MCQ Detailed View

The Z number of an element represents its atomic number, which is defined as the number of protons present in the nucleus of an atom. Since atoms are electrically neutral, the number of protons is equal to the number of electrons in the atom. This atomic number is a unique identifier for each element in the periodic table.

For chlorine (Cl), the atomic number (Z) is 17. This means:

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  Chlorine has 17 protons in its nucleus.

  
  


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  It also contains 17 electrons surrounding the nucleus in neutral state.

  
  


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  Its electronic configuration is 1s² 2s² 2p⁶ 3s² 3p⁵, which shows chlorine belongs to Group 17 (Halogens) and Period 3 of the periodic table.

  
  

Chlorine has a mass number of about 35.5 because it exists as a mixture of two stable isotopes:

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  Chlorine-35 (about 75%)

  
  


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  Chlorine-37 (about 25%)

  
  

The Z number is important because it determines the chemical identity and reactivity of the element. Chlorine, with its 17 protons and 7 valence electrons (in the 3p orbital), has a strong tendency to gain one electron to complete its octet. This is why chlorine commonly forms the chloride ion (Cl⁻) with a stable noble gas configuration similar to argon.

Key facts about chlorine:

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  Symbol: Cl

  
  


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  Atomic number (Z): 17

  
  


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  Atomic mass: ≈ 35.5 u

  
  


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  Group: Halogens (Group 17)

  
  


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  Period: 3

  
  


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  Common oxidation states: –1 (most common), +1, +3, +5, +7

  
  


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  Found in nature as sodium chloride (NaCl), also known as common salt.

  
  

Thus, the Z number for chlorine is 17, which defines its position in the periodic table and explains its strong non-metallic and reactive halogen behavior.