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Choose a suitable word to fill in the blanks in the following statements.
  
1. What is the valency of phosphorus with atomic number \(15\)?
   
 
2. Lithium (Li), Sodium(Na) and Potassium(K) form a Dobereiner’s triad. The atomic masses of lithium and potassium are \(7\) and \(39\) respectively. Predict the atomic mass of sodium.