It is possible for many different elements to have the same number of nucleons. It all comes down to isotopes. For example, Nickel has many stable isotopes such as 59Ni, 60Ni etc... What this means is that in each case the number of protons is the same but the number of neutrons changes.
And here I notice a mistake, in that I used Nickel 59, which is an unstable isotope, and it should have been Nickel 58 which is the most abundant isotope. Still the conclusion is the same, as cobalt is lower in the table than Nickel though is more massive because of more neutrons. Remember though that what defines the position of an element on the periodic table is not its total mass number, but its atomic number (that is the number of protons and electrons in the atom). It is this number that defines the chemical properties of the element and hence its position. For nuclei, you have to consider the total number of nucleons, and so the nuclear properties (akin to chemical reactions, but are instead nuclear reactions) is defined by the nucleon (or mass) number.
The reason why Cobalt has more neutrons than Nickel comes down to quantum mechanics, in particular to do with pauli exclusion principle. its rather difficult to explain, but it comes down to the "shell model" of the nucleus, where the number of protons and neutrons required to form a stable nucleus depends upon how each fill the proton and neutron and coupled proton/neutron shells. This is similar to the shell model of electron orbitals, though a bit more complex since you are dealing with two types of indistinguishable fermi-dirac particles.
In a very broad sense, inside the nucleus there are two competing forces, namely the mutual repulsion from electrostatic charges of the protons, and the attractive strong force between those protons, and the neutral neutrons. If you were to make a recipe of making nuclei, too many protons will cause the mixture to be unstable (the electrostatic force over comes the strong force), and to stabilise the mixture, you add neutrons. Sometimes though too many neutrons will make the mixture too flat, and so less are required. What defines the number of protons and neutrons in the recipe is quantum mechanics, and it inst always intuitive (Nickel is a nice example, as the 58 and 60 weight isotopes are stable, but the 59 mass isotope is not).
So many other nuclei can have the same number of nucleons (but different combionations). They are called isobars. Whilst some isobars will be stable and some unstable, some will have long half lives. An example is above where I mistakenly took Nickel59 and not Nickel 58.
I do remember that, but I dont think I have that data handy anymore. If I did, it would be on my previous and now expired computer. if I remember correctly the speed of sound values came from a journal article i found for you. I know for a fact that i do not have a copy of that article. I will though have a route around and see if I can find the information again.
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