# Chemistry - Is an isobar the same as an isotope?

## Solution 1:

Not quite, an isotope has same number of protons ($$A- N = Z = \mathrm{constant}$$), but a different number of neutrons ($$\mathrm N$$ varies; e.g. $$\ce{^3_\color{red}{1}H}$$ and $$\ce{^2_\color{red}{1}H}$$, or $$\ce{^235_\color{red}{92}U}$$ and $$\ce{^238_\color{red}{92}U}$$ are isotopes).

An isobar has a fixed number of total nucleons ($$Z + N = A = \mathrm{constant}$$; e.g. $$\ce{^\color{red}{40}_19K}$$ and $$\ce{^\color{red}{40}_20Ca}$$, or $$\ce{^\color{red}{3}_2He}$$ and $$\ce{^\color{red}{3}_1H}$$ are isobars). Not nearly as mainstream as isotopes, but isobars are important to consider when doing mass spectroscopy.

Extra fact: For nuclei of the same number of neutrons ($$A - Z = N = \mathrm{constant}$$), the term is isotones.

## Solution 2:

I believe the definition you found may have been a little bit misleading. Here is another definition of isobar I found: each of two or more isotopes of different elements, with the same atomic weight. An isobar is referring to completely different elements. The prefix iso- means only one component must be the same between the different elements, and in the case of an isobar: mass.

I saw in your question the example that an isobar of carbon-13 would look like so $$\ce{^13C(p:7, n:6)}$$; however, this is not proper notation, as an isobar cannot be of the same element. Once the number of protons changes, your element is no longer the same.

Isobars are simply two different elements with the same mass while isotopes are two of the same elements with different masses.

## Solution 3:

Isobar is of more interest to physics than chemistry.

As others have explained, your definition is confusing. The one here may be clearer: isobar (Wikipedia).

In chemistry, the number of protons is most significant since it determines the number of electrons and hence the chemical behaviour. The number of neutrons is relatively unimportant: variants may be useful for labeling and may have slightly different behaviour (most noticeable for $$\ce{^1H}$$ and $$\ce{^2H}$$). So, isotope is used fairly frequently to discuss these variants of the elements. Isobars may have very different chemical behaviour and are unlikely to be an interesting grouping.

In nuclear physics, the number of neutrons and protons have a similar significance hence the term nuclide (Wikipedia) for atoms with a specific number of protons and neutrons is useful. Isobars have a closer connection with each other than isotopes since isobars can interconvert via beta processes relatively easily. It is rare for an atom to decay into an isotope. For example, $$\ce{^{14}C}$$ decays into $$\ce{^{14}N}$$ rather than $$\ce{^{13}C}$$ .

Isobars will be relevant in some specific situations in chemistry such mass spectroscopy. So, it is still useful to understand the concept.