MCQ Detailed View

The Earth's crust is made up of many chemical elements, but some are far more abundant than others. The major elements that dominate the Earth's crust include oxygen, silicon, aluminum, iron, calcium, sodium, potassium, and magnesium. Together, these elements form over 99% of the crust's composition, mainly in the form of various minerals and rocks.

Among the given options:

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  Silicon (Si) is the second most abundant element in the Earth's crust, making up about 27% by weight. It is a key component of silicate minerals, which form most rocks and sand.

  
  


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  Aluminum (Al) is the third most abundant element, present mainly in minerals like bauxite, feldspar, and clay.

  
  


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  Carbon (C) is not as abundant as silicon or aluminum but still occurs naturally in the crust. It is found in compounds like carbonates (limestone, dolomite) and in organic matter like coal and petroleum deposits.

  
  

Radium (Ra), on the other hand, is a rare radioactive element found only in trace amounts in uranium and thorium ores. Its concentration in the Earth's crust is extremely low, typically around 1 part per trillion, making it one of the least abundant naturally occurring elements.Because radium is rare and unstable, it does not exist freely in significant amounts in the Earth's crust. It decays quickly into other radioactive elements, mainly radon gas. In contrast, silicon, aluminum, and carbon are relatively common and widespread in rocks, soil, and sediments.

Thus, the element not abundantly present in Earth's crust is Radium, as it occurs only in minute trace amounts compared to the other listed elements.