CfE Higher Chemistry

Unit One – Chemical Changes and Structure

Chapter Three – Trends in the Periodic Table

Physical Properties of the Elements

There are variations in the physical properties of the elements across the periods and down the groups. These trends include; melting point, boiling point, density, covalent radius, ionisation energy and electronegativity.

Melting and Boiling Points

Melting and boiling points decrease down group one of the periodic table. This occurs as there is a decrease in the electrostatic force of attraction between the positive metal core and the negatively charged delocalised electrons.

Melting and boiling points increase down groups seven and zero of the periodic table. This occurs as there is an increase in the London Dispersal Force of attraction between the molecules in group seven elements and between the atoms in group zero with their increased molecular mass.

Density

In any period in the Periodic Table the density first increases from group one to a maximum value in the centre of the Periodic Table before decreasing again towards group zero.

In any group the density of the elements increases as the atomic number increases.

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Covalent Radius – a measure of atomic size.

Covalent Atomic Radius is defined as half the distance between the nuclei of two covalently bonded atoms of an element.

The distance between the two nuclei can be measured by X-ray diffraction which determines the bond length. The covalent radius is half the bond length.

For example, X-ray diffraction determines the bond length between two hydrogen atoms in a molecule of hydrogen as 0.74 x 10-10m. Therefore the covalent radius of a hydrogen atom is half this value at 0.37 x 10-10m.

0.37 x 10-10m is equivalent to 37 x 10-12m or 37 pm, where pm is a picometre (10-12).

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Covalent Radius (atomic size) is a periodic property as there are two trends associated with it;

 Decreasing covalent radii across a period.  Increased covalent radii down a group.

Elements increase in atomic number (number of protons) across a period in the Periodic Table, therefore their positive nuclear also increases. Elements in the same period of the Periodic Table have the same number of occupied energy levels.

This increased positive nuclear charge exerts an increased attraction force on its outer negatively charged energy level, resulting in the covalent radius decreasing in size.

Down any group in the Periodic Table the covalent radii increases as an extra energy level is added each period down the group.

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Ionisation Energy

First Ionisation Energy

The first ionisation energy of an element is the enthalpy change per mole required to remove one mole of electrons from one mole of atoms in their gaseous state. It is an endothermic process.

For example

+ - -1 Na(g) → Na (g) + e ΔH = +502KJmol

Ionisation Energy is a periodic property as there are two trends associated with it;

 Increasing Ionisation Energy across a period.  Decreasing Ionisation Energy down a group.

Across a period in the Periodic Table Ionisation Energy increases as the positive nuclear charge increases, so, attracting the outer electrons more strongly. As a result more energy is required to remove these electrons.

Down a group in the Periodic Table Ionisation energy decreases as the outer electrons are further away from the positive nucleus (extra energy levels) and there is a screening effect from the inner electron energy level, so, the attraction for their outer electrons is weaker in strength. As a result less energy is required to remove these electrons.

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Second Ionisation Energy

The Second Ionisation Energy of an element is the enthalpy change per mole associated with the removal of a second electron from each gaseous one plus ion. It is an endothermic process.

For example

+ 2+ - -1 Na (g) → Na (g) + e ΔH = +4560KJmol

The Third Ionisation Energy can be defined in a similar way.

+ - First Ionisation Energy X(g) → X (g) + e

+ 2+ - Second Ionisation Energy X (g) → X (g) + e

2+ 3+ - Third Ionisation Energy X (g) → X (g) + e

Calculation

Use your data booklet to calculate the energy required for the following change?

3+ - Al(g) → Al (g) + 3e

+ - -1 Al(g) → Al (g) + e ΔH = +584KJmol

+ 2+ - -1 Al (g) → Al (g) + e ΔH = +1830KJmol

2+ 3+ - -1 Al (g) → Al (g) + e ΔH = +2760KJmol

Add

3+ - -1 Al(g) → Al (g) + 3e ΔH = +5434KJmol

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The First Ionisation Energy for Sodium is;

+ - -1 Na(g) → Na (g) + e ΔH = +502KJmol

The Second Ionisation Energy for Sodium is;

+ 2+ - -1 Na (g) → Na (g) + e ΔH = +4560KJmol

Why?

The first electron to be removed from the group one sodium atom is in the outer energy level.

The second electron is removed from an energy level nearer the positive nuclear charge (greater electrostatic force of attraction) and is subject to a lesser degree of electron shielding (one less energy level) so requires a lot more energy to remove it.

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Electronegativity

A covalent bond is formed by sharing electrons between two atoms. The relative powers of the individual atoms to attract bonding electrons to themselves are different and are defined as their electronegativities.

Electronegativity is a periodic property as there are two trends associated with it;

 Increasing Electronegativity across a period.  Decreasing Electronegativity down a group.

Across a period in the Periodic Table Electronegativity increases as the positive nuclear charge increases, so, attracting the bonded electrons more strongly.

Down a group in the Periodic Table Electronegativity decreases as the outer electrons are further away from the positive nucleus (extra energy levels) and there is a screening effect from the inner electron energy level, so, the attraction for bonded electrons is weaker in strength.

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