Isotopes of an element have the same number of ?

Isotopes are different forms of the same chemical element that have the same number of protons but a different number of neutrons in their atomic nuclei. The defining feature of any element is its number of protons — also called the atomic number — which determines the element’s identity on the periodic table. Therefore, isotopes of an element must always have the same number of protons.

For example, all atoms of carbon contain 6 protons. However, carbon can exist in several isotopic forms such as Carbon-12, Carbon-13, and Carbon-14. These isotopes differ by their number of neutrons:

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  Carbon-12 has 6 neutrons,

  
  


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  Carbon-13 has 7 neutrons, and

  
  


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  Carbon-14 has 8 neutrons.

  
  

Despite these differences in neutron count, they are all forms of carbon because their proton number remains the same — 6. This shared proton number ensures that they have identical chemical behavior, bonding characteristics, and placement in the periodic table.

The varying number of neutrons, however, affects the atomic mass of each isotope, giving rise to small differences in physical properties. For example, some isotopes may be stable, while others are radioactive and decay over time, releasing radiation. Radioactive isotopes like Carbon-14 are useful in radiocarbon dating, while stable isotopes are used in medical imaging and scientific research.

Electrons, on the other hand, determine how atoms bond, but their number can change if atoms gain or lose charge (forming ions). Neutrons can vary without altering the element’s identity, which is why isotopes exist.

Hence, isotopes of an element always have the same number of protons, confirming their chemical identity while differing only in their neutron count.