MCQ Detailed View

The atomic mass of an atom is concentrated in its nucleus. The nucleus, located at the center of the atom, contains protons and neutrons, collectively known as nucleons. Since the mass of electrons is extremely small compared to protons and neutrons, the contribution of electrons to the overall atomic mass is negligible.

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  Proton mass ≈ 1 atomic mass unit (amu)

  
  


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  Neutron mass ≈ 1 atomic mass unit (amu)

  
  


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  Electron mass ≈ 1/1836 of a proton (negligible)

  
  

This means that nearly the entire atomic mass comes from protons and neutrons, both of which are found inside the nucleus.

For example:

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  Hydrogen atom (¹H): It has 1 proton and no neutrons, so its mass is almost equal to 1 amu.

  
  


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  Carbon atom (¹²C): It has 6 protons and 6 neutrons, making its atomic mass approximately 12 amu.

  
  

The shells, orbits, and orbitals mentioned in the options are regions outside the nucleus where electrons are found. While these regions determine the atom’s chemical properties and bonding behavior, they do not contribute significantly to its mass.

Why not the other options?

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  Shell (option 1): Electron shells contain electrons, which have almost no mass.

  
  


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  Orbit (option 2): Similar to shells, orbits represent paths of electrons, but mass is not located there.

  
  


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  Orbital (option 4): Orbitals describe regions of space where electrons are likely to be found, but again, electrons are nearly massless compared to nucleons.

  
  

Key facts:

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  More than 99.9% of the atom’s mass lies in its nucleus.

  
  


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  The nucleus is extremely dense, with a radius about 100,000 times smaller than the atom’s total size.

  
  


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  Atomic mass is often expressed in atomic mass units (amu), where 1 amu = 1/12th of the mass of a carbon-12 atom.

  
  

Thus, the nucleus is the true mass center of the atom, making option 3 the correct choice.