Question

the table below provides some information about an unidentified element
physical properties:

• shiny silver - colored solid
• easily flattened with small hammer
• denser than water

chemical properties:

• does not react with sodium
• reacts with oxygen

based on this information, the unidentified element is best classified as which of the following?
o k (potassium - atomic number 19)
o cl (chlorine - atomic number 17)
o h (hydrogen - atomic number 1)
o he (helium - atomic number 2)

Explanation:

• Potassium (K):
• Physical properties: Metals are shiny (lustrous). Potassium is a metal and is a shiny solid. Metals are malleable (can be flattened), and potassium is a metal so it can be flattened with a hammer. Potassium has a density of \(0.86\space g/cm^{3}\) which is less than water's density (\(1\space g/cm^{3}\)), but let's check chemical properties. Sodium (\(Na\)) and potassium (\(K\)) are both alkali metals (Group 1). They have similar chemical properties and react with each other. But the element in question does not react with sodium. However, potassium reacts with oxygen (\(4K + O_{2}=2K_{2}O\) or \(K + O_{2}=KO_{2}\) depending on conditions).
• Chlorine (\(Cl\)):
• Physical properties: Chlorine is a non - metal. Non - metals are usually not shiny solids (chlorine is a green - yellow gas at room temperature). So it does not match the physical property of being a shiny silver - colored solid.
• Hydrogen (\(H\)):
• Physical properties: Hydrogen is a non - metal and is a gas at room temperature. So it does not match the physical property of being a shiny silver - colored solid.
• Helium (\(He\)):
• Physical properties: Helium is a noble gas (non - metal) and is a gas at room temperature. So it does not match the physical property of being a shiny silver - colored solid. Also, noble gases are inert and do not react with oxygen under normal conditions.

Answer:

K (Potassium - Atomic number 19)