Physical Properties of Period 3 Elements (Oxford AQA International A Level Chemistry)

Period 3 Trend: Atomic Radius

• The atomic radius is the distance between the nucleus and the outermost electron of an atom

• The atomic radius is measured by taking two atoms of the same element, measuring the distance between their nuclei and then halving this distance

  • In metals, this is also called the metallic radius

  • In non-metals, this is also called the covalent radius

Atomic radii of Period 3 elements table

Graph of atomic radii across Period 3

• Across Period 3, the atomic radii decreases because:

  • The number of protons (nuclear charge) and the number of electrons increases by one every time you go an element to the right

  • The elements in a period all have the same number of shells (so the shielding effect is the same)

  • So, as you go across the period the nucleus attracts the electrons more strongly and pull them closer to the nucleus

Explaining why atomic radius decreases across Period 3

Period 3 Trend: First Ionisation Energy

• Ionisation is the process by which an electron is removed from an atom or a molecule

• The first ionisation energy is the energy required to remove one mole of electrons from one mole of atoms of an element to form one mole of 1+ ions

  • E.g. the first ionisation energy of gaseous sodium

Na (g) → Na⁺ (g) + e^-          IE₁ = + 496 kJ mol^-1

Ionisation Process

• The first ionisation energy generally increases for each element going across Period 3 because:

  • The nuclear charge (the number of protons in the nucleus) increases

  • This causes the atomic radius of the atoms to decrease, as the outer shell is pulled closer to the nucleus

  • Therefore, the distance between the nucleus and the outer electrons decreases

  • The shielding by inner shell electrons remains reasonably constant across the period as electrons are being added to the same shell

  • So, it becomes harder to remove an electron as you move across the period so more energy is needed

Graph to show ionisation energies of Period 3 elements

• There are two elements which do not follow the general trend:

  • Aluminium has a lower first ionisation energy than magnesium

  • Sulfur has a lower first ionisation energy than phosphorus

• This can be explained by looking closely at their electronic configurations

Period 3 Trend: Melting Point

• It can be more difficult to observe the trend in melting points across Period 3

• A general increase in melting point for the Period 3 elements up to silicon is observed

  • Silicon has the highest melting point

• After the Si element, the melting points of the elements decrease significantly

Melting points of the elements across Period 3 table

Graph of melting points across Period 3

• The trends in melting point can be explained by looking at the bonding and structure of the elements

Na- Al

• Sodium, magnesium and aluminium all have metallic bonding

  • There are strong electrostatic forces of attraction between positive metal ions and the 'sea' of delocalised electrons

• The electrons in this sea are those from the outer shell of the atoms

  • Na will donate one electron into the sea of delocalised electrons, Mg will donate two and Al three electrons

• The metallic bonding is therefore stronger in Al

  • The electrostatic forces between a 3+ ion and the larger sea of delocalised electrons are much stronger compared to a 1+ ion and the smaller number of delocalised electrons in Na

Si

• Silicon has the highest melting point due to its giant molecular structure

• Each Si atom is held to its neighboring Si atoms by four strong covalent bonds

P-Ar

• P, S, Cl and Ar are non-metallic elements

• They exist as simple molecules (P₄, S₈, Cl₂ and Ar)

• The covalent bonds within the molecules are strong but there are only weak van der Waals' forces between the molecules

• Little energy is needed to overcome these van der Waals' forces so the melting points of these substances is low

• The strength of the van der Waal's forces is dependent on the number of electrons and size of the molecules

• The melting points of these elements from highest to lowest is S₈, P₄, Cl₂, Ar