s & p Block Elements (SL IB Chemistry)

What determines the strength of metallic bonds? 

• Not all metallic bonds are equal
• There are several factors that affect the strength of a metallic bond:

The charge on the metal ion

• The greater the charge on the metal ion, the greater the number of electrons in the sea of delocalised electrons and the greater the charge difference between the ions and the electrons
• A greater charge difference leads to a stronger electrostatic attraction, and therefore a stronger metallic bond
• This effect can be seen in melting point data across a period, as the charge on the metal ion increases without a significant change in ionic radius:

Melting point data of the Period 3 metals

Group	1	2	3 (13)
Metal	Sodium	Magnesium	Aluminium
Melting point / K	371	923	933

 
The melting point of the metal increases moving across a period, from left to right

The radius of the metal ion

• Metal ions with smaller ionic radii exert a greater attraction on the sea of delocalised electrons
• This greater attraction means a stronger metallic bond, requiring more energy to break
• This can be seen in data from metals, descending a group, where the charge on the ion remains constant but the ionic radius increases:

Melting point data of the Group 1 metals

Period	3 (13)	4	5
Metal	Sodium	Potassium	Rubidium
Melting point / K	371	336	312

 
The melting point of the metal decreases moving down a group

Trends in Melting Points of Metals

• The strength of electrostatic attraction can be increased by:
  • Increasing the number of delocalised electrons per metal atom
  • Increasing the number of positive charges on the metal centres in the lattice
  • Decreasing the size of the metal ions
• These factors can be seen in the trends across a period and down a group

Melting points of metals across a period

• If you compare the electron configuration of sodium, magnesium and aluminium you can see the number of valence electrons increases
  • Na = 1s²2s²2p⁶3s¹
  • Mg = 1s²2s²2p⁶3s²
  • Al  = 1s²2s²2p⁶3s²3p¹
• Aluminium ions are also a smaller size than magnesium ions or sodium ions and these two factors lead to stronger metallic bonding which can be seen in the melting points
• The stronger the metallic bonding, the more energy is needed to break the metallic lattice and so the higher the melting point
• As we go across Period 3, we can see the effect of stronger metallic bonding on the metals
  • Remember: Only the first three elements have metallic bonding in this graph

Melting point of elements across a period chart

Melting points as you go across a period. The metallic bonding gets stronger from Na to Al

Melting points of metals down a group

• As you go down the group, the size of the cation increases
  • This decreases the attraction between the outer electrons and the metallic lattice
  • Therefore, there is a reduction in the melting point

Melting point of metals down a group chart

Melting points as you go down a group of metals. The metallic bonding gets weaker from Li to Cs