# Why do different elements have different number of isotopes?

For example: Carbon-12,Carbon-13 and Carbon-14 are three isotopes of the element carbon with mass numbers 12, 13 and 14 respectively. Lithium-6 and Lithium-7 for lithium,etc.

My question is that are the number of isotopes for an element a random fact or there is a better explanation?

• Pedantically speaking, every element has an infinite number of isotopes. For example, there is a theoretical C-6, C-7, C-8, ... But we only refer to isotopes that we can isolate long enough to measure them. These need a "middle ground" of number of neutrons to be stable. – Ian Jan 9 '18 at 17:39
• Look up Nuclear Drip Line – Jim Garrison Jan 9 '18 at 18:59
• I've asked something related to this once in this site, and the full answer can be a bit complicated. In my question, the answer had this helpful link: www-nds.iaea.org/relnsd/vcharthtml/VChartHTML.html. – Vendetta Jan 9 '18 at 19:52
• @Ian: I'm not that sure. When the halflife is lower than the time it takes light to travel from one side of the "nucleus" to the other, you don't have interaction between all neutrons involved. – MSalters Jan 10 '18 at 10:23
• @Ian: Pedantically speaking, there's only a finite number of particles in the observable universe. So you won't find a $\mathrm{C-10^{81}}$ anytime soon. – Eric Duminil Jan 10 '18 at 11:21