Let’s look at the terminology using the helium (He) as an example.
The most common isotope of He can be written as ⁴He or ₂⁴He as in the example.
An isotope is where the atom has the same number of protons but a different number of neutrons.
He has 9 known isotopes ²He, ³He,⁴ He, ⁵He,⁶ He, ⁷He, ⁸He, ⁹He and ¹⁰He.
Each isotope of helium has 2 protons and number of neutrons varies for 0 to 8.
There are 92 naturally occurring elements ending in uranium where the most stable isotope is ²³⁸U mentioned in my previous post.
²³⁸U has 92 protons hence 238-92 = 146 neutrons.
Technically Np (neptunium) and Pu (plutonium) the two elements which follow U in the periodic table are also naturally occurring but only in trace amounts, their amounts have increased due to nuclear bomb testing last century.
Protons and neutrons are held together in the nucleus by the strong nuclear force but as more protons are in the nucleus the electrostatic repulsion between protons becomes significant and for manmade elements well beyond U in the periodic table the nucleus can undergo spontaneous fission and have extremely short half lives.
Another cause for instability is when a nucleus can absorb a neutron.
The most stable nuclei are when there are an even number of protons and neutrons, the least stable is when there are an odd number of protons and neutrons.
From my previous post ²³⁸U +n → ²³⁹U → ²³⁹Np + β⁻ → ²³⁹Pu → U²³⁵+ α.
²³⁸U is quite stable as it has an even number of protons and neutrons and has a half life of 4.5 billion years, when it absorbs a neutron ²³⁹U is produced but there is now an even odd number of protons and neutrons respectively.
As a result a neutron is converted into a proton creating ²³⁹Np and β⁻ radiation composed of electrons and anti-neutrinos are emitted.
²³⁹Np is also unstable which leads to the final equation and my question to you.
²³⁹Pu → U²³⁵+ α
²³⁹Pu has 94 protons in the nucleus and electrostatic repulsion of the protons has become significant resulting in α radiation where the ²³⁹Pu nucleus ejects an α particle composed of 2 protons and 2 neutrons and forms a ²³⁵U nucleus.
This has a half life of 24,110 years and a 4.5 billion year old earth explains why naturally occurring ²³⁹Pu occurs only in trace amounts.
My question is how can a 6000 year old earth result in these trace amounts when there has not been enough time to reduce ²³⁹Pu to trace amounts?