Isobars

Do you know isobars were originally called as “isobares”? Isobars are atoms or nuclides of separate chemical elements having the same number of nucleons (protons+ neutrons). The name was given by Alfred Walter Stewart in 1918. It is originally taken from the combination of Greek words- isos means equal and bar means weight.

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More About Isobars

Atoms of chemical elements having same atomic mass but a different atomic number are called Isobars. The sum of the number of protons and neutrons together form the atomic mass. Therefore, we can also say the number of nucleons present in the nucleus is equal to the atomic mass of an atom. It will have the same number of nucleons.

The number of protons and neutrons alone will vary but the number of nucleons or the sum of protons and neutrons in isobars will always be same. Isobars always have different atomic structure because of the difference in atomic numbers. The number of neutrons makes up the difference in the number of nucleons. Therefore, they are always different chemical elements having same atomic masses. Thus, isobar has different chemical properties.

Learn the concept of an Atomic number here.

By now we know that the atomic masses in isobars are same. Therefore, they have similar physical properties. For example, the isobars iron and nickel. Iron and nickel have atomic number 26 and 28 respectively. However, the mass number is 58.

Another example is argon and calcium with atomic number 18 and 20, respectively. However, they have the same atomic mass 40. Furthermore, sometimes there are series of an element that has same atomic mass. For example, Cobalt (Co), Nickel (Ni), Copper (Cu), and Iron (Fe) have same atomic mass 64 but the atomic number varies.

Examples

• ₁₈Ar⁴⁰ ₁₉K⁴⁰ ₂₀Ca⁴⁰
• ₃₂ Ce⁷⁶, ₃₄Se⁷⁶
• ₁₁Na²⁴ ₁₂Mg²⁴
• ₂₆Fe⁵⁸, ₂₇ Ni⁵⁸ 
• ₂₇Co⁶⁴ ₂₈Ni⁶⁴

Browse more Topics under Structure Of Atom

• Introduction: Structure of Atom
• Atomic Number
• Bohr’s Model of Atom
• Charged Particles in Matter
• Isotopes
• Mass Number
• Neutrons
• Rutherford’s Model of an Atom
• Thomson’s Model of an Atom
• Valency
• How are Electrons Distributed in Different Orbits (Shells)?
• Sub-Atomic Particles
• Atomic Models
• Shapes of Atomic Orbitals
• Energies of Orbitals
• Quantum Numbers
• Development Leading to Bohr’s Model of Atom
• Emission and Absorption Spectra
• Towards Quantum Mechanical Model of Atom

You can download Structure of Atom Cheat Sheet by clicking on the download button below

Difference between Isotopes and Isobars

Isotope	Isobar
Isotopes are atomic structures of same elements having a different mass number/atomic mass	Isobars are different chemical elements having same atomic mass.
Atomic numbers of all isotopic forms of a single element are equal.	Atomic numbers of isobars vary from each other
They are the same chemical element but their forms are different	They are different elements altogether.
All isotopic forms of single elements have different physical properties.	Physical properties  can be similar to each other

Conclusion

Isotope and isobar indicate the relationships between chemical elements. Therefore, it is necessary to understand the difference between them. Isotopes provide the relationship of different forms of the single chemical element. Isobars indicate the relationship between different chemical elements but having the same atomic mass and thus the same physical properties.

Solved Questions for You

Q1: Isobars have the same number of ______?

1. Protons
2. Electrons
3. Neutrons
4. Nucleons

Ans: The answer is option 4 (Nucleons). The number of protons and neutrons alone will vary in isobars but the number of nucleons or the sum of protons and neutrons will always be same.

Q2. Which of the following statements about Isobars is false?

1. Their atomic numbers vary from each other.
2. Chemically they are the same element but their forms are different.
3. Physical properties can be similar to each other.
4. They are different chemical elements having same atomic mass.

Ans: The statement 2 is false. Isobars are different elements altogether.