period. The outer electrons are also shielded by this extra full shell of negative charge from the effect of the positive nucleus. The effective nuclear charge can be considered to be equal to the number of outer electrons and this is effectively the same for all the elements in a group.

  • However, it is spread over an increasingly larger 'outer surface', down the group, so steadily reducing the electric field effect of the positive nucleus attracting the outer negative electrons. Therefore the outer electron is less and less strongly held by the nucleus and this combination of factors means the outer electron is increasingly more easily lost, (e.g. the M⁺ ion more easily formed), and so less energy is needed remove it.

  • The 'diluting effect' of the increasing outer surface area on the strength of the electron attracting electric field effect, outweighs the increase in atomic/proton number down the group.

• Atomic radius of an element

  • Can be defined as volume within which 95% of the electron charge exists on a time averaged basis.

  • Atomic radius increases down the group as an extra inner shell of electrons is added per period.

  • The argument is almost identical to that for ionisation energy above.

  • As you go from one period to another an extra full inner shell of electrons is added and the outer electrons are increasingly further away, increasingly shielded by more filled inner shells and hence less strongly attracted to the nucleus

  • Therefore the volume occupied by the electrons expands as you go down the group.

• Ionic radii

  • The ionic radius increases down the group as an extra inner shell of electrons is added per period.

  • For the trend we assume all the ions have the same charge for the same group e.g. Gp1/2 form M⁺/M²⁺ ions or Gp7 form X^- ions and the argument is identical to that for the atomic radius increase.

  • The reasons are identical to the argument set out for atomic radius.

• Metallic or non-metallic character of an element:

  • Elements tend to get more metallic down a group.

  • The trend doesn't show up strongly in Gps 1, 2 (both all metals) and 7 Halogens (all non-metals), but in Gp 4 you start with non-metallic carbon at the top and very metallic lead at the bottom.

  • The principal reason is the lowering of ionisation energies as you descend the group.

• Electronegativity of an element (usually the Pauling scale)

  • The power (electric field effect) of an atom to attract electron charge towards it, in the context of the electrons of a covalent bond linking it to another atom.

  • Electronegativity decreases down a group.

    • The reason are essentially the same as the lowering of the 1st ionisation energy due to increase in atomic radius and the outer electrons being less strongly held, even in the context of a bonding situation.

    • So, in this case, the arguments support the notion that the nucleus has a lesser and lesser effect in attracting the outer electrons including a pair of bonding electrons.

    • This weakens the bond and less thermal energy is required effect a change of state melting or boiling.

• Melting points and boiling points of elements:

  • Trends can be a bit complicated due to significant structural changes from one element to another in the same group.

  • However for Groups 1 and 2 the general trend is for melting points and boiling points to decrease down the group.

    • As the atomic radii increase down the group, the positive nucleus to delocalised distance increases.

    • This decreases the electrical attractive force between the immobile ions in the metal lattice and the free mobile delocalised electrons.

  • For Groups 7/17 Halogens and Group 0/18 Noble Gases the melting points and boiling points steadily increase down the group.

  • As the molecule gets bigger with more electrons, the Van der Waals forces (transient dipole - induced dipole) increase in strength so more energy is required to overcome them i.e. a higher thermal energy - increased temperature.

• Relative electrical conductivity:

  • Generally increases as the metallic character increases, which can, as in the case of Group 5,

• Density

  • Generally increases as the metallic character increases, which can, as in the case of Group 5,

  • However, even if they are all metals (Groups 1& 2) or all non-metals (Groups 7/17 & 0/18) generally speaking density does increase down a group.

• Oxidation states

  • In terms of the general principles of the Periodic Table i.e. group similarity, you would expect all the elements to exhibit the same oxidation states in their compounds, and generally this is true except for oxygen in Group 6/16 and fluorine in Group 7/17.

  • However, down a group you can get a significant change in the relative stabilities of different oxidation states.

    • This is can be seen as increasing stability of a lower oxidation state with respect to a higher oxidation state e.g.

      • In Group 3/13 the +1 state becomes more stable with respect to the +3 state

      • In Group 4/14 the +2 state becomes more stable with respect to the +4 state

      • In Group 4/14 the +2 state becomes more stable with respect to the +4 state

• The character of the chlorides

  • For Groups 1 and 2 the halides become more ionic as the difference in electronegativity increases and the steadily decreasing polarising power of the cation as its radius increases down the group.

  • Trends become complicated if two different oxidation states are involved.

• The character of the oxides

  • For Groups 1 and 2 the halides become more ionic as the difference in electronegativity increases and the steadily decreasing polarising power of the cation as its radius increases down the group.

  • Trends become complicated if two different oxidation states are involved.

• The acid-base character of oxides: 

  • More importantly, oxides get more basic or less acidic down a group.

    • e.g. Group 5/15

    • At the top of the group the oxides of nitrogen and phosphorus are strongly acidic,

    • arsenic(III) oxide is weakly acidic,

    • antimony(III) oxide is amphoteric,

    • and bismuth(III) oxide at the bottom of the group is basic.

• -

Diagrammatic summary of three important Periodic Table Trends

and the so-called 'diagonal relationship of some elements'

WHAT NEXT?

See also 4.1 Period 2 Survey of the individual elements, 4.2 Period 2 element trends and explanations of physical properties * 4.3 Period 2 element trends in bonding, structure, oxidation state, formulae & reactions, 5.1 Period 3 survey of elements, 5.2 Period 3 element trends & explanations of physical properties, 5.3 Period 3 element trends in bonding, structure, oxidation state, formulae & reactions, 6.1 Survey of Period 4 elements, 6.2 Period 4 trends in physical properties, 6.3 Period 4 trends in bonding, formulae and oxidation state

INORGANIC Part 6 Period 4 survey & group trends page sub-index: 6.1 Survey of Period 4 elements * 6.2 Period 4 trends in physical properties * 6.3 Period 4 trends in bonding, formulae and oxidation state * 6.4 Important element trends down a Group

Advanced Level Inorganic Chemistry Periodic Table Index * Part 1 Periodic Table history * Part 2 Electron configurations, spectroscopy, hydrogen spectrum, ionisation energies * Part 3 Period 1 survey H to He * Part 4 Period 2 survey Li to Ne * Part 5 Period 3 survey Na to Ar * Part 6 Period 4 survey K to Kr and important trends down a group * Part 7 s-block Groups 1/2 Alkali Metals/Alkaline Earth Metals * Part 8  p-block Groups 3/13 to 0/18 * Part 9 Group 7/17 The Halogens * Part 10 3d block elements & Transition Metal Series * Part 11 Group & Series data & periodicity plots

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