Chemistry (Part-II)

A pure element is a substance composed of only one type of atom. Among the options: 

• Glass is a mixture of silica and other oxides.
• Cement is a complex mixture of compounds, including calcium silicates and aluminates. 
• Sodium is a chemical element with the symbol Na and atomic number 11. It exists as a pure substance consisting only of sodium atoms. 
• Steel is an alloy of iron and carbon, often containing other elements.

Therefore, the correct answer is Sodium.

Alkaline earth metals are elements in Group 2 of the periodic table. Among the options:

• Cobalt (Co) is a transition metal.
• Magnesium (Mg) is an alkaline earth metal.
• Copper (Cu) is a transition metal.
• Platinum (Pt) is a transition metal (noble metal).

Therefore, the correct answer is Magnesium.