Which of the following elements does not give the Borax bead test?

The Borax bead test is a classic qualitative analysis method used in inorganic chemistry to detect the presence of certain metal ions based on the color they produce when heated with borax (sodium tetraborate, Na₂B₄O₇·10H₂O). When borax is heated, it first loses water of crystallization and then forms sodium metaborate (NaBO₂) and boric anhydride (B₂O₃). The boric anhydride acts as a flux and dissolves metal oxides to form metal borates, which impart characteristic colors to the bead when hot and cold.

For example:

• 
  

  Copper (Cu²⁺): blue-green bead (hot), green (cold)

  
  


• 
  

  Chromium (Cr³⁺): green bead

  
  


• 
  

  Nickel (Ni²⁺): brownish-red bead

  
  

However, aluminium (Al³⁺) does not give any color in the borax bead test. This is because aluminium is not a transition metal and lacks partially filled d-orbitals that are responsible for producing colored compounds. When aluminium oxide (Al₂O₃) reacts with borax, it forms aluminium borate (Al(BO₂)₃), which is colorless and infusible, meaning it does not form a clear glassy bead or develop color under flame.

This difference highlights a key distinction between main group elements and transition elements in inorganic chemistry. Transition metals form colored coordination compounds due to d–d electronic transitions, whereas aluminium, being a p-block element with a fully filled 3p orbital, does not exhibit such properties.

Hence, aluminium (Al) is the correct answer as it does not give a borax bead test, whereas copper, chromium, and nickel show characteristic colors due to their transition metal nature. This principle is widely used in qualitative inorganic analysis to identify metal ions in laboratory testing