Both argon-38 and argon-40 have 18 protons but they have different atomic masses. The atomic mass of argon-38 is 38u while the atomic mass of argon-40 is 40u. Thus, they are isotopes.
What are isotopes?
- Some atoms contain the same number of protons (i.e. same atomic number) but they have a different number of neutrons (i.e. different mass numbers). These types of atoms are called isotopes.
- Argon element has three isotopes namely, argon-36, argon-38 and argon-40.
- All the three isotopes of argon have the same number of protons i.e. 18. But, all have different numbers of neutrons and thus, different mass numbers.
- Argon-36 has 18 neutrons, argon-38 has 20 neutrons and argon-40 has 22 neutrons.
- Thus, the mass numbers of argon-36, argon-38 and argon-40 are 36u, 38u and 40u respectively.
Thus, the structure of an atom of argon-38 and argon-40 is slightly different as they are isotopes. Argon 40 has more atomic mass than argon-38. But, both contain the same number of protons. They have similar chemical properties but different physical properties.
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